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

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.