Dry hot rock is a special geothermal resource featuring large reserves, wide distribution, clean energy generation, and recyclability. The United States, France and other countries have successfully developed and utilized dry hot rock to generate electricity and heat. Abundant dry hot rock resources have been developed in southern Tibet, western Yunnan, and the coastal areas of the southeast China, but they have not yet been exploited effectively. The efficient development of dry hot rock is of great significance in adjusting energy structure, reducing environment pollution, and coping with climate change challenges. Proper hydraulic fracturing can generate a complex fracture system in thermal reservoirs that will enhance heat exchange volume, which is the key element in the transformation from dry hot rock resources into energy. Based on the research status of dry hot rock fracturing technologies worldwide, this paper analyzed the characteristics of foreign main dry hot rock fracturing technology, and pointed out the situation and difficulties of fracturing faced in the development of dry hot rock in China. It is recommended in this paper that research focus on high temperature dry hot rock mechanics and in-situ stress characteristics, rupture and extension mechanisms of frac under thermal stress, ultra-high temperature staged/zonal fracturing tools, volume fracturing design method for high temperature hard formation and software development, and long-term real-time monitoring of fractures so as to develop the matched fracturing technology as soon as possible. At that point it can then promote the development and utilization of dry hot rock resources.

Deep marine shale gas in the southern Sichuan Basin has a deep burial depth, high formation temperature and formation pressure, poor rock drillability and thin high-quality reservoirs. Further, the contradictions between fast drilling and borehole quality, shale collapse and drilling cost, time-controlled drilling and the variable probability of encountering high-quality reservoirs have hindered its effective development. To this end, based on the experience of conventional shale gas development, through the differentiated casing program and borehole trajectory design, by adopting bent-housing mud motor BHA, optimization of drilling parameters, conducting vertical drilling and gas drilling technical tests, development of high-performance water-based drilling fluid and the borehole trajectory control, a set of key drilling technologies suitable for the effective development of deep shale gas in the Southern Sichuan were formed. Those technologies were applied in 27 deep marine shale gas wells in the Southern Sichuan, resulting in the fact that the the drilling cycle was shortened by 46.0% and the rate of high-quality encountering economic reservoirs reached 93.17%. The results indicated that those key drilling technologies could sustain the effective development of deep marine shale gas in southern Sichuan, and they could provide references for the development of similar shale gas reservoirs at home and abroad.

The Yingqiong Basin in western South China Sea is characterized by complex geological conditions, high bottomhole temperatures and pressures, and lost circulation which occurs frequently during drilling into the target layer. In order to solve the problem of frequent lost circulation in target layer, the main causes of lost circulation were analyzed, the idea of combining the high temperature resistant rigid plugging materials with high temperature resistant elastic plugging materials was followed, and a plugging slurry was prepared by adding DXD (a high temperature resistant rigid plugging material) and TXD (a high temperature resistant elastic graphite plugging material) into drilling fluid. The performance evaluation showed that the density of plugging slurry could reach 2.40 kg/L and the temperature resistance could reach 200 °C. The DXD could form bridges in the induced fractures, while the TXD could enter the residual pores of induced fractures under pressure difference, preventing the induced fractures from further opening and expanding, blocking the induced fractures and improving the pressure-bearing capacity of formation. This plugging slurry had been applied in several high temperature and high pressure (HTHP) wells in the Yingqiong Basin with the success rate of plugging increasing from 30% to 80%, indicating that the plugging slurry could block the induced fractures in the target layer of the Yingqiong Basin, improve the pressure-bearing capacity of formation and the success rate of plugging, and solve the problem of frequent lost circulation in the target layer.

The F Gas Field in Yinggehai Basin of the South China Sea is an HTHP gas field, characteristics of high temperature, high pressure and high CO₂ content has been the challenge to the integrity of wellbore, so the completion method was selected pertinently according to the characteristics of reservoirs. Based on the principle of keeping both the safety and the economy, the tubing/casing has been made by modified 13Cr material, and designed the different types of production strings. In terms of the characteristics of gas field and the development requirements, the selection of wellhead Christmas trees and downhole tools was carried out properly, the perforation pipe string was designed, and the proper annulus protection fluid was developed. In the end, it formed a completion technology suitable for the development of HTHP offshore gas fields with high acidic gas content. This completion technology has been applied in more than 10 wells of this gas field, and no sustained annulus pressure was observed during the production process, which indicated that the developed completion technology could effectively ensure the wellbore integrity of such gas fields, and provide supports for the large-scale development of HTHP gas fields in the Yinggehai Basin.

