Mechanisms Analysis And Numerical Modeling Of Deformation And Failure Of Low Permeability Rock Under Thermal-Hydraulic Loading Condition

Geological rock structures in tectonic stress,fluid pressure and temperature environment are subject to complex mass flow,heat conduction,deformation and failure.The relevant studies are related to the geological energy engineering fields,such as oil and gas exploitation,geothermal,nuclear waste landfill,etc.The temperature and fluid pressure changes induced by engineering factors(collectively referred to as thermal-hydraulic loading)form the coupling of temperature,fluid pressure,and mechanical deformation,also known as the THM coupling problem,which has been concerned by people all the time.Based on the multi-field coupling theory of porous continuum,through the theoretical model,petrophysical experiment,and numerical simulation,this paper studies the THM coupling response characteristics of granite and tight mudstone under thermal-hydraulic loading during two working conditions of geothermal recovery by cold water injection and heavy oil thermal recovery by steam injection.The main research content includes:1.Based on the mixture theory and the framework of the porous continuum,under the assumptions of the consistency of gas and liquid mass transfer and pore volume change and local thermal equilibrium,the THM coupling equations including an unsaturated gas-liquidsolid three-phase system mass,momentum and energy conservation are established.Based on the previous formula,the THM coupling equation of saturated liquid-solid two-phase system and the elastoplastic stress-strain constitutive equation of solid-phase are given.The finite element numerical calculation formula of the THM equation in Galerkin form is derived.Time scale difference and time step estimation of heat conduction and fluid flow properties in low permeability porous media are discussed.2.To study the wellbore structural stability of tight granite geothermal development,the temperature/hydration physical and mechanical properties of granite are studied experimentally,and the plastic thermal damage model is established based on the experimental data.Cold water is injected into the wellbore as a thermal-hydraulic loading.The THM coupling mechanical model of casing-cement sheath-granite formation wellbore structure is established.The mechanical response of wellbore structure during the process of cold water injection with a heat exchange temperature difference of 160 ℃ is studied,and the destructive instability of wellbore caused by excess pore pressure caused by a temperature change at the joint of open hole completion and cased completion is mainly analyzed.The results show that the temperature difference between the injection of cold water and the heat formation heat exchange generates a redistribution of stress in the wellbore area,which is the main controlling factor for the instability and failure of the geothermal wellbore structure.3.The discontinuous mudstone interlayer distribution in heavy oil reservoirs has an important influence on the migration of steam and crude oil in steam assisted gravity drainage(SAGD)thermal recovery.A case study was conducted in Haqian 1 block of Chunhui,Xinjiang.Through reservoir numerical simulation,the dynamic characteristics of pressure,temperature,fluid flow,and other parameters in the development of homogeneous reservoir and mudstone interlayered reservoir are studied and compared.The results show that the existence of mudstone interlayer hinders the vapor-liquid transmission,but has little influence on the heat conduction and exchange.The uneven thermal-hydraulic(gradient)distribution in the local reservoir promotes the mudstone cracking and has a significant impact on the SAGD production effect.4.Through the self-developed high-temperature and high-pressure triaxial rock mechanics experimental system,the changes of strength and mechanical properties of mudstone under different heat rates were experimentally studied,and the effects of thermal pore pressure(expansion)induced tension and shear mechanism on mudstone cracking under hightemperature were studied.Combined with the reservoir numerical simulation and the mudstone THM coupling cracking analysis,the critical temperature of mudstone thermal cracking and the distribution of thermal fractures along the horizontal wellbore induced by thermal recovery are studied,which provides a reference for the optimization of the SAGD production process.