Simulation Of Rock Resistivity Under High Temperature And High Pressure

In recent years,major breakthroughs have been made in the exploration of high temperature and high pressure oil and gas reservoirs around the world.A number of large high temperature and high pressure oil and gas fields with temperatures up to 150-250 ° C and pressures of 50-150 MPa have been discovered.Moreover,deep oil and gas research has become an important area for breakthrough discovery and scale storage of onshore oil and gas exploration in China.The key to reservoir evaluation is the identification of pore fluid properties of reservoir resistivity characteristics.Therefore,under high temperature and high pressure conditions,the microscopic pore structure and pore fluid properties of rock are quite different from those under normal temperature and pressure conditions.In order to improve the accuracy of deep oil and gas reservoir evaluation,it is necessary to quantitatively study the rock resistivity characteristics under reservoir conditions.Compared with indoor petrophysical experiments,digital petrophysical experiments can not only quantitatively characterize reservoir microstructure,but also break through laboratory temperature and pressure limits,and the time cost of numerical simulation experiments is low.In this Letter,digital rock physics experiment method is used to simulate the influence of temperature and pressure on the resistivity characteristics of rock.First,the three-dimensional digital cores of different microstructures were established by the sedimentation process,including a single mineral component,a multi-mineral component,and a model of developmental joints.On the basis of the three-dimensional digital core,the finite element method and the discrete element method are used to solve the strain of each voxel in the digital core under pressure,and then the variation law of the microscopic pore structure is quantitatively analyzed.The finite element method is used to calculate the resistivity of three-dimensional digital core under stress,and the influence of temperature and pressure on the resistivity of rock is analyzed.It is of great significance to improve the evaluation of deep oil and gas saturation.Through the research of this paper,the following conclusions are obtained.(1)The discrete element method is used to simulate the crack initiation and propagation process in digital cores,and the finite element method is used to simulate the stress distribution in digital cores.The location where the crack is generated is substantially the same as the stress concentration region.Two numerical simulation algorithms verify each other,which shows that it is feasible to study the influence of temperature and pressure on pore structure by numerical simulation algorithm.Compared with the traditional finite element method,this method can more realistically describe nonlinear behaviors such as large deformation and crack generation in the core.(2)Numerical simulations show that pressure has an important effect on the pore structure:(1)Simulation results for the absence of joint and development single-joint digital cores indicate: When there is no joint,the microcracks are more at the end and distributed throughout the sample.When the joints with a dip angle of 50° are developed,the microcracks are mainly distributed near the joint surface,and the cracks are generated along the shear plane of the joint surface,and the rock samples are destroyed.(2)When a single-component rock breaks,the microcrack expands toward both ends along the direction of the applied force.The multi-component three-dimensional digital core microcracks containing clay start at the clay position and extend toward both ends in the direction of force application.(3)As the pressure increases,the porosity of the rock gradually decreases,and the value of the initial porosity increases.And the looser the rock,the more sensitive the porosity is to pressure.Under the same conditions,the porosity change value of carbonate rock is smaller than that of sandstone.The higher the clay content,the larger the porosity change value of rock.(4)At the same pressure,the effect of temperature on the pore structure is not obvious.(3)The numerical simulation results of resistivity show that:(1)The resistivity of a three-dimensional digital core increases with increasing pressure,mainly due to an increase in porosity resulting in a decrease in porosity.(2)As the pressure increases,the rock resistivity exhibits significant anisotropy.