碳酸盐岩气藏应力敏感特征及微观机理

Stress sensitivity of carbonate gas reservoirs and its microscopic mechanism

在模拟原始地层温度、应力条件下开展储集层应力敏感测试,评价碳酸盐岩岩心的应力敏感特征;在毛管压力曲线标定下,引入变分形维数建立了弛豫时间与孔径的转换公式,基于核磁共振定量分析了不同尺度孔喉条件下因应力敏感造成的孔隙体积损失,厘清了碳酸盐岩气藏应力敏感的微观机理。研究发现,裂缝会显著改变碳酸盐岩储集层的应力敏感特征,随着初始渗透率的增加,孔隙型储集层的应力敏感系数先减小后增大,裂缝-孔隙型储集层则单调增大;应力敏感造成的孔隙体积损失主要来源于中孔尺度(0.02~0.50μm)孔隙,贡献率超过50%,单条高角度裂缝对应力敏感、不可逆伤害的贡献率分别为9.6%和15.7%;碳酸盐岩气藏应力敏感的微观机理主要为裂缝闭合、孔隙弹性收缩和骨架塑性变形。

In order to evaluate the stress sensitivity of carbonate reservoirs, a series of rock stress sensitivity tests were carried out under in-situ formation temperature and stress condition. Based on the calibration of capillary pressure curve, the variable fractal dimension was introduced to establish the conversion formula between relaxation time and pore size. By using the nuclear magnetic resonance(NMR)method, the pore volume loss caused by stress sensitivity within different scales of pore throat was quantitatively analyzed, and the microscopic mechanism of stress sensitivity of carbonate gas reservoirs was clarified. The results show that fractures can significantly affect the stress sensitivity of carbonate reservoirs. With the increase of initial permeability, the stress sensitivity coefficient decreases and then increases for porous reservoirs, but increases monotonously for fractured-porous reservoirs. The pore volume loss caused by stress sensitivity mainly occurs for mesopores(0.02–0.50 μm), contributing more than 50% of the total volume loss. Single high-angle fracture contributes 9.6% of the stress sensitivity and 15.7% of the irreversible damage. The microscopic mechanism of the stress sensitivity of carbonate gas reservoirs can be concluded as fracture closure, elastic contraction of pores and plastic deformation of rock skeleton.