| Hydraulic fracturing can produce a wide range of complex fracture networks in shale gas reservoirs,but the gas in matrix pores is still difficult in extraction.The heat treatment on the reservoir can promote the development of secondary fractures in shale matrix,forming multi-scale gas-water seepage channels,strengthening gas desorption and enhancing transmission capacity.This can enhance the gas production in the same mining life.Therefore,the heat treatment stimulation on a shale gas reservoir is a multi-physical and multi-phase coupling process,which is of scientific and engineering significance.In this thesis,laboratory tests,theoretical analysis and numerical simulation are comprehensively employed to solve the problem of thermal fracturing for the permeability enhancement of heterogeneous shale reservoirs.Based on the analysis of mineral compositions and pore-fracture structures,a permeability evolution model is proposed to consider the thermal fracturing of matrix,and a fractional-order analytical solution for gas-water two-phase coupling flow is obtained.The thermal-gas-liquid-solid coupling mechanisms of shale reservoir volumetric fracturing and permeability enhancement are explored.Based on these investigations,following innovative results have been achieved:(1)Experimental investigations observe the variations of shale mineral composition,pore-fracture structure and permeability evolution under different thermal environments.In oder to simulate different heat treatment conditions of shale reservoirs,the shale samples were thermally treated with 11 kinds of temperature treatments from low temperature(-196 ℃)to high temperature(300 ℃).Then,X-ray diffraction,electron microscopy scanning and high-pressure mercury intrusion test were used to test the evolution of shale mineral composition and matrix pore structure.It is found that during the heating process,the shale successively undergoes the process of mesopore development due to thermal volatilization,a substantial increase in the proportion of micropores due to thermal expansion of minerals,and the development and penetration of mesopores caused by thermal fracture of matrix.The permeability increases first and then decreases slightly,and increases sharply after 200 ℃.During the freezing process,the matrix shrinkage before-38 ℃ results in a smaller total pore volume,and then due to the unconsistent deformation of different mineral particles,a large number of mesopores and macropores are developed,and the permeability increases significantly.From low temperature to high temperature,the content of muscovite changes is consistent with permeability behavior,and quartz is more sensitive to high temperatures.(2)A new three-parameter permeability evolution model is proposed to consider the thermal fracturing of shale matrix.The rock reservoir is divided into "soft" parts(fissures)and "hard" parts(matrices).They follow Hooke’s laws with natural strain and engineering strain,respectively.Then,a three-parameter permeability evolution model is proposed based on the classic matchbox model.This model includes these inconsistent deformations in soft and hard parts as well as the thermal fracturing process of fractured rock.It can describe the compaction of primary fissures and the formation of secondary fissures.The influences of effective stress,temperature change,cracks and matrix constitutive behavior on permeability evolution are described.(3)An analytical model is established for the thermal-gas-water coupling flow and the productivity prediction from a tortuous holes-fissures system.A shale gas-water two-phase flow model was established to consider the solubility of shale gas in water.Then,iterative analytical solutions of gas-water pressure and production were obtained without neglecting the nonlinear terms in the two-phase flow coupling model.The effects of non-linear terms,gas solubility and capillary pressure on shale gas productivity were investigated.Further,considering temperature and tortuosity of the seepage path in fractured rock reserviors,the coupling between heat conduction and two-phase flow was described in the form of fractional time and space derivatives.The coupling model of shale fractional heat conduction and fractional heat-gas-water flow was further established.Analytical solutions of fluid temperature,gas and water production were derived using fractional order traveling wave method and variational iteration method.(4)The thermal-gas-liquid-solid coupling mechanisms of fracture and permeability enhancement of heterogeneous shale reservoirs are revealed.A thermo-gas-liquid-solid multi-physical coupling numerical model for shale gas production was developed.This model couples the equation of shale reservoir deformation involving in both gas adsorption and thermal expansion,the gas-liquid two-phase continuity equation considering gas solubility,and the heat transfer equation for heat conduction and convection.The interactions among stress,temperature and seepage in a heterogeneous shale reservoir were studied.The influences of heat injection temperature on gas production during the heat treatment of the reservoir were analyzed,and the effectiveness of reservoir heat treatment for shale gas production enhancement and the production life prolong was evaluated.This dissertation includes 54 figures,12 tables and 188 references. |
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