深层页岩剪切滑移裂缝渗透率变化规律

为了明确深层页岩裂缝渗透率在剪切滑移过程中的变化规律,选用四川盆地涪陵地区下志留统龙马溪组页岩和美国宾夕法尼亚地区Marcellus页岩露头岩样,进行了不同法向应力和滑移速率条件下的裂缝剪切滑移实验,并应用脉冲衰减法采集了裂缝渗透率的变化数据,分析了法向应力、岩石矿物组分和滑移速率等因素对深层页岩裂缝长期导流能力影响的规律。研究结果表明:①页岩剪切滑移裂缝的渗透率受到裂缝表面凸起被剪切破坏和剪胀作用两种因素的综合影响;②裂缝表面凸起被剪切破坏致使裂缝的孔隙度下降、有效水力开度减小、渗透率降低,其中渗透率的变化主要受到页岩矿物组分、法向应力和滑移速率等因素的综合影响;③层状硅酸盐含量高的页岩裂缝在高法向应力和高滑移速率下,剪切作用导致裂缝表面凸起被破坏,较之于网状硅酸盐含量高的页岩,前者裂缝渗透率的下降幅度更大;④滑移裂缝的剪胀作用使得裂缝的有效水力开度增大、渗透率升高,其中渗透率的变化主要受到滑移距离、剪胀角、页岩矿物组分、法向应力等因素的影响;⑤网状硅酸盐含量高的页岩裂缝在低法向应力和低滑移速率下,剪胀作用会使得裂缝渗透率出现小幅度的上升;⑥压裂施工前期可考虑大排量、高注入压力形成规模复杂缝网,后期可考虑采用较低的流体注入速率和注入压力,使压裂裂缝和天然裂缝产生一定程度地滑移,从而有效提高裂缝渗透率和储层整体渗透性。

Evolution laws of fracture permeability of deep shale in the process of shear slip

To investigate the evolution laws of fracture permeability of deep shale in the process of shear slip, this paper selected the Lower Silurian Longmaxi Formation shale samples from the Puling area of the Sichuan Basin and the Marcellus shale outcrop samples from Pennsylvania of the USA as the research objects to perform fracture shear slip experiments under different normal stresses and slip rates. Then, the variation data of fracture permeability was collected using the pulse-decay method. Finally, the influential laws of normal stress, shale mineralogy and slip rate on the long-term flow conductivity of fractures in deep shale were analyzed. And the following research results were obtained. First, the shale fracture permeability in the process of shear slip is influenced comprehensively by two factors, i.e., shear failure of fracture surface asperity and shear dilation. Second, the shear failure of fracture surface asperity results in the decrease of porosity, effective hydraulic aperture and permeability. And the variation of permeability is mainly under the comprehensive effect of shale mineralogy, normal stress and slip rate. Third, under high normal stress and high slip rate, the surface asperity of the shale fracture with a high phyllosilicate content is damaged by the shear action, and its permeability reduction amplitude is larger than that of the shale fracture with a high tectosilicate content. Fourth, shear dilation of slip fractures leads to the increase of fracture permeability and effective hydraulic aperture. And the variation of permeability is mainly influenced by slip distance, dilation angle, shale mineralogy and normal stress. Fifth, under low normal stress and low slip rate, the permeability of the shale fracture with a high tectosilicate content is increased slightly due to shear dilation. Sixth, it is recommended to adopt large displacement and high injection pressure in the early stage of hydraulic fracturing to form large-scale complex fracture networks and apply lower fluid injection rate and injection pressure in the later stage to drive the slip of hydraulic fractures and natural fractures to a certain degree, so as to improve fracture permeability and overall reservoir permeability effectively.