变速率剪切滑移过程中页岩裂缝稳定性-渗透率演化规律

Friction stability and permeability response of shale fractures during shear slip

页岩气资源开采的过程中,水力压裂技术常作为增产的基本手段。但是,大量流体注入地层后可能影响储层岩石的整体性、稳定性和渗透特性。同时,裂缝的渗透率是决定储层产量的关键条件。因此,选取了中国川东南龙马溪组页岩、美国Green River页岩和Marcellus页岩,通过进行变滑移速率的页岩裂缝剪切滑移实验,同时测量了页岩裂缝滑移过程中的摩擦因数和渗透率变化,基于速率状态定律评估了3种页岩裂缝的剪切滑移稳定性,分析研究了岩裂缝稳定性-渗透率演化规律。在实验结果观察的基础上,通过建立页岩裂缝滑移过程中的微观物理模型,讨论了页岩矿物组分对剪切裂缝剪切强度、稳定性和渗透率演化的微观控制机制。研究结果表明:页岩裂缝面的摩擦强度随网状硅酸盐质量分数的增加而增大,随层状硅酸盐质量分数的增加而减小;页岩裂缝剪切滑移稳定性参数(a-b)值随层状硅酸盐质量分数增加而增大,倾向于发生稳定性蠕变滑移,网状硅酸盐质量分数的增加使(a-b)值减小,更易产生非稳定性地震滑移;在含有高网状硅酸盐质量分数的裂缝中,滑移速率突然增加导致的剪胀作用导致裂缝渗透率的增大,并且伴随着非稳定性地震滑移;高层状硅酸盐裂缝滑移速率突然增大时,裂缝面的加速破坏导致裂缝渗透率进一步降低,同时伴随着稳定的蠕变滑移。研究结果对于控制水力压裂诱发页岩地层非稳定滑移提供借鉴。

During the shale gas production,hydraulic fracturing is a basic stimulation technology to enhance the reservoir permeability.However,after a large amount of high-pressure fluid injection into the underground,the integration,stability,and permeability of the reservoir may also change.Moreover,the fracture permeability is critical in determining the reservoir permeability and production.Therefore,three kinds of shale samples,i.e.,the Longmaxi shale in Southern Lower Silurian Formation,the Green River shale,and the Marcellus shale,were selected for the fracture shear slip experiments under various slip rates.The fracture permeability was concurrently measured by steady-state flow methods.The rate-state friction law was used to describe the stability and permeability evolution of shale fractures during the slip process.Based on the experimental observations,a microscopic physical model of shale fracture slip was established to show the effect of mineralogical composition on the shear strength,stability,and permeability feedback of shear fractures during slip.The research results show that the friction strength of the shale fracture surface increases with the increase of tectosilicate content,and decreases with the increase of phyllosilicate content;the stability parameter(a-b)increases with the increase of phyllosilicate content,and stable creep is favored.In the shale reservoir with high tectosilicate content,the sudden increase of slip rate may result in the permeability enhancement due to the shear dilation,which accompanies with unstable dynamic slip.While in the shale reservoir with high phyllosilicate content,the slip rate increase may induce further permeability decay,which accompanies with stable creep slip.The results provide a basic understanding of injection-induced fracture slip in shale reservoirs.