2015 Vol. 43 No. 5
Abstract：
Tight oil resources are abundant in China, but they are only developed in small scale presently, understanding of the reservoir and engineering technologies are still at initial stage of development. On the basis of introducing the characteristics of tight oil reservoirs and Sinopec’s current development, the major challenges and opportunities for development of tight oil reservoirs were analyzed. It is indicated that main challenges in developing tight oil reservoir include complex geologic conditions, development technologies need to be improved, and the ways to develop tight oil reservoirs efficiently under the situation of low oil prices. Moreover, it is proposed in five aspects for developing tight oil reservoir in China, i.e. (1) identifying enrichment laws, (2) analyzing development mechanism, (3) further exploring the engineering technologies, (4) making more efforts in EOR technologies, and (5) establishing development and management modes, which will play an important part for efficient development of tight oil reservoirs, and keep national energy supply and security in China.
Tight oil resources are abundant in China, but they are only developed in small scale presently, understanding of the reservoir and engineering technologies are still at initial stage of development. On the basis of introducing the characteristics of tight oil reservoirs and Sinopec’s current development, the major challenges and opportunities for development of tight oil reservoirs were analyzed. It is indicated that main challenges in developing tight oil reservoir include complex geologic conditions, development technologies need to be improved, and the ways to develop tight oil reservoirs efficiently under the situation of low oil prices. Moreover, it is proposed in five aspects for developing tight oil reservoir in China, i.e. (1) identifying enrichment laws, (2) analyzing development mechanism, (3) further exploring the engineering technologies, (4) making more efforts in EOR technologies, and (5) establishing development and management modes, which will play an important part for efficient development of tight oil reservoirs, and keep national energy supply and security in China.
Abstract：
This paper identifies and discusses the multiple geologic engineering challenges involved in shale gas exploitation which include reservoir characterization, safe and fast drilling, environmentally-friendly and efficient exploitation, and it puts them within a context of China’s experience in exploring for and exploiting shale gas.The paper elaborates upon the key fundamentals, which include geologic mechanical behavior and prediction theories of non-linear shale engineering, safe and quality shale oil and gas well drilling theories based on multiple coupling, dynamic and random fracture control mechanisms and non-aqueous fracturing technologies of shale beds, and multiscale seepage characteristics and exploitation theories of shale oil and gas. The following advanced theoretical issues should be solved as soon as possible. It was necessary to investigate the evolution rules of physical, chemical and mechanical properties of shale reservoirs and carry out mathematical characterization, analyze the interaction between discontinuous surrounding rocks and drilling and completion fluids under multiple coupling conditions, develop dynamic and random fracture control methods, long-term effective diverting mechanisms and non-aqueous fracturing technologies for shale reservoirs, explore micro-nano scale adsorption and desorption mechanisms of shale, and study multi-phase seepage theories with upscaling and multiple coupling. The paper concludes with a summary of research progress and development trends in the key advanced mechanical issues.This paper serves as a reference and guidance for the scientific and efficient development of shale oil and gas in China.
This paper identifies and discusses the multiple geologic engineering challenges involved in shale gas exploitation which include reservoir characterization, safe and fast drilling, environmentally-friendly and efficient exploitation, and it puts them within a context of China’s experience in exploring for and exploiting shale gas.The paper elaborates upon the key fundamentals, which include geologic mechanical behavior and prediction theories of non-linear shale engineering, safe and quality shale oil and gas well drilling theories based on multiple coupling, dynamic and random fracture control mechanisms and non-aqueous fracturing technologies of shale beds, and multiscale seepage characteristics and exploitation theories of shale oil and gas. The following advanced theoretical issues should be solved as soon as possible. It was necessary to investigate the evolution rules of physical, chemical and mechanical properties of shale reservoirs and carry out mathematical characterization, analyze the interaction between discontinuous surrounding rocks and drilling and completion fluids under multiple coupling conditions, develop dynamic and random fracture control methods, long-term effective diverting mechanisms and non-aqueous fracturing technologies for shale reservoirs, explore micro-nano scale adsorption and desorption mechanisms of shale, and study multi-phase seepage theories with upscaling and multiple coupling. The paper concludes with a summary of research progress and development trends in the key advanced mechanical issues.This paper serves as a reference and guidance for the scientific and efficient development of shale oil and gas in China.
Abstract：
During the fracturing stimulation of unconventional oil and gas reservoirs (e.g. shale gas), various challenges are encountered, including the large amount of water consumption, difficult disposal of flow-back waste fluid and formation/reservoir damage. It is meaningful to explore a new non-aqueous fracturing technology to replace the traditional hydraulic fracturing technology for development and production of unconventional oil and gas reservoirs. The dialkyl phosphate gelling agent was formed through the reaction among phosphorus pentoxide (P2O5), triethyl phosphate (TEP) and mixed alcohols, it was mixed with a modified complex iron crosslinking agent in the low-carbon alkane to produce the low-molecular alkane non-aqueous fracturing fluid. By analyzing its basic performance and rheological property,the result showed that the fracturing fluid gel was prepared by the crosslinking ratio 100.0:3.5 and gelling agent mass fraction 1.5%), which can meet the requirements of reservoir fracturing operations with good temperature resistance, shear resistance and proppant carrying capacity.The viscous elasticity and shear thinning capacity of crosslinking gel tend to vary with the mass fraction of gelling agent, with obvious regularity. The rheological curves of the low-molecular alkane fracturing fluid can be characterized by the non-linear co-rotational Jeffreys constitutive equation, which provides the theoretical basis for the new fracturing fluid.
