页岩储层应力敏感性的时间效应

Time Effects of Stress Sensitivity in Shale Reservoirs

有效应力及其作用时间是控制岩石孔隙和裂缝变形的重要外部因素。页岩储层的多尺度孔隙结构特征及孔缝配置关系使得其应力敏感时间效应具有特殊性,然而,以往研究尚未考虑有效应力加载及卸载时间对页岩应力敏感行为的影响。以川南威荣气田龙马溪组页岩气藏为研究对象,钻取不同孔缝发育程度的页岩样品,设计并测试了变有效应力状态下变加载及卸载持续稳定时间对岩样渗透率的影响实验。结果表明:1)有效应力从5 MPa升至35 MPa时,微裂缝岩样和基块岩样渗透率分别降低了66.82%和27.19%,卸载过程中渗透率恢复率为74.73%和79.82%。页岩渗透率越低,有效应力滞后效应越显著;2)恒定有效应力加载持续稳定时间从2.5 h延长至10.0 h,应力敏感系数增加3.76%∼80.17%,且当有效应力较小时的时间效应更显著。研究认为,页岩组分及多尺度孔缝结构是导致其孔缝变形行为差异性的主因。基于实验研究结果,推荐页岩样品老化处理为6.0 h,提出了优化页岩储层敏感性评价实验方案和优化气井工作制度的建议。

The effective stress and its action time are important external factors controlling the deformation of pores and fractures in rock.The characteristics of multi-scale pore structure and pore fracture configuration make the stress sensitive time effect of shale reservoirs special.However,the influence of effective stress loading and unloading time on the stress sensitive behavior of shale reservoirs has not been considered in previous studies.In this paper,the Longmaxi Formation shale gas reservoir in Weirong Gas Field,south Sichuan is taken as the research area.The shale samples with different development degrees of pores and fractures are drilled,and the experiment of the influence of variable loading and unloading sustained stability time on permeability of rock samples under variable effective stress is designed and tested.The results show that:1)when the effective stress loading from 5 MPa to 35 MPa,the permeability of micro fracture sample and matrix sample decreases by 66.82%and 27.19%,respectively,74.73%and 79.82%recovery rate of permeability during unloading.The lower the shale permeability is,the more significant the effective stress hysteresis effect is.2)when the continuous stability time of constant effective stress loading is extended from 2.5 h to 10.0 h,and stress sensitivity index increases by 3.76%∼80.17%.and the time effect is more significant when the effective stress is small.It is concluded that shale composition and multi-scale pore structure are the main reasons for the difference of pore deformation behavior.Based on the experimental results,the effective stress loading pretreatment time of shale samples is determined to be 6 h,and some suggestions are put forward to optimize the experimental scheme of shale reservoir sensitivity evaluation and optimize the working system of gas wells.