Experimental Research On Pore Elasto-Plastic Deformation And Effective Stress Coefficient Of Ultra-low Permeability Rock

Ultra-low permeability and unconventional reservoir plays an increasingly important role in the development of oil and gas resources in China and the world.One of the key issues is related to pore properties.Especially in the aspects of pore deformation,permeability and fluid pressure effect,the ultra-low permeability rocks show different properties from conventional rocks with medium and high permeability,and their laws are not well understood.It is of great significance to study the elastoplastic deformation and effective stress characteristics of ultra-low permeability rock.The ultra-low permeability limestone and shale outcrops are taken as the objects investigated.Their micropore structure was analyzed.The conventional triaxial loading experiment and hydrostatic loading experiment for effective stress research were carried out to study pore elastoplastic deformation and hydraulic transportation characteristics,the effect of fluid saturation on failure mode of ultra-low permeability rocks under loading,and effective stress behavoir.Detailed results and research conclusions are as follows:(1)Ultra-low permeability rocks generally are characterized by small size of pores diameter,complex pore structure and poor connectivity.Systematic research on the micro pore characteristics of ultra-low permeability rocks are carried out by means of optical,microscopic scanning electron microscopy,mercury pressure testing and nuclear magnetic resonance,combined with a large number of relevant published paper.The results show that ultra-low permeability rock is characterized by wide pore size distribution,poor connectivity and strong heterogeneity.(2)Elastoplastic deformation will occur in ultra-low permeability under stress.Elastoplastic deformation of pore in ultra-low permeability rock not only causes pore volume changes,but it also changes pore connectivity greatly.The reduction of permeability is related to pore volume and connectivity.The pore structure of ultra-low permeability rocks is complex.The stiffness of different types of pores is different,with different deformation laws.The narrow pore is characterized by strong stress sensitivity.It leads to the decrease of pore connectivity under loading.The decrease in permeability is affected by both a reduction in pore volume and a decrease in pore connectivity.The loading under high confining pressure can cause pore collapse or particle crushing,which greatly reduces pore connectivity,resulting in a significant decrease in permeability.(3)It will cause local pore pressure in shale saturated withs shale under stress and will affect microscopic fracture.Parts of pore in shale will become isolated,disconnected pore under stress.The isolated pores saturated with fluid cause high local pore pressure under loading,which is easier to lead to multi-ple local micro-failures.It increases the complexity of the fracture system and alters the fracture mode of the shale.(4)The heterogeneous pore structure has an important effect on the effective stress coefficient of ultra-low permeability rock,which is related to pore connectivity.The results show that effective stress coefficient associated with permeability and porosity of medium and high permeability rock is less than 1,while effective stress coefficient associated with permeability and porosity is more than 1 for ultra-low permeability limestone with non-uniform pore structure characteristics.The heterogeneous pore structure pore structure of ultra-low permeability rocks is easy to cause the change of pore connectivity,resulting in greatly increasing the effect of pore pressure,and the effective stress coefficient.The conventional porelastic theory based on pore connectivity can not well explain the characteristics of effective stress coefficient more than 1,and the effect of stress on pore connectivity of ultra-low permeability should be considered.