Digital Core Construction Methods for High Stress in Deep and Ultra-Deep Oil and Gas Reservoirs
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摘要:
深层超深层油气藏由于埋藏深，其地应力达200 MPa，会显著改变储层岩石孔隙的微观结构。数字岩心是孔隙尺度数值模拟的重要载体，但是现有数字岩心重构方法是基于常温常压下岩心的扫描图像重构，不能反映高应力下的孔隙结构。为此，提出了一种基于离散元法考虑高应力影响的数字岩心重构方法。首先，采用分水岭算法分割CT图像，利用球面谐波分析方法建立轮廓数据库，并在PFC3D中建立Clump（团簇）模板库；然后，根据孔隙度和粒径分布使用模板库中的Clump建立离散元模型，并用两点相关和线性路径相关函数曲线评价模型的准确性；随后，标定颗粒间微观力学参数，并加载应力模拟得到不同应力下的数字岩心；最后，分析了不同应力下数字岩心的孔隙几何拓扑结构，计算孔隙度和渗透率。以Bentheim砂岩为例，构建了其不同应力下的数字岩心，研究结果表明，应力增大，导致孔隙和喉道半径缩小、喉道伸长、连通性变差、孔隙度和渗透率减小。研究结果为深层超深层油气藏孔隙尺度模拟提供了技术途径。
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关键词:
- 油气藏 /
- 深层超深层；数字岩心重构 /
- 离散元法 /
- 孔隙几何拓扑结构 /
- 渗透率
Abstract:Deep and ultra-deep oil and gas reservoirs buried at significant depths and subjected to ground stresses of 200 MPa, undergo notable changes in the pore microstructure of reservoir rocks. Digital core modeling serves as a crucial tool for pore-scale numerical simulations. However, current digital core reconstruction methods are based on scanning image reconstruction under normal temperature and pressure conditions and thus they fail to reflect the pore structure under high pressure conditions. Therefore, a digital core reconstruction method based on the discrete element method (DEM) was proposed by considering the effect of high stress. Initially, the watershed algorithm was employed to segment computed tomography (CT) images, and the contour database was established by the spherical harmonic analysis method. The Clump template library was established in PFC3D. Then, according to porosity and particle size distribution, the Clump in template library was used to build a discrete element model. After, the accuracy of the model was evaluated via calculations of two-point correlation and linear path correlation function curves. Next, the micromechanical parameters between particles were calibrated, enabling simulation of the digital core under varying stress conditions. Finally, the pore geometry topology of the digital core under different stresses was analyzed, and porosity and permeability were calculated. Bentheim sandstone was taken as an example to construct digital cores under different stresses.. The research results show that high stress leads to reduced pore and throat radius, elongated throats, diminished connectivity, and lower porosity and permeability. The results provide technical support for pore-scale simulations of deep and ultra-deep oil and gas reservoirs..
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图 2 常温常压数字岩心离散元建模流程
Figure 2. Flowchart of digital core modeling at normal temperature and pressure by DEM
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图 3 样品粒径及颗粒形状分析结果
Figure 3. Analysis results of particle size and particle shapes of the sample
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图 4 球面描述符与SH阶数的关系曲线
Figure 4. Relationship between the spherical descriptor and SH degree
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图 5 常温常压数字岩心离散元建模结果
Figure 5. Results of digital core modeling at normal temperature and pressure by DEM
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图 7 重构岩心与真实岩心的两点相关和线性路径相关函数曲线对比结果
Figure 7. Comparison of two-point correlation and linear path correlation function curves between reconstructed and real cores
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图 8 颗粒间微观力学参数标定模型及偏应力-应变曲线
Figure 8. Micromechanical parameter calibration model among particles and the deviational stress-strain curve
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图 9 相同围压、不同轴压下的数字岩心孔隙几何拓扑结构
Figure 9. Pore geometry topology of digital core under same confining pressure and different axial pressures
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图 10 相同轴向应力、不同围压下的数字岩心孔隙几何拓扑结构
Figure 10. Pore geometry topology of digital core under same axial stress and different confining pressures
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图 11 不同应力组合作用下数字岩心孔隙度和渗透率的分布
Figure 11. Porosity and permeability distribution of digital core under different stress combinations
表 1 不同应力组合下的数字岩心构建结果
Table 1 Digital core construction results under different stress combinationss
水平应力 σz = 120 MPa σz = 150 MPa σz = 180 MPa σx = 50 MPa
σy = 70 MPa
σx = 70 MPa
σy = 90 MPa
σx = 90 MPa
σy = 110 MPa
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