The goal was to improve the rock-breaking efficiency of the pointed rotary steering drilling system. To do so, a new process was developed, based on the research of bit kinematics, the digital PDC bit model and the digital rock model. The team used Matlab software, which, combined with the discretization processing of rock model, was able to simulate the interaction between PDC bit and rock under the condition of rotary steering drilling, and further obtain the quantitative calculation method of rock-breaking efficiency. The influence of the rotary speeds of the internal and external rings of pointed rotary steering system on rock-breaking efficiency of PDC bit was analyzed, and the rock-breaking law at different time steps was obtained. The results showed that for the rock with a shear strength of 11 MPa, an internal friction angle of 22º and a friction coefficient of 0.2, it had an optimal rock-breaking speed ratio of about 1.0, and the pointed rotary steering system presented a steady state cutting during the drilling process. Its rock-breaking efficiency was closely related to the speed ratio of the inner and outer eccentric rings. As the speed ratio increased, the rock-breaking efficiency also increased, but it eventually stabilized. The research results demonstrated a possible theoretical guidance for improving the drilling efficiency of pointed rotary steering system.

Oil-based drilling fluids generally have various disadvantages, for example, the base oil is difficult to degrade and non-renewable. Therefore, an investigation on the renewable biosynthesis oil-based drilling fluid system was carried out. The biosynthetic base oils were synthesized by the catalytic hydrogenation and molecular isomerization of natural bio-oils and fats. Based on the biosynthetic base oil and modified organic soil, the renewable biosynthesis-based drilling fluid system was formed through adding other drilling fluid additives and the optimized dosage, and its performance was evaluated. The biosynthetic base oil was a mixture of C₁₂–C₂₄ branched isoparaffins, which had excellent safety, environmental protection and viscosity-temperature properties. The high temperature and high pressure filtration loss of the renewable biosynthesis-based drilling fluid system was lower than 12 mL, the sedimentation stability was good, the demulsification voltage was up to 768 V, and the 96 h semi-lethal concentration was greater than 1 000 000 mg/L. The biosynthetic oil was able to resist the invasion of 20% formation water and 10% inferior soil, and the rolling recovery rate of cuttings reached 98.06%; the permeability recovery rate of the contaminated cores was up to 83.5%–92.3%. The research results showed that the oil with a biosynthetic base possessed the advantages of low toxicity, environmental protection, degradability and regenerability. The prepared drilling fluid exhibited good properties in emulsion stability, anti-pollution, lubrication, inhibition, reservoir protection, safety and environmental protection, which fully met the needs of drilling fluids under complex geological conditions.

In order to address the problems of borehole caving, blockage and sticking while drilling the Paleogene shale formation in the Xijiang Oilfield of the South China Sea, technical research has been carried out on anti-sloughing drilling fluids. Through analyses of stratigraphic mineral composition, physicochemical properties and mechanical parameters, the mechanisms of wellbore instability in the Paleogene shale formation have been clarified, and have established the relationship chart between the drilling fluid density required for maintaining wellbore stability and rock cohesion, so as to determine the minimum rock cohesion index required to sustain wellbore stability. In order to improve the strength of surrounding shale immersed in drilling fluid, a new anti-sloughing drilling fluid formula was obtained through selection of proper inhibitor and plugging agent as well as their optimal dosages. The research showed that the intrusion of drilling fluid filtrate would lead to the reduction of shale strength, which is the main reason for the wellbore instability of Paleogene shale formation in this oilfield; After immersing for 10 days in the new anti-sloughing drilling fluid added by 2.0% polyammonium salt, 0.5% nano-silica and 3.0% calcium carbonate into KCl-polymer drilling fluid, the rock sample still has the cohesive force of 8.8 MPa, which satisfies the required rock cohesion of greater than 8.7 MPa in the expected period. According to the comprehensive analysis, the new anti-sloughing drilling fluid featured by good inhibition, plugging and anti-sloughing effect, it can solve the problems encountered during Paleogene shale drilling in the Xijiang Oilfield of the South China Sea, and effectively control the borehole enlargement rate.