During the fracturing stimulation of unconventional oil and gas reservoirs (e.g. shale gas), various challenges are encountered, including the large amount of water consumption, difficult disposal of flow-back waste fluid and formation/reservoir damage. It is meaningful to explore a new non-aqueous fracturing technology to replace the traditional hydraulic fracturing technology for development and production of unconventional oil and gas reservoirs. The dialkyl phosphate gelling agent was formed through the reaction among phosphorus pentoxide (P2O5), triethyl phosphate (TEP) and mixed alcohols, it was mixed with a modified complex iron crosslinking agent in the low-carbon alkane to produce the low-molecular alkane non-aqueous fracturing fluid. By analyzing its basic performance and rheological property,the result showed that the fracturing fluid gel was prepared by the crosslinking ratio 100.0:3.5 and gelling agent mass fraction 1.5%), which can meet the requirements of reservoir fracturing operations with good temperature resistance, shear resistance and proppant carrying capacity.The viscous elasticity and shear thinning capacity of crosslinking gel tend to vary with the mass fraction of gelling agent, with obvious regularity. The rheological curves of the low-molecular alkane fracturing fluid can be characterized by the non-linear co-rotational Jeffreys constitutive equation, which provides the theoretical basis for the new fracturing fluid.
Abstract：
In order to study the hole stability of horizontal wells in a shale formation, it is necessary to accurately and completely evaluate the rock strength in shale reservoirs. Based on the characteristics of shale reservoirs in the Longmaxi Formation, multiple evaluation methods (e.g. compression test, buckle hardness test and scratch test) were used to evaluate and analyze the compressive strength of shale. It is shown that the compressive strength of Longmaxi shale is generally in the range of 100-300 MPa, with an average of 123.71 MPa. After the shale has a contact with water, its compressive strength around the micro cracks fall by about 80%. Research showed that the compressive strength of Longmaxi shale was more influenced by the relative enrichment of mineral composition,the formations with high content of quartz and low content of clay mineral have higher compressive strength.The effect of micro cracks on strength of shale is a dominant characteristic. When water based drilling fluids are used for drilling horizontal wells in shale formation, it is necessary to strengthen the sealing capacity in micro cracks so as to effectively lower the risk of hole instability.
In order to study the hole stability of horizontal wells in a shale formation, it is necessary to accurately and completely evaluate the rock strength in shale reservoirs. Based on the characteristics of shale reservoirs in the Longmaxi Formation, multiple evaluation methods (e.g. compression test, buckle hardness test and scratch test) were used to evaluate and analyze the compressive strength of shale. It is shown that the compressive strength of Longmaxi shale is generally in the range of 100-300 MPa, with an average of 123.71 MPa. After the shale has a contact with water, its compressive strength around the micro cracks fall by about 80%. Research showed that the compressive strength of Longmaxi shale was more influenced by the relative enrichment of mineral composition,the formations with high content of quartz and low content of clay mineral have higher compressive strength.The effect of micro cracks on strength of shale is a dominant characteristic. When water based drilling fluids are used for drilling horizontal wells in shale formation, it is necessary to strengthen the sealing capacity in micro cracks so as to effectively lower the risk of hole instability.
Abstract：
Due to rock friability and the strong difference of rock mechanical characteristics between parallel and vertical bedding directions of the horizontal bedding shale, existing in-situ stress calculation methods could not meet the accuracy requirements and it is difficult to perform laboratory in-situ tests on the cores. Therefore, shale in-situ stress evaluation methods were studied in this paper so as to provide the effective guidance for well drilling and fracturing design. The shale elasticity parameters in different bedding directions were obtained by means of uniaxial compression tests, and the transversely isotropic characteristics were presented obviously. Based on the constitutive relation of transversely isotropic materials, the model for horizontal bedding shale in-situ stress inverted from its adjacent sand-shale in-situ stress was established, after it was assumed that there was no relative displacement between the formations in the process of the deposition and later tectonic movement. An analysis was conducted on sensitivity factors and influence laws. It is shown that the shale in-situ stress was controlled by its own elastic parameters and the elastic modulus, Poisson’s ratio and in-situ stress value of its adjacent sand-shale beds. The two horizontal in-situ stresses calculated with this method were higher than those obtained with the Terzaghi and Newberry models, and they were between the upper and lower limits of Huang Rongzun model.The research results in this paper provided a new method for evaluating the in-situ stress of horizontal bedding shale.
Due to rock friability and the strong difference of rock mechanical characteristics between parallel and vertical bedding directions of the horizontal bedding shale, existing in-situ stress calculation methods could not meet the accuracy requirements and it is difficult to perform laboratory in-situ tests on the cores. Therefore, shale in-situ stress evaluation methods were studied in this paper so as to provide the effective guidance for well drilling and fracturing design. The shale elasticity parameters in different bedding directions were obtained by means of uniaxial compression tests, and the transversely isotropic characteristics were presented obviously. Based on the constitutive relation of transversely isotropic materials, the model for horizontal bedding shale in-situ stress inverted from its adjacent sand-shale in-situ stress was established, after it was assumed that there was no relative displacement between the formations in the process of the deposition and later tectonic movement. An analysis was conducted on sensitivity factors and influence laws. It is shown that the shale in-situ stress was controlled by its own elastic parameters and the elastic modulus, Poisson’s ratio and in-situ stress value of its adjacent sand-shale beds. The two horizontal in-situ stresses calculated with this method were higher than those obtained with the Terzaghi and Newberry models, and they were between the upper and lower limits of Huang Rongzun model.The research results in this paper provided a new method for evaluating the in-situ stress of horizontal bedding shale.
Abstract：
It is necessary to accurately predict the fracture initiation pressure at different positions when a staged hydraulic jet fracturing program is designed and optimized. In this paper,therefore, a fracture initiation model which takes the induced stress field into consideration is established for the staged hydraulic jet fracturing on the basis of Westergaard theory. The effect of the induced stress of the first fracture on the following initiation fracture was investigated and compared with the actual fracturing data. In addition, an analysis was conducted to determine the effect of fracture height, net fracture pressure, minimum horizontal in-situ principal stress and fracture spacing from the first to the second one on the initiation pressure of the second fracture.The calculated results showed that the model calculations and the test values were maintained identical. In situations with practical parameters, the following initiation pressure increased by 3 MPa if the net pressure at the fracture plane increased by 5 MPa. It increased by 2 MPa, and the influential range of induced stress field enlarged by 30 m if the height of first fracture increased by 10 m. The following initiation pressure increased with the shortening of the spacing to the first fracture, with maximum increase ratio 21%. The minimum horizontal in-situ principal stress had no effect on the influential range of induced stress field. It was shown that the fracture initiation model established in this paper was better accordant with the field testing data. The following fracture initiation pressure was significantly affected by the net fracture pressure, the fracture height and the spacing to the first fracture, but was hardly affected by the minimum horizontal in-situ principal stress. The research results in this paper played an instructive role for the design and optimization of staged hydraulic jet fracturing programs.
