Research On Mechanical Softening Characteristics Of Shale Under Hydration

Shale gas acts as a low-carbon,clean emerging energy source.Its exploration and development have always been the focus of international energy.At present,"horizontal well+volume fracturing+factory operation" is the basic mode for the efficient development of shale gas.In recent years,in the practice of hydraulic fracturing,it has been found that in the volume fracturing process of unconventional reservoirs,compared to conventional reservoir reformation,there is a problem of large liquid volumes.Each well of a shale gas well in North America is involved.The amount of liquid used is as high as 2×104m3.Because of the special pore structure characteristics of shale,the return flow rate is generally low during the flowback process.This results in a large amount of fracturing fluid remaining in the reservoir.A large number of fracturing fluids remain in the reservoir,and the water and rock will inevitably produce hydration reactions.According to the principle of hydration interaction,water absorption promotes the change of the microstructure of the rock,resulting in the softening of the mechanical properties of the rock,which can then induce new ones.The cracks create or open natural fractures that were originally closed.This has an important impact on the construction of a series of projects such as the fracture pattern formed in fracturing,the conductivity of the reservoir,and the stability of the reservoir.Therefore,the research on the mechanical softening characteristics of shale under hydration conditio ns has the following features:the optimized flowback system in oil and gas exploration and design,and the pump injection design in fracturing and construction design,and provides better connectivity for the fluid flow in the reservoir fluid.significance.This article takes the core of the Longmaxi formation in section of Chongqing Fuling as the research object,adopts the free water immersion experiment method,and uses a series of microscopic instruments and macroscopic test instruments to analyze the shale under different hydration time under hydration.The changes in hydration microstructure,pore diameter change,and micro-cracks propagation behavior were analyzed.Based on the triaxial compression test after water immersion treatment,the damage constitutive structure under the action of hydration-load coupling was established by means of continuous damage mechanics and statistical theory.The model studies the evolution process of the damage effect of mechanical properties of shale under hydration.The main findings are as follows.(1)Through X-ray diffraction and environmental electron microscope tests of shale,the original dense layered structure gradually loosens with the increase of the immers ion time,and the degree of cementation between minerals gradually decreases,and the pores and cracks increase.Through the inversion analysis of nuclear magnetic lateral relaxation time T2 spectral area and mercury intrusion test,hydration promoted the expansion of the original small pores,the permeability increased,the original porous dense interconnected,and accompanied by the production of new cracks.Through the CT technique,a method to evaluate the mesoscopic damage of shale after flooding was established.It was revealed that under the effect of hydration,the shale under the hydration,due to the effect of capillary force,the microstructure and secondary cracks in the matrix experienced the emergence of oblique seam?oblique cross seam expansion?substrate microcracks appear?diagonal seam expansion acceleration?microcrack extension bifurcation?hydration cracks?microcracks propagation(2)By conducting indoor triaxial compression tests,the law of mechanical softening of shale under hydration was revealed.The test results show that the effect of water immersion on the rock is gradual,and the initial impact is more significant.With the prolongation of the immersion time,the softening effect is gradually weaker.Shale softness is used to characterize the degree of influence of rock strength under hydration.The time of rock has a significant effect on the destruction of rock.In the early stage of hydration,the characteristics of shale damage are generally manifested as fracture rupture.Under high confining pressure,the plasticity of rock is obviously enhanced with the increase of hydration time.(3)Coupling the hydration-load interaction into a statistical damage constitutive model,applying the continuous damage mechanics and statistical mechanics theory,establishing the damage evolution equation through the Weibull distribution of the rock microelement strength,and deducing the derivation of the rock under the action of hydration-load The damage constitutive model describes the shale degradation process.The prolongation of hydration time and the effect of confining pressure will affect the values of the parameters m and F0.At the initial stage of hydration,the hydration time has agreat influence on the parameter.