Experimental Study On Microcracks Propagation Of Shale Hydration

Horizontal well completion and large-scale hydraulic fracturing are the main ways to develop shale gas reservoirs.The low rate of fracturing fluid return has always been a problem that cannot be ignored in the process of reservoir exploitation.The retention of fracturing fluid in the reservoir will cause the permeability of the reservoir to decrease.However,some field data indicate that the gas produced by the well with low backwash rate is relatively large,which is inextricably linked with the hydration of shale.This paper focuses on the transformation of shale hydration to reservoir core permeability.The main shale producing cores of the Lower Silurian Longmaxi Formation in the Zhaotong area of northern Guizhou were used to perform core hydration experiments under conditions with external effective stress using nuclear magnetic resonance and tomography techniques.The effect of hydration on the pore-fracture structure of the shale reservoir core was studied,and the tomographic imaging technique and the fractal analysis method of cracks were used to evaluate the expansion of shale hydration microcracks.The study shows that within a certain pressure range,the shale core hydration can offset the decrease of permeability caused by stress sensitivity,and increase the complexity of core fractures due to the expansion of pore-crack structure,the generation of new micro-cracks and inter-seam intersections.,pore-crack volumes and their connectivity,and increase core permeability;hydration damage mainly along the direction of bedding or pristine fracture development;hydration has the most significant impact on the more developed core of pore-crack structure,and the pore-The influence of cores with high degree of fissure structural development is relatively small,and the impact on pure pore cores is minimal;the strength of shale core hydration is affected by the development of pore-fracture,clay mineral content and brittleness index.When the pressure is increased to a certain extent,the core hydration of shale is inhibited and the stress-sensitive effect cannot be compensated.As a result,the shale cracks close and the core permeability decreases.Based on the above research results,it is suggested that according to the physical characteristics of shale reservoirs during fracturing of reservoirs,the flowback system should be optimized through the design of pump injection mode,shut-in wells,or control of small oil nozzles during the renovation process to improve the complexity of the reform cracks.And its connectivity,enhance the effect of reservoir volume transformation.