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中文核心期刊

页岩储层应力敏感性定量评价: 思路及应用

QUANTITATIVE EVALUATION OF STRESS SENSITIVITY IN SHALE RESERVOIRS: IDEAS AND APPLICATIONS

  • 摘要: 应力敏感性定量评价是页岩油气勘探开发中公认的关键工程难题之一, 然而变应力条件下页岩孔缝尺寸及渗透率下降规律等问题至今尚无定论, 亟待深入探索. 本文从Griffith经典弹性力学解出发, 在充分刻画非均质页岩孔隙和微裂缝的基础上, 通过建立横截面为椭圆的柱体管束模型, 推导得到应力作用下岩石渗透率保持水平的计算公式, 基于此给出适宜于非均质页岩油储层的应力敏感性评价思路及覆压渗透率计算公式, 最后在中国西部和中部典型页岩油储层中开展了工程应用. 研究显示: (1) 相同应力状态下, 页岩油储层应力敏感程度由储渗空间初始长短轴比值、杨氏模量及泊松比共同控制; (2) 裂缝型页岩由于发育长短轴比值较高, 其应力敏感程度略高于裂缝发育程度较低的基质型页岩, 且杨氏模量越小, 两类页岩应力敏感程度差异性越大; (3) 在40 MPa的有效应力作用下, 裂缝型和基质型页岩油储层渗透率损耗程度最高值分别不足10%和8%, 证明页岩应力敏感程度总体较低. 应力敏感性对于页岩油原位储量及实际产能的影响程度亟需在工程实际中予以重新审视. 研究成果将为页岩油储量的精确评估和采收率的高效提升提供新的理论与实践依据.

     

    Abstract: Quantitative evaluation of stress sensitivity is one of the recognized key engineering problems in shale oil and gas exploration and development. The problems of shale pore size and permeability decline under the condition of variable stress have not been settled yet, and need to be explored further. Based on Griffith's classical elasticity solution, fine characterization of the pores and microcracks of heterogeneous shale, the formula of the retention of rock permeability under stress is derived by establishing a cylinder tube bundle model with elliptical cross-section. Then the stress sensitivity evaluation method and the calculation formula of overburden permeability suitable for heterogeneous shale oil reservoirs are given, respectively. Finally, it has been applied in typical shale oil reservoirs in western and central China. The results show that: (1) under the same stress, the stress sensitivity of shale oil reservoir is jointly controlled by the ratio between the initial major and minor axes of storage and seepage space, Young's modulus, and Poisson's ratio, and has nothing to do with the initial porosity and permeability of shale; (2) The stress sensitivity of fracture-type shale is slightly higher than that of matrix-type shale due to the development of microcracks with high ratio of major and minor axes, and the smaller the Young's modulus, the greater the difference between above two types of shale; (3) Under the effective stress of 40 MPa, the maximum permeability loss of fracture-type and matrix-type shale oil reservoirs is less than 10% and 8%, respectively, which proves that the stress sensitivity of shale is generally low. The impact of stress sensitivity on in-situ reserves and actual productivity of shale oil needs to be reexamined in engineering practice. The conclusion provides the new theoretical and practical basis for the accurate evaluation of shale oil reserves and the efficient improvement of oil recovery.

     

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