Well MaHW1602 in Mahu Oilfield owns complex formation conditions, the target layer in the Triassic Baikouquan Formation contains hard mudstone and loose conglomerate, and downhole events such as collapse and caving are prone to occur during horizontal section drilling. Due to the insufficient collapse resistance of the " desulfurization” drilling fluid applied in this area in recent years, through analyzing the formation characteristics and the challenges in drilling fluids, the activity modifier, plugging agent and inhibitor were preferably selected in the laboratory, and the non-sulfonated low-activity drilling fluid system was further developed. The results of indoor test show that the plugging rate of low-activity drilling fluid exceeds 90%, and the average water activity is 0.863, which meets the requirements of field drilling. The drilling fluid was applied in the third spud section of Well MaHW1602, there was no obvious hole enlargement in the loose and collapsing-prone section, the wellbore was regular, and the average hole enlargement rate was 6.5%. In addition, there was no obvious weight stacking during drilling, the electrical logging and casing RIH were performed smoothly without any problems such as the rapid changes in drilling fluid properties, lost circulation during RIH or pump starting, etc. Research and field application suggest that the low-activity drilling fluid has good plugging and anti-sloughing properties, which can meet the requirements for safe drilling of long lateral horizontal wells in the Mahu Oilfield.

In view of the fact of low accuracy of wellbore fluid temperature prediction and difficulty in identifying the position of the thief zone when the circulation loss occurs, a model of wellbore temperature field under lost circulation was established based on the analysis of wellbore heat transfer laws by comprehensively considering the influences of heat source items and variable mass flow on wellbore temperature. The reliability of this model was verified by field measured data, and the influences of the leakage rate and the thief zone on the temperature distribution laws of wellbore were analyzed. The numerical simulation results show that when compared with the model from Chen, the outputs from the new model are closer to the measured temperature, with the average relative error of 2.1%. The leakage rate imposes much greater influence on the bottom hole fluid temperature than the wellhead fluid temperature. In addition, when the leakage occurs in the upper open hole section, there is an inflection point on the temperature gradient distribution curve of the annulus fluids, and the position of the point is consistent with that of thief zone. The research results suggest that the model can accurately predict wellbore temperature distribution under lost circulation conditions, and the position of the thief zone can be determined in the field according to the annulus temperature gradient distribution curve.

Slim liner cementing in ultra-deep wells with narrow annulus in the No.1 District of the Shunbei Block faces a series of technical challenges such as weak cement sheath, high displacement pumping pressure, low displacement efficiency, high bottomhole temperature and the development of high-pressure brine layer, which compromises the cementing quality. In order to solve those problems, on the basis of summarizing the previous operation experiences, small liner cementing technology for ultra-deep wells with narrow annulus in the No. 1 District of the Shunbei Block was formed by improving wellbore preparation technologies, optimizing the high temperature/gas channeling-proof elastic toughness cement slurry system, conducting cement stone sealing integrity research and carrying out cementing rheology design and stable-killing gas channeling-proof optimization. It has been applied in 3 wells in this area with good cementing quality. The subsequent operations were free from water intrusion, ensuring the long-term sealing of section with a narrow annulus. This cementing technology can not only effectively solve the cementing problems of this block, but also effectively ensure the safe and efficient development of the block.

Given the TIV anisotropy characteristics of low porosity/permeability formations, the accurate calculation of horizontal in-situ stress is essential for the perforation and fracturing designs of those reservoirs. In order to more accurately and effectively calculate the horizontal in-situ stress of a TIV formation, considering the low-pressure characteristics of such formation, the Bowers method was used to obtain the formation pore pressure. Then, the array acoustic logging data was used to obtain the anisotropic rock mechanical parameters (the vertical/ horizontal Young’s modulus of elasticity and Poisson's ratio) of TIV formation. Considering the influence of bedding plane occurrence on horizontal in-situ stress, the traditional Sn model was improved and a new calculation model for the horizontal in-situ stress of TIV formation was established. The horizontal in-situ stresses of the TIV formations in the Chang 6, Chang 7 and Chang 8 sections of the Heshui area of the Ordos Basin were calculated by this new model. The maximum relative errors between the calculated and the measured maximum/minimum horizontal in-situ stresses were 8.70% and 7.86%, respectively, which were smaller than those of in-situ stresses calculated by Sn model. The results showed that the horizontal in-situ stress calculated by this new model was more in line with the variation laws of the vertical and horizontal distributions of the actual in-situ stress, which could provide a more reliable reference for the hydraulic fracturing design of tight oil reservoirs.