It is necessary to accurately predict the fracture initiation pressure at different positions when a staged hydraulic jet fracturing program is designed and optimized. In this paper,therefore, a fracture initiation model which takes the induced stress field into consideration is established for the staged hydraulic jet fracturing on the basis of Westergaard theory. The effect of the induced stress of the first fracture on the following initiation fracture was investigated and compared with the actual fracturing data. In addition, an analysis was conducted to determine the effect of fracture height, net fracture pressure, minimum horizontal in-situ principal stress and fracture spacing from the first to the second one on the initiation pressure of the second fracture.The calculated results showed that the model calculations and the test values were maintained identical. In situations with practical parameters, the following initiation pressure increased by 3 MPa if the net pressure at the fracture plane increased by 5 MPa. It increased by 2 MPa, and the influential range of induced stress field enlarged by 30 m if the height of first fracture increased by 10 m. The following initiation pressure increased with the shortening of the spacing to the first fracture, with maximum increase ratio 21%. The minimum horizontal in-situ principal stress had no effect on the influential range of induced stress field. It was shown that the fracture initiation model established in this paper was better accordant with the field testing data. The following fracture initiation pressure was significantly affected by the net fracture pressure, the fracture height and the spacing to the first fracture, but was hardly affected by the minimum horizontal in-situ principal stress. The research results in this paper played an instructive role for the design and optimization of staged hydraulic jet fracturing programs.
Abstract：
In order to reveal the microscopic mechanical mechanisms of rock stress sensitivity and to further understand stress sensitivity, a geometry and topology structure analysis was carried out using CT scanning technology which could show the actual internal structure characteristics of rocks, and can be combined with digital cores and a pore network model. Following this procedure, the relationships between effective stress,pore structure and permeability were obtained, and pore structure parameters and percolation capacities were compared during the increase and decrease of effective stress. It is shown that with the increasing of effective stress, the pore radius distribution curve shifted to the left, the shape factor probability curve shifted to the right, the pore throat connectivity became worse resulting in a decrease in permeability, and vice versa. The pore structure deformation was characterized by stress sensitivity hysteresis. After the effective stress dropped, permeability could not fully recover. In summary, it is necessary to pay attention to the effect of stress sensitivity and fluid flow on the deformation of rock pores.
In order to reveal the microscopic mechanical mechanisms of rock stress sensitivity and to further understand stress sensitivity, a geometry and topology structure analysis was carried out using CT scanning technology which could show the actual internal structure characteristics of rocks, and can be combined with digital cores and a pore network model. Following this procedure, the relationships between effective stress,pore structure and permeability were obtained, and pore structure parameters and percolation capacities were compared during the increase and decrease of effective stress. It is shown that with the increasing of effective stress, the pore radius distribution curve shifted to the left, the shape factor probability curve shifted to the right, the pore throat connectivity became worse resulting in a decrease in permeability, and vice versa. The pore structure deformation was characterized by stress sensitivity hysteresis. After the effective stress dropped, permeability could not fully recover. In summary, it is necessary to pay attention to the effect of stress sensitivity and fluid flow on the deformation of rock pores.
Abstract：
Drilling footage and service life of cone bits were short and the comprehensive benefits were low in the Yuanba Gas Field,due to the fact that the Ziliujing and Xujiahe Formations in deep onshore regions are tight, hard and highly abrasive. Based on practical drilling situations, high-efficiency drill bits were selected through the analysis of the formations’ drillability and rock breaking principles. Appropriate dynamic drilling tools were selected according to the matching principle of drill bits and drilling tools.Then, field tests were conducted on the following three drill bits and tools combinations, including imported impregnated diamond bits and high-speed turbines, impregnated diamond bits and screws, and cones and PDC. Drill bits were selected on the basis of specific energy for unit footage consumption, and the three combinations were economically evaluated using breakeven analysis. Based on field test results, adoption was recommended for the homemade impregnated diamond bit and high-speed screw drilling for high-pressure and highly abrasive tight formations, and the impregnated diamond bit and high-speed turbo drilling for Zhenzhuchong Formation with high gravel content, and the cone and PDC drilling for high-pressure tight formations with more interbeds of sandstones and mudstones. Studies showed that the application of homemade impregnated diamond drill bit and high-speed screw drilling gave the best economic benefit contributions, followed by cone and PDC drilling followed by the imported impregnated diamond drill bit and high-speed turbo drilling.
Drilling footage and service life of cone bits were short and the comprehensive benefits were low in the Yuanba Gas Field,due to the fact that the Ziliujing and Xujiahe Formations in deep onshore regions are tight, hard and highly abrasive. Based on practical drilling situations, high-efficiency drill bits were selected through the analysis of the formations’ drillability and rock breaking principles. Appropriate dynamic drilling tools were selected according to the matching principle of drill bits and drilling tools.Then, field tests were conducted on the following three drill bits and tools combinations, including imported impregnated diamond bits and high-speed turbines, impregnated diamond bits and screws, and cones and PDC. Drill bits were selected on the basis of specific energy for unit footage consumption, and the three combinations were economically evaluated using breakeven analysis. Based on field test results, adoption was recommended for the homemade impregnated diamond bit and high-speed screw drilling for high-pressure and highly abrasive tight formations, and the impregnated diamond bit and high-speed turbo drilling for Zhenzhuchong Formation with high gravel content, and the cone and PDC drilling for high-pressure tight formations with more interbeds of sandstones and mudstones. Studies showed that the application of homemade impregnated diamond drill bit and high-speed screw drilling gave the best economic benefit contributions, followed by cone and PDC drilling followed by the imported impregnated diamond drill bit and high-speed turbo drilling.
