Experimental study on porosity and permeability and stress sensitivity of shale under pressurization
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摘要:
为了研究页岩在应力作用下的孔渗变化及孔隙结构特征，采用四川盆地昭通区块页岩样，不同压力条件下对页岩的孔隙度和渗透率进行实验分析，建立了页岩样孔隙度、渗透性与净覆压之间的相关关系和模型；采用渗透率损害率和应力敏感系数分析了页岩储层的应力敏感性。研究结果表明，页岩基质孔隙度和渗透率随有效应力的增加呈负指数函数规律降低，渗透率与孔隙结构有关，页岩地层中包括基质孔隙和裂隙共同发育的双重介质体系。当净覆压小于5 MPa时，页岩储层应力敏感系数变化较大，应力敏感性强；当净覆压大于5 MPa时，页岩储层应力敏感系数随有效应力的增加下降速度整体减缓，且存在波动变化，应力敏感性减弱，渗透率损害率随有效应力的增大而缓慢增加。研究发现，不同孔径的孔隙度随应力的增大而减小，反映了页岩中不同孔径对孔隙度的协同效应，对揭示页岩储层的孔径变化，指导深部页岩储层的物性特征具有一定的实际意义。
Abstract:In order to study the pore and permeability changes and pore structure characteristics of shale under stress, the porosity and permeability of shale in net confining stress were analyzed through experiment using the shale samples in Zhaotong area, Sichuan Basin.The stress sensitivity of shale reservoir was analyzed by permeability damage rate and stress sensitivity coefficient.The results indicate that the porosity and permeability of shale matrix decrease by a negative exponential function with the increasing effective stress, and permeability is related to pore structure.Shale formation consists of a dual media system including matrix pores and fractures.If the net confing stress is less than 5 MPa, the stress sensitivity coefficient of shale reservoir varies significantly and the shale reservoir is highly sensitive to stress.If the net confing stress is greater than 5 MPa, the stress sensitivity coefficient of the shale reservoir decreases as the effective stress increases slowly, and there is a fluctuation.Stress sensitivity weakens and permeability damage rate increases slowly with the increase of effective stress.It is found that the porosity of different pore sizes decreases with the increase of stress, which reflects the synergistic effect of different pore sizes on porosity in shale, and has certain practical significance to reveal the pore size change of shale reservoir and guide the physical properties of deep shale reservoir.
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Keywords:
- pressurization /
- shale /
- porosity /
- permeability /
- stress sensitivity
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致谢: 感谢中国石油集团安全环保技术研究院在研究过程中提供的经费、场地、实验原料等帮助；中国地质大学(北京)水资源与环境学院辜海林、梁峰、李颖等同学在野外调查和取样过程中完成部分工作，特此表示衷心感谢。
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图 1 水力压裂开采增加页岩储层压力过程示意图
P1—一定深度的页岩储层压力；P2—水力压裂作用下的压力；ΔP—地层实际增加的净覆压
Figure 1. Schematic diagram of hydraulic fracturing increasing the pressure of shale reservoir
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图 2 页岩孔隙度、渗透率与净覆压力之间的关系
Figure 2. Relationship between porosity, permeability of shale and the net confining pressure
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图 3 页岩渗透压缩率(PPR)、应力敏感系数(SSC) 与净覆压的关系
Figure 3. Relationship between permeability damage rates(PPR), stress sensitivity coefficient(SSC)and the effective stress
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图 4 页岩孔隙度压缩系数和净覆压的关系
Figure 4. Relationship between pore compressibility factor and the effective stress
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图 5 覆压条件下页岩孔隙度与渗透率的关系
Figure 5. Relationship between porosity and permeability of shale under stress
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图 6 覆压条件下页岩孔渗双对数曲线
Figure 6. Logarithmic curve of pore and permeability of shale under stress
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图 8 样品14在电镜扫描下裂缝的存在
Figure 8. The existence of cracks in sample 14 under electron microscope scanning
表 1 实验样品基础数据
Table 1 Basic data of the shale samples
样品编号 直径/cm 长度/cm 孔隙度/% 渗透率/10-3μm2 密度/(g·cm-3) 样品描述 1 2.457 6.452 5.33 0.08371 2.58 平行层理 6 2.462 4.597 7.49 0.0333 2.56 平行层理 8 2.472 4.966 7.47 0.04242 2.53 平行层理 12 2.477 3.813 5.85 0.025 2.58 垂直层理 14 2.474 5.210 4.59 0.00016 2.58 垂直层理 17 2.474 5.242 4.99 0.00061 2.59 垂直层理 
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表 2 页岩样品孔隙度、渗透率和净覆压力之间的统计分析结果
Table 2 Statistic analysis results of relationship between porosity and permeability of shale and the effective confining pressure
样品编号 压缩系数(Cp)/(MPa-1) 孔隙度(φ0)/% 相关系数(R12) 渗透率应力敏感系数(a)/(MPa-1) 渗透率(K0)/mD 相关系数(R22) 1 0.026 5.333 0.959 0.280 0.0837 0.987 6 0.049 7.494 0.983 0.090 0.033 0.991 8 0.012 7.479 0.536 0.012 0.042 0.857 12 0.017 5.811 0.823 0.263 0.025 0.945 14 0.032 4.591 0.835 — 0.00016 — 17 0.039 4.989 0.778 — 0.00061 —
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