The deformation characteristics of low permeability reservoir rocks under confining pressure

For low permeability reservoir rocks,the hypothesis that linear poroelastic deformation occurs when rocks are compressed does not coincide with the laboratory findings.For this reason,various experimental methods,including the CMS300 test under confining pressure,rock mechanics test,mercury injection and cast thin section analysis,were adopted to study the deformation characteristics of low permeability reservoir rocks under confining pressure.The experimental results show that the rock sample has first experienced soft plastic deformation,and then elastic deformation,and finally plastic deformation as the stress increases.The strong stress-sensitivity of low permeability rock samples in the experiment under confining pressure is determined by their deformation characteristics.Subtle soft plastic deformation of rocks at the initial stage of confining pressure increasing is the main cause of significant decrease of permeability,and it is related with the relatively high contents of shale and cements as well as narrow pore throat of low permeability rocks.Based on the experimental results,a new coefficient of stress sensitiveness is defined to express the relationships between the permeability and the effective confining pressure.It has better correlativity with the experimental data,and can be used to conveniently convert the permeability of conventional rock samples into the permeability under reservoir conditions.