Abstract：
With the application of low density cement slurries, the existing cementing quality evaluation criteria are no longer applicable, so it is necessary to amend the criteria after sufficiently studying the various influencing factors. A downhole acoustic field model for cased wells is established in this paper through analysis of acoustic field. Combining the effect of different factors on acoustic characteristics of set cement with the basis of evaluation criteria for cementing quality, the improvement methods for the criteria was proposed. Furthermore, the operation procedure of improved evaluation criteria for cementing quality was provided.The logging evaluation index for improved cementing quality was obtained by comparing the logging response after improving the conditions with that in conventional conditions.The fitting functional equations of acoustic amplitudes were made by using different densities of cement slurry, the correlation coefficients for all equations are higher than 0.93. Through the comparison of linear coefficients of matching equations, it is shown that the matching coefficient of micro-silica low density cement slurry is higher than that of floating bead micro-silica low density cement slurry, and the lowest one is from the low-density coal ash cement slurry. After waiting on the cement for 48 hours, the upper coefficient limits of their acoustic amplitudes are 20.74, 19.15 and 18.82, and the lower coefficient limits are 20.85, 14.81 and 12.96 respectively. The cementing quality evaluation indices of all low-density cement slurries decrease with increasing density, and they have a very strong linear relationship between them. In light of the improved evaluation criteria, the evaluation indices can be calculated accurately by density of various cement slurries. This method can play an important role for the improvement of cementing quality evaluation criteria.
With the application of low density cement slurries, the existing cementing quality evaluation criteria are no longer applicable, so it is necessary to amend the criteria after sufficiently studying the various influencing factors. A downhole acoustic field model for cased wells is established in this paper through analysis of acoustic field. Combining the effect of different factors on acoustic characteristics of set cement with the basis of evaluation criteria for cementing quality, the improvement methods for the criteria was proposed. Furthermore, the operation procedure of improved evaluation criteria for cementing quality was provided.The logging evaluation index for improved cementing quality was obtained by comparing the logging response after improving the conditions with that in conventional conditions.The fitting functional equations of acoustic amplitudes were made by using different densities of cement slurry, the correlation coefficients for all equations are higher than 0.93. Through the comparison of linear coefficients of matching equations, it is shown that the matching coefficient of micro-silica low density cement slurry is higher than that of floating bead micro-silica low density cement slurry, and the lowest one is from the low-density coal ash cement slurry. After waiting on the cement for 48 hours, the upper coefficient limits of their acoustic amplitudes are 20.74, 19.15 and 18.82, and the lower coefficient limits are 20.85, 14.81 and 12.96 respectively. The cementing quality evaluation indices of all low-density cement slurries decrease with increasing density, and they have a very strong linear relationship between them. In light of the improved evaluation criteria, the evaluation indices can be calculated accurately by density of various cement slurries. This method can play an important role for the improvement of cementing quality evaluation criteria.
Abstract：
In order to best tap the potential of shale reservoirs and optimize the placement of horizontal wells during exploration and development, an integrated steering drilling technology was researched and developed in this paper. First, after an in-depth understanding of the shale reservoir was achieved, a high-resolution 3D fine geologic model was developed that could accurately predict "sweet spots". Second, optimization design was conducted on the well trajectory on the basis of the formation parameters extracted from the 3D geologic model. Third, microstructure change of target zones was ascertained by using reservoir prediction technologies, so as to provide the geologic basis for the well trajectory prediction. Fourth, the position of the bit was predicted in the hole accurately by means of geological target tracking and trajectory adjustment technologies. And finally, real-time modification was conducted on the geologic model by estimating the real formation dip, so that the well trajectory could be extended reasonably in the reservoirs. The technique was applied in Well NH2-4 during the drilling of its horizontal section. And based on the technique, it was 35.00 m from the horizontal section to bottom boundary of high-quality shale, and the corrected formation dip was 6.48°. The geological model was real-time modified and the trajectory parameters were adjusted for time, so high quality reservoir drilling rate achieved 94.5%. It is shown that the integrated steering drilling technology for geologic engineering provided apractical and feasible integration technology for the development of shale gas.
In order to best tap the potential of shale reservoirs and optimize the placement of horizontal wells during exploration and development, an integrated steering drilling technology was researched and developed in this paper. First, after an in-depth understanding of the shale reservoir was achieved, a high-resolution 3D fine geologic model was developed that could accurately predict "sweet spots". Second, optimization design was conducted on the well trajectory on the basis of the formation parameters extracted from the 3D geologic model. Third, microstructure change of target zones was ascertained by using reservoir prediction technologies, so as to provide the geologic basis for the well trajectory prediction. Fourth, the position of the bit was predicted in the hole accurately by means of geological target tracking and trajectory adjustment technologies. And finally, real-time modification was conducted on the geologic model by estimating the real formation dip, so that the well trajectory could be extended reasonably in the reservoirs. The technique was applied in Well NH2-4 during the drilling of its horizontal section. And based on the technique, it was 35.00 m from the horizontal section to bottom boundary of high-quality shale, and the corrected formation dip was 6.48°. The geological model was real-time modified and the trajectory parameters were adjusted for time, so high quality reservoir drilling rate achieved 94.5%. It is shown that the integrated steering drilling technology for geologic engineering provided apractical and feasible integration technology for the development of shale gas.
Abstract：
Hard brittle clay shale has micro-fractured bedding and tends to cause side-wall collapse. Particularly when being invaded by foreign fluids,the rock has obvious rheological effects. Taking hard brittle shale as the object of study, uniaxial creep experiments on water saturation samples of formations with different degrees of mineralization were conducted. Research results showed that under the same externally applied loads, the creep deformation degree of the formation water saturated rock samples with a salinity of 6.0×104 mg/L was the least (0.001-0.014) and the creep deformation degree of the formation water saturated rock samples with a salinity of 8.0×104 mg/L was a little greater (0.005-0.024), while the creep deformation degree of the formation water saturated rock sample with a salinity of 4.0×104 mg/L was the greatest (0.010-0.030).Learning from the modeling ideas of classic element combination models, the creep model reflecting three creep stages was established.Three important parameters (instantaneous elastic modulus, viscous coefficient, accelerated creep viscous coefficient) in the model tended to decrease obviously. Test results showed that the relation between rock mineralization and creep deformation degree was quadratic non-linear, and that too high or too low salinity would have the greatest influence on the mechanical properties of rocks. The study suggested that the influences of drilling fluid salinity on borehole wall stability should be considered in drilling operations.