Improvements on the effect of fracturing stimulation is essential for the economic and effective development of tight reservoirs, and it is necessary to consider the distribution characteristics of complex fracture networks while evaluating the effect of fracturing stimulation. Based on microseismic data, distribution characteristics of secondary fractures were obtained by using a fractal random fracture networks generation algorithm. By considering the physical properties variation of the near borehole region of fractured horizontal wells, a mathematical model of fluid flow in multi-zone porous media of fractured horizontal wells was established and then resolved. Based on the model, the influences of the complexity of secondary fractures on the flow were studied. The results demonstrate that the complexity of secondary fractures has a significant impact on the entire flow stage. The findings include the larger the fractal dimension, the higher the yield; the wider the stimulated zone, the longer the steady-state turbulence early linear flow and the bilinear flow will last. The fracturing stimulated area with the distributed secondary fractures is the main area in the early to middle stage of production, and it makes the largest contribution to the production. Thus, during actual production, the best approach is to increase the stimulated volume so as to improve the early production and to maximize the EOR of reservoirs. The research results can provide a theoretical basis ofr the evaluation of a volume fracturing effect of tight reservoirs and the optimization of fracturing design.

In order to overcome the deficiencies of conventional horizontal well staged fracturing technologies, including complicated operations, the inability to achieve full drift size and limited number of fracturing stages, a horizontal well staged fracturing technology with an RFID intelligent sliding sleeve was proposed. The pipe string structure and technical challenges of RFID intelligent sliding sleeve fracturing technology were studied, on the basis of analyzing the characteristics of horizontal staged fracturing technologies and experiments carried out scheme optimization, simulation analysis, unit test and indoor prototype test for an RFID intelligent sliding sleeve structure. The test results showed that the RFID intelligent sliding sleeve could be smoothly opened to the preset openness under the temperature of 120 °C and pressure difference of 0–70 MPa. The RFID tag ball can control the window of RFID intelligent sliding to the preset openness, with RFID communication having an excellent success rate. The reesults indicated that RFID intelligent sliding sleeve-based horizontal well staged fracturing technology could reduce the cost of horizontal well fracturing, improve the efficiency of oil and gas development, and provide technical support for the intelligent, cost-saving and efficient development of oil and gas, with good foundation for future application.

In view of the low efficiency, high risk and high labor intensity of coiled tubing fishing with a conventional tubing string, research was conducted to investigate the current status of coiled tubing operation technologies. The process of coiled tubing fishing with coiled tubing was proposed, and a specific tool was developed. By virtue of its modular design, the tool was able to integrate the functions of fish top rotation entrance, fish top detection, fish top capture and shearing into the tool, so that the tool could adapt to the operation conditions of coiled tubing fishing. The results of simulation calculation and laboratory test verified that the performance of this tool met the desired design requirements. This fishing tool was used in Well X–1 of Tarim Oilfield on coiled tubing, and it finished 13 fishing jobs in ϕ88.9 mm production string. In total 2 851.87 m of ϕ38.1 mm coiled tubing that had been seriously stuck in the hole was retrieved, and the retrieval rate was up to 100%, while the fishing efficiency was four times higher than that of the conventional fishing string. The field application also verified the stability of tool’s performance. Studies suggested that this coiled tubing fishing tool could also be effectively applied in coiled tubing fishing, and it could provide technical references in the future for fishing in coiled tubing.

