低渗致密砂岩渗透率应力敏感性试验研究

储层岩石中孔隙流体压力的变化会引起有效应力发生变化，进而导致渗透率的改变，引发储层岩石渗透率应力敏感现象。以试验研究了不同围压下渗透率随孔隙流体压力的变化规律，试验包含了老化处理和升降内压4个回路，每个回路是在围压不变降低和增加孔隙流体压力下实现的，采用稳态法采集每个测试点的数据。试验结果表明，渗透率随着孔隙流体压力的降低而减小，随着孔隙流体压力的增加而增加；在低围压下渗透率的变化幅度较大，在高围压下，则变化幅度较小；在不同围压回路下，净应力相等的点对应的渗透率不相等；渗透率在随孔隙流体压力的变化中呈现出“台阶式”变化。根据Bernabe的模型计算了渗透率有效应力系数，结合Bernabe的观点分析计算结果发现渗透率呈“台阶式”变化是微裂缝随应力的变化而发生变形的表现。对试验岩样进行了储层岩石应力敏感性评价，结果表明，该低渗致密砂岩储层表现为中等应力敏感。

Laboratory study of stress sensitivity to permeability in tight sandstone

The effective stress will vary with pore pressure in reservoir rocks, which then will induce the change of permeability and the stress sensitivity to permeability. Laboratory study of the stress sensitivity to permeability has been carried out. Experiments include aging treatment and four cycles. Each cycle is run under constant-confining-stress through decreasing and increasing pore pressure. Experimental data are collected with steady-state method. The results show that permeability varies with pore pressure. At low confining stress, the variation range of permeability is larger, while at high confining stress, it is smaller. For equivalent net stress in different cycles, permeability is different. The change of permeability, with the change of pore pressure at some cycles, belongs to a typical step change. Meanwhile, the effective stress coefficients for permeability are calculated by Bernabe’ model. The calculated results，according to Benabe’s viewpoint, show that the step change of permeability is resulted from the change of microcrack deformation with stress change. The stress sensitivity evaluation to permeability has done; and the results show that the tight sandstone reservoir studied puts up the moderate stress sensitivity.