Hard brittle clay shale has micro-fractured bedding and tends to cause side-wall collapse. Particularly when being invaded by foreign fluids,the rock has obvious rheological effects. Taking hard brittle shale as the object of study, uniaxial creep experiments on water saturation samples of formations with different degrees of mineralization were conducted. Research results showed that under the same externally applied loads, the creep deformation degree of the formation water saturated rock samples with a salinity of 6.0×104 mg/L was the least (0.001-0.014) and the creep deformation degree of the formation water saturated rock samples with a salinity of 8.0×104 mg/L was a little greater (0.005-0.024), while the creep deformation degree of the formation water saturated rock sample with a salinity of 4.0×104 mg/L was the greatest (0.010-0.030).Learning from the modeling ideas of classic element combination models, the creep model reflecting three creep stages was established.Three important parameters (instantaneous elastic modulus, viscous coefficient, accelerated creep viscous coefficient) in the model tended to decrease obviously. Test results showed that the relation between rock mineralization and creep deformation degree was quadratic non-linear, and that too high or too low salinity would have the greatest influence on the mechanical properties of rocks. The study suggested that the influences of drilling fluid salinity on borehole wall stability should be considered in drilling operations.
Abstract：
Low reservoir volume and low single well production are bottlenecks that restrict coal bed methane(CBM) development, so how to improve the drilling efficiency and how to control the costs are the keys to the effective development of CBM. After summarizing the previous drilling experiences in South Yanchuan CBM block, the well factory drilling model was successfully applied and many technical research and field applications focused on reducing the cost and increasing benefits were carried out. Techniques included cluster well optimization, preventive deviation anticollision, navigational BHA, individualized bit design, efficient leak stoppage, etc. Then, the learning curve was used to optimize and improve key technologies and develop low cost and efficient drilling technology. Low cost and efficient drilling technology that was developed in the South Yanchuan CBM Block was applied to 808 wells and the operations were performed with good quality and high efficiency, in which the average single well drilling cycle was shortened by more than 30.3%, and productivity construction and penetration rate were improved by over 51.6%. The application showed that the well factory drilling mode could be employed in the field of CBM development to significantly control drilling costs, improve the efficiency to cost-effectively develop CBM.
Low reservoir volume and low single well production are bottlenecks that restrict coal bed methane(CBM) development, so how to improve the drilling efficiency and how to control the costs are the keys to the effective development of CBM. After summarizing the previous drilling experiences in South Yanchuan CBM block, the well factory drilling model was successfully applied and many technical research and field applications focused on reducing the cost and increasing benefits were carried out. Techniques included cluster well optimization, preventive deviation anticollision, navigational BHA, individualized bit design, efficient leak stoppage, etc. Then, the learning curve was used to optimize and improve key technologies and develop low cost and efficient drilling technology. Low cost and efficient drilling technology that was developed in the South Yanchuan CBM Block was applied to 808 wells and the operations were performed with good quality and high efficiency, in which the average single well drilling cycle was shortened by more than 30.3%, and productivity construction and penetration rate were improved by over 51.6%. The application showed that the well factory drilling mode could be employed in the field of CBM development to significantly control drilling costs, improve the efficiency to cost-effectively develop CBM.
Abstract：
It is much more difficult to identify fractures in marine-continental transitional facies tight clastic rocks from well logging,so a study was proposed to carry out quantitative identification on single-well fracture development sections in marine-continental transitional facies clastic formations by using array sonic logging data, combined with the Gassmann equation, adaptive matrix mineral and skeleton modulus extraction method, and DEM theoretical model. It is shown that the extracted Ko(matrix mineral bulk modulus of clastics)ranges from 13 to 58 GPa, which is slightly higher than Ks (bulk modulus)and Kd (dry rock skeleton bulk modulus), and μo(matrix mineral shear modulus) ranges from 5 to 18 GPa, which is slightly higher than μs (shear modulus). Shear wave time difference can be predicted effectively with relative error 3.5% from adaptive method. Fractures can be predicted by introducing modulus parameters extracted by the adaptive method into the DEM theoretical model, and its coincidence rate is apparently superior to that of the conventional logging method, the fracture parameter method,the multi-parameters probability discriminance method and the R/S method. In conclusion, the newly proposed fracture identification method is reliable, with its calculation results highly coincident with practical value.
It is much more difficult to identify fractures in marine-continental transitional facies tight clastic rocks from well logging,so a study was proposed to carry out quantitative identification on single-well fracture development sections in marine-continental transitional facies clastic formations by using array sonic logging data, combined with the Gassmann equation, adaptive matrix mineral and skeleton modulus extraction method, and DEM theoretical model. It is shown that the extracted Ko(matrix mineral bulk modulus of clastics)ranges from 13 to 58 GPa, which is slightly higher than Ks (bulk modulus)and Kd (dry rock skeleton bulk modulus), and μo(matrix mineral shear modulus) ranges from 5 to 18 GPa, which is slightly higher than μs (shear modulus). Shear wave time difference can be predicted effectively with relative error 3.5% from adaptive method. Fractures can be predicted by introducing modulus parameters extracted by the adaptive method into the DEM theoretical model, and its coincidence rate is apparently superior to that of the conventional logging method, the fracture parameter method,the multi-parameters probability discriminance method and the R/S method. In conclusion, the newly proposed fracture identification method is reliable, with its calculation results highly coincident with practical value.
Abstract：
In order to satisfy the requirement for real-time formation pressure monitoring and geomechanical parameters calculation, an acoustic nipple while drilling was developed based on the analysis of key technologies of acoustic measurement while drilling. The internal and external diameters of the nipple are 57.2 mm and 171.0 mm,respectively. It has one emitter and two receivers in operation mode. The emitting acoustic system is equipped with one monopolar sub-emitting transducer, and the receiving acoustic system is equipped with two receivers with four broad-band receiving transducers for each. The emitting and receiving acoustic systems consist of two separated parts. The spacing between the emitter and the receiver can be changed, but it is fixed at 200 mm among receivers. Based on the test of acoustic measurement,acoustic excitation frequency of emitting acoustic system is 12.92 kHz and its directivity pattern is similar to an ellipse. In the receiving acoustic system, the resonance frequency of eight receiving transducers is in the range of 30.84-33.53 kHz, averaging 32.23 kHz. The full wave form of the cased hole in the test was recorded by using the nipple and the acoustic measurement circuit while drilling, and the calculated acoustic velocity was 5 100 m/s. The successful development of the acoustic nipple while drilling will provide significant technical data for the development in China of acoustic logging devices while drilling.
In order to satisfy the requirement for real-time formation pressure monitoring and geomechanical parameters calculation, an acoustic nipple while drilling was developed based on the analysis of key technologies of acoustic measurement while drilling. The internal and external diameters of the nipple are 57.2 mm and 171.0 mm,respectively. It has one emitter and two receivers in operation mode. The emitting acoustic system is equipped with one monopolar sub-emitting transducer, and the receiving acoustic system is equipped with two receivers with four broad-band receiving transducers for each. The emitting and receiving acoustic systems consist of two separated parts. The spacing between the emitter and the receiver can be changed, but it is fixed at 200 mm among receivers. Based on the test of acoustic measurement,acoustic excitation frequency of emitting acoustic system is 12.92 kHz and its directivity pattern is similar to an ellipse. In the receiving acoustic system, the resonance frequency of eight receiving transducers is in the range of 30.84-33.53 kHz, averaging 32.23 kHz. The full wave form of the cased hole in the test was recorded by using the nipple and the acoustic measurement circuit while drilling, and the calculated acoustic velocity was 5 100 m/s. The successful development of the acoustic nipple while drilling will provide significant technical data for the development in China of acoustic logging devices while drilling.
Abstract：
Coiled tubing has been widely used in shale gas development in China, there have been significant difficulties, such as tubing collapse, deformation, self-locking, low efficiency in drilling and milling, getting stuck and falling into the hole. Aiming to mitigate or eliminate these problems, relative analysis and technical investigation have been conducted at home and abroad, especially by using the high performance compound connector with small size, anti-drop PDM, five-blade concave mill, hydraulic oscillator and fishing accelerator, a integrated coiled tubing technology for supplementary fracturing and gas testing has been formed in shale gas development, including plug setting in long horizontal section, a drilling bridge plug, perforating, fishing and sand washing in an extended horizontal well. Field application demonstrated that by using the new techniques, the time of drilling one plug was reduced from 70 min to 40 min, the drilling plug cycle in a well reduced from 9 days to 5 days, drilling and removing rate of plugs raised from 97% to 100%, tubing collapse has not happened in perforating, the success rate of fishing and setting plug reached to 100%. All in all, the practice showed that the integrated technology could improve the success rate of fracturing and gas testing in shale gas wells.
Coiled tubing has been widely used in shale gas development in China, there have been significant difficulties, such as tubing collapse, deformation, self-locking, low efficiency in drilling and milling, getting stuck and falling into the hole. Aiming to mitigate or eliminate these problems, relative analysis and technical investigation have been conducted at home and abroad, especially by using the high performance compound connector with small size, anti-drop PDM, five-blade concave mill, hydraulic oscillator and fishing accelerator, a integrated coiled tubing technology for supplementary fracturing and gas testing has been formed in shale gas development, including plug setting in long horizontal section, a drilling bridge plug, perforating, fishing and sand washing in an extended horizontal well. Field application demonstrated that by using the new techniques, the time of drilling one plug was reduced from 70 min to 40 min, the drilling plug cycle in a well reduced from 9 days to 5 days, drilling and removing rate of plugs raised from 97% to 100%, tubing collapse has not happened in perforating, the success rate of fishing and setting plug reached to 100%. All in all, the practice showed that the integrated technology could improve the success rate of fracturing and gas testing in shale gas wells.
Abstract：
It is difficult to forecast the production capacities of stepped horizontal wells when producing multiple reservoirs at multiple sections. On the basis of the relevant theories such as seepage mechanics, engineering fluid mechanics, reservoir engineering and numerical analysis, a new detailed coupled model of stepped wells and thin interbedded reservoirs was established by utilizing discretization processing and considering factors such as anisotropy, filtering interference, pipe flow pressure drop and contamination caused by drilling and/or completion. This productivity calculation model of multilayer commingled production of stepped wells has a unique solution. Taking a three thin interbedded reservoir as an example, we analyzed with this model the wellbore radial flow rate, wellbore flow and wellbore flow pressure distribution rules of stepped horizontal wells. Numerical simulation results showed that flow friction in the wells should not be overlooked, and the stepped horizontal well flow rate distribution presented the shape of a high-order polynomial, the flow rate distribution adhered to cubic polynomial law, flow pressure distribution stayed within parabolic limits, and pressure dropped faster at the heel section. The entire well production calculated by this model is just 1.79% less than Joshi’s. It demonstrated that the calculation results for production were reasonable and provided a new method for productivity prediction of stepped horizontal wells.
It is difficult to forecast the production capacities of stepped horizontal wells when producing multiple reservoirs at multiple sections. On the basis of the relevant theories such as seepage mechanics, engineering fluid mechanics, reservoir engineering and numerical analysis, a new detailed coupled model of stepped wells and thin interbedded reservoirs was established by utilizing discretization processing and considering factors such as anisotropy, filtering interference, pipe flow pressure drop and contamination caused by drilling and/or completion. This productivity calculation model of multilayer commingled production of stepped wells has a unique solution. Taking a three thin interbedded reservoir as an example, we analyzed with this model the wellbore radial flow rate, wellbore flow and wellbore flow pressure distribution rules of stepped horizontal wells. Numerical simulation results showed that flow friction in the wells should not be overlooked, and the stepped horizontal well flow rate distribution presented the shape of a high-order polynomial, the flow rate distribution adhered to cubic polynomial law, flow pressure distribution stayed within parabolic limits, and pressure dropped faster at the heel section. The entire well production calculated by this model is just 1.79% less than Joshi’s. It demonstrated that the calculation results for production were reasonable and provided a new method for productivity prediction of stepped horizontal wells.
Abstract：
For low production, high fluid loss, and low pressure gas formations, the application of conventional workover fluid may result in drainage difficulties, formation damage and production recovery. A low density and solid-free foam workover fluid was developed by adding high effective amphoteric ionic foaming agent and foam stabilizer in the circulating foam drilling fluid to make it form micro bubble materials with a special structure. Its density is between 0.50 and 0.95 kg/L, viscosity is less than 90 mPa·s, and stabilization time is 24 h under 100 ℃. Comprehensive experimental results indicated that the kind of workover fluid has a slight damage to payzone, with low filtration rate and good plugging effect, the influence of temperature and pressure on the workover fluid is not serious. Its application in Well Ba-II Xi 80104 of Xinjiang indicated that this bubble workover fluid performed very well to ensure smooth completion and workover operation and might be used in high fluid loss, strong water-sensitive, and water lock low pressure gas reservoirs.
For low production, high fluid loss, and low pressure gas formations, the application of conventional workover fluid may result in drainage difficulties, formation damage and production recovery. A low density and solid-free foam workover fluid was developed by adding high effective amphoteric ionic foaming agent and foam stabilizer in the circulating foam drilling fluid to make it form micro bubble materials with a special structure. Its density is between 0.50 and 0.95 kg/L, viscosity is less than 90 mPa·s, and stabilization time is 24 h under 100 ℃. Comprehensive experimental results indicated that the kind of workover fluid has a slight damage to payzone, with low filtration rate and good plugging effect, the influence of temperature and pressure on the workover fluid is not serious. Its application in Well Ba-II Xi 80104 of Xinjiang indicated that this bubble workover fluid performed very well to ensure smooth completion and workover operation and might be used in high fluid loss, strong water-sensitive, and water lock low pressure gas reservoirs.
Abstract：
In situ combutiom (ISC) technology of heavy oil and oil sand reservoirs presents low cost, high utilization rate of thermal energy and high recovery rate. currently,it is deemed a technical research hotspot in heavy oil and oil sands development. Based on technical investigations, combustion principles and types, technical characteristics of conventional ISC were summarized and analyzed in this paper. The latest research on ISC, oil displacement principles of ISC, well patterns, applicability, advantages and disadvantages of the technology, and engineering difficulties were introduced in detail. These technologies include COFCAW, THAI, COGD, CAGD, and SAGDOX. The studies suggested that the innovation research of ISC technology mainly focused on well pattern and integration with other projects and well stimulation measures. As a result, it had no such technical difficulties as in traditional ISC, which promoted its rapid development and field application. On the other hand, it could integrate the technical characteristics of steam injection method and in-situ combustion method to improve oil displacement effects and heavy oil and oil sands reservoir recovery efficiency. The innovation and development of ISC technology would provide guidance in the exploitation of heavy oil in oil sands in China and also provide technical references for the subsequent technology reserve and innovation of ISC.
In situ combutiom (ISC) technology of heavy oil and oil sand reservoirs presents low cost, high utilization rate of thermal energy and high recovery rate. currently,it is deemed a technical research hotspot in heavy oil and oil sands development. Based on technical investigations, combustion principles and types, technical characteristics of conventional ISC were summarized and analyzed in this paper. The latest research on ISC, oil displacement principles of ISC, well patterns, applicability, advantages and disadvantages of the technology, and engineering difficulties were introduced in detail. These technologies include COFCAW, THAI, COGD, CAGD, and SAGDOX. The studies suggested that the innovation research of ISC technology mainly focused on well pattern and integration with other projects and well stimulation measures. As a result, it had no such technical difficulties as in traditional ISC, which promoted its rapid development and field application. On the other hand, it could integrate the technical characteristics of steam injection method and in-situ combustion method to improve oil displacement effects and heavy oil and oil sands reservoir recovery efficiency. The innovation and development of ISC technology would provide guidance in the exploitation of heavy oil in oil sands in China and also provide technical references for the subsequent technology reserve and innovation of ISC.
Abstract：
In order to solve reducing WOB stack of horizontal and directional wells,increase the ROP and shorten the period of drilling in Block 40 Linpan Oilfield, an impactor reducing WOB stack has been tested and applied. The feasibility of the impactor in horizontal and directional wells has been verified through the impactor simulation in horizontal wells, and its performance parameters and life evaluation have been tested. The test results showed the impactor had an impact force of 20-160 kN,impact frequency of 8-20 Hz,and working life of more than 120 h. According to the design of the wells, the friction of the drill string was simulated. On this basis, the BHA and drilling parameters were optimized. Field test of three sets of reducing weight stack preventing impactors were conducted in Well Pan 40-Xie 501 with a total footage of 1 080 m, and an effective working life of more than 100 h, and significant friction drag reduction. The results of application showed that the impactor was an effective tool to solve the friction problem in long horizontal section of horizontal and directional wells.
In order to solve reducing WOB stack of horizontal and directional wells,increase the ROP and shorten the period of drilling in Block 40 Linpan Oilfield, an impactor reducing WOB stack has been tested and applied. The feasibility of the impactor in horizontal and directional wells has been verified through the impactor simulation in horizontal wells, and its performance parameters and life evaluation have been tested. The test results showed the impactor had an impact force of 20-160 kN,impact frequency of 8-20 Hz,and working life of more than 120 h. According to the design of the wells, the friction of the drill string was simulated. On this basis, the BHA and drilling parameters were optimized. Field test of three sets of reducing weight stack preventing impactors were conducted in Well Pan 40-Xie 501 with a total footage of 1 080 m, and an effective working life of more than 100 h, and significant friction drag reduction. The results of application showed that the impactor was an effective tool to solve the friction problem in long horizontal section of horizontal and directional wells.
2015, 43(5): 116-122.
DOI: 10.11911/syztjs.201505020
Abstract：
Because drilling problems can occur when hydraulic oscillators are used to increase the rate of penetration (ROP) in directional and horizontal sectiondrilling, the application and research status of hydraulic oscillators were studied. Firstly, analysis was performed on various hydraulic oscillators in other parts of the world to determine advantages and disadvantages based on their structures. Then, a comparison study was carried out on the velocity improvement between hydraulic oscillators and rotary steering tools based on their field application. And finally, a series of recommendations were proposed. Hydraulic oscillators can decrease friction, improve ROP, shorten drilling cycle and reduce drilling cost. Compared with rotary steering tools, hydraulic oscillators can increase ROP by 29.8% and reduce drilling cost by RMB 380 000 Yuan. In practical operation, however, the application of the hydraulic oscillators isstrongly influenced by improper pumping rate and tools setting, and due to its high pressure consumption, precision instruments (e.g. MWD) may be damaged and resistance to erosion is not satisfactory. In order to solve these problems,it is necessary to continuously improve hydraulic oscillators.
Because drilling problems can occur when hydraulic oscillators are used to increase the rate of penetration (ROP) in directional and horizontal sectiondrilling, the application and research status of hydraulic oscillators were studied. Firstly, analysis was performed on various hydraulic oscillators in other parts of the world to determine advantages and disadvantages based on their structures. Then, a comparison study was carried out on the velocity improvement between hydraulic oscillators and rotary steering tools based on their field application. And finally, a series of recommendations were proposed. Hydraulic oscillators can decrease friction, improve ROP, shorten drilling cycle and reduce drilling cost. Compared with rotary steering tools, hydraulic oscillators can increase ROP by 29.8% and reduce drilling cost by RMB 380 000 Yuan. In practical operation, however, the application of the hydraulic oscillators isstrongly influenced by improper pumping rate and tools setting, and due to its high pressure consumption, precision instruments (e.g. MWD) may be damaged and resistance to erosion is not satisfactory. In order to solve these problems,it is necessary to continuously improve hydraulic oscillators.
Abstract：
In order to study the plugging mechanisms and optimize the structure of foam metal screen,the plugging process for foam metal screens was simulated by a large-scale sand control simulation device. With the screen plugging degree evaluating the specific productivity index, the plugging degree and mechanism under the different shale content, the screen precision and the thickness of the foam metal were analyzed. The experimental results showed that surface particle bridging was the main cause of foam metal screen plugging. The specific productivity index exponentially decreases with the increase of shale content where it finally stabilized at 0.075 m3/(d·MPa·m), which decreased by 78% compared with the index where there was no shale content. When the pore size of foam metal is increased, the specific productivity index increases. The increase of foam metal thickness had slight impact on the specific productivity index. The experimental study of plugging mechanism and influencing factors of foam metal screen provided theoretical basis for the optimization of screen structure.
In order to study the plugging mechanisms and optimize the structure of foam metal screen,the plugging process for foam metal screens was simulated by a large-scale sand control simulation device. With the screen plugging degree evaluating the specific productivity index, the plugging degree and mechanism under the different shale content, the screen precision and the thickness of the foam metal were analyzed. The experimental results showed that surface particle bridging was the main cause of foam metal screen plugging. The specific productivity index exponentially decreases with the increase of shale content where it finally stabilized at 0.075 m3/(d·MPa·m), which decreased by 78% compared with the index where there was no shale content. When the pore size of foam metal is increased, the specific productivity index increases. The increase of foam metal thickness had slight impact on the specific productivity index. The experimental study of plugging mechanism and influencing factors of foam metal screen provided theoretical basis for the optimization of screen structure.
Abstract：
The Sulige gas field belongs to the largest tight sandstone gas reservoir in China, and Su 53 is the only block in which horizontal wells have been implemented in integrated development. In order to improve the development efficiency at a low cost, and following the previous practice and experience of advanced factory operations experience both domestically and abroad, suitable factory drilling and completion techniques for Sulige gas field have been determined by regional geological research, which optimize the design scheme and the construction management. The key technologies include geosteering for horizontal wells, drilling and reservoir stimulation. Geosteering technology can better guide horizontal wells accurately into the target zones with high efficiency by improving the geological model and adjusting the well trajectory. Drilling technology includes optimizing the casing program and well trajectory, optimizing PDC bits and drilling fluid system as well as designing the drilling rig move on a rail system. Reservoir stimulation technique integrates the volume fracturing into synchronous fracturing to improve producing reserves according to the regional geological characteristics and drilling parameters. Through the implementation of factory drilling and completion in Su 53 Block, the drilling cycle of the average single well was shortened for 15.98 days, the drilling payzone rate raised 4.9% points, and the gas production increased 4 900 cubic meters per day respectively compared with traditional horizontal wells. The factory drilling and completion techniques could provide a new paradigm for domestic unconventional gas reservoir development with horizontal wells.
The Sulige gas field belongs to the largest tight sandstone gas reservoir in China, and Su 53 is the only block in which horizontal wells have been implemented in integrated development. In order to improve the development efficiency at a low cost, and following the previous practice and experience of advanced factory operations experience both domestically and abroad, suitable factory drilling and completion techniques for Sulige gas field have been determined by regional geological research, which optimize the design scheme and the construction management. The key technologies include geosteering for horizontal wells, drilling and reservoir stimulation. Geosteering technology can better guide horizontal wells accurately into the target zones with high efficiency by improving the geological model and adjusting the well trajectory. Drilling technology includes optimizing the casing program and well trajectory, optimizing PDC bits and drilling fluid system as well as designing the drilling rig move on a rail system. Reservoir stimulation technique integrates the volume fracturing into synchronous fracturing to improve producing reserves according to the regional geological characteristics and drilling parameters. Through the implementation of factory drilling and completion in Su 53 Block, the drilling cycle of the average single well was shortened for 15.98 days, the drilling payzone rate raised 4.9% points, and the gas production increased 4 900 cubic meters per day respectively compared with traditional horizontal wells. The factory drilling and completion techniques could provide a new paradigm for domestic unconventional gas reservoir development with horizontal wells.