Having a closely detailed description of the nonlinear relationship between flow velocity and pressure gradient in low permeability reservoir is necessary for accurately developing the frac design, and calculating the production of a group (or unit) of wells that have been hydraulically fractured. Therefore, based on the description of the nonlinear seepage characteristics of low permeability reservoir, a nonlinear mathematical model of coupling low permeability reservoir and hydraulic fractures was established, which divided the seepage process into the nonlinear seepage stage and quasi-linearity stage according to the seepage characteristics. The Taylor expansion was used to linearize the nonlinear mathematical model, and established the finite difference equations, and then formed the computer solving model. The results of example analysis showed that the distributions of formation pressure and saturation calculated by the nonlinear mathematical model were in line with the actual situations of the stratum; the fracture flow conductivity of injection well in the fractured five-spot well pattern decreased with the formation closure, which led to poor water injection effect and low oil well production. Thus, the fracture design should be modified in accordance with the study’s results. The study results indicated that the nonlinear mathematical model and hydraulic fracture coupling could accurately describe the nonlinear relationship between flow velocity and pressure gradient in low-permeability reservoir. This breakthrough establishes a foundation to calculate the production of fractured well group in low-permeability reservoir accurately, and provides a guidance for water flooding development of low permeability reservoir.

During drilling the deep shale gas horizontal wells in the Weirong Shale Gas Field in southern Sichuan, it is very difficult to accurately identify the marker bed, determine the bit position and judge the penetrating state of bit. To solve this problem, the X-ray fluorescence (XRF) element mud logging data was used to establish a three-terminal chart, an element intersection chart and a three-dimensional element display chart, as well as a quantitative calculation model of evaluation parameters such as rock density and gas content. The Geosteering Technology has been formed by such principle, including that fact that a three-terminal chart was used to identify the key marker beds before window-entering, the element intersection chart was used to guide the accurate window-entering of horizontal well, the three-dimensional element display chart was used to determine the penetration of the bit in a horizontal section, and the shale density and gas content etc., were used to guide the optimization of horizontal wellbore trajectory. This technology has been applied in 18 horizontal shale gas wells in the Weirong Shale Gas Field, with the average drilling rate of the target up to 98.06%. Research results showed that the technology could improve the rate of encountering high-quality reservoirs by accurately identifying the marker bed, determining the bit position and judging the penetrating state of the bit.

The fluctuation of interval transit time data can be caused by factors such as variable formation lithology which affects the accurate identification of abnormally compacted strata by interval transit time. Therefore, the fluctuation of interval transit time data in normally compacted sections was analyzed according to the relationship between the interval transit time and well depth at normally compacted mudstone section, by eliminating the invalid data points using a density data clustering method, made possible by means of wavelet theory and the probability analysis method. Research results showed that the density clustering method could effectively remove the anomalous data points. One wavelet decomposition could meet the requirement to describe the fluctuation of interval transit time data at the normally compacted section; the distribution fitting test indicated that the fluctuation of interval transit time data was consistent with tLocation–Scale probability distribution. Thus the abnormally compacted section could be quantitatively identified by constructing a probability calculation formula. The calculation results showed that the abnormally compacted section could be quantitatively identified by calculating the fluctuation probability of interval transit time, improve the identification accuracy of abnormally compacted section, and avoid the blindness and randomness of analysis. Similar analyses can be performed for other conventional logging data analyses based on the normal compaction curve of shale.

Seeking to solve the problem of low transmission rate of existing LWD data and the demands on real-time gamma imaging, sector gamma-ray imaging characteristics were simulated and analyzed. This occurred while a near bit gamma-ray imaging tool crossing inclined interface moving from low-level radioactive sandstone to high-level radioactive mudstone, and thus a rapid sinusoidal fitting method based on the sector gamma imaging was proposed. Based on the studies, a rapid sinusoidal fitting firmware algorithm for near-bit gamma imaging logging was designed. This algorithm combined the least squares frequency estimation and 3-parameter sine fitting to obtain the 4 parameters of the sinusoid, such as amplitude, frequency, phase and DC component, hence obtaining the multi-sector gamma imaging through fitting inversion. By using the fast sinusoidal fitting method, the gamma data was acquired in the simulated wellbore fabricated with the standard rock samples and layered dipping strata, and the fitting of multi-sector gamma imaging measurement was realized with a small fitting error. The results showed that the sinusoidal features of the 8-sector gamma image obtained by fitting inversion were clear and they could accurately reflect the information of the inclined strata interface, which verified the correctness and feasibility of the fast sinusoidal fitting method.