Supercritical CO₂-induced Deformation Strain On Shale And Supercritical CO₂ Migration Law In Shale

Shale gas is an unconventional but promising gas resource that has been used with significant success in China.The efficient production of shale gas has become a great strategic demand for China to make full use of resources and ensure energy security.The studies of domestic and foreign scholars have shown that,supercritical CO₂ fluids can solve the swelling of clay meeting with water and improve the reservoir seepage channel,at the same time,the low viscosity of it can cause numerous microcracks of reservoir,which can maximize the communication of the natural cracks to enhance the cracks conductivity to improve the gas recovery efficiency.However,the change of shale properties under carbon dioxide and its migration in shale directly affect the efficiency of crushing.In this paper,the shale samples used in laboratory experiments are collected from Longmaxi Formation of lower Silurian in Funing City Sichuan Province.The law of shale adsorbing supercritical CO₂,the law of the deformation of shale under the action of supercritical CO₂ and He,and the law of the migration of supercritical CO₂ migration in the shale formation were investigated.The research achievements are as follows:?1?Based on volume method and stress-strain metering method,an equipment for testing shale deformation under high temperature and pressure was independently developed,which could be used to measure adsorption and deformation of block sample?effective size is ?50mm×100mm?at the same time in air environment.The maximum testing pressure and highest test temperature could reach 35 MPa and 95? respectively,which allows the effective simulation of high pressure and temperature environment where the shale gas reservoir lies.Error analysis showed that the maximum error of this equipment is around 10%,able to meet the testing requirements.?2?The adsorption and deformation characteristics of shale has been measured under different temperature,pressure,and gases,and the deformation rules under CO₂ was verified.Under low pressure condition,change of surface potential energy of shale caused by the adsorption of CO₂ further leads to its dilatancy.The bulk phase density is far less than adsorbed phase density,which results to linear increase of deformation/adsorption with the increase of CO₂ pressure.With the further increase of pressure,CO₂ enters into super critical state,and the bulk phase density is equivalent to its adsorption phase density.Then the adsorption and deformation no longer significantly increases when the pressure increases,while the compressive deformation under gas pressure gradually increases,which causes the decrease of shale dilatancy with the increase of CO₂ pressure.?3?Dual calculation model of adsorption-deformation and stress-strain of shale under CO₂ condition was established.By considering shale as a material of transverse isotropy,the adsorption-deformation calculation model based on adsorption density of CO₂ was built according to energy conservation,thermodynamics and principles of surface physical chemistry.While the stress-strain calculation model of shale under gas pressure was built based on principle of effective stress and the consideration of influence of pore pressure on Biot coefficient.Comparing with test results,this model could be relatively accurate to describe the deformation of shale under CO₂ pressure.?4?The influence mechanism of adsorption deformation on gas migration law was disclosed.Under low pressure below 3Mpa,diffusion is the main form of migration for CO₂ in shale,and its proportion increases when gas pressure increases.However,with the increase of CO₂ pressure,adsorption deformation of shale increases,and collision between gas molecule is strengthened,which caused the rapid decrease of CO₂ diffusion ability and a stable lower proportion.In this situation,transfusion is the main form of migration for CO₂ in shale.Therefore,when the pressure increases under shale deformation condition,transmittability of CO₂ in shale will initially decrease and then recover gradually.The results of this research provide adsorption-deformation on shale.Enrich the new idea of high effective development of shale gas.So it has a certain value for the scientific research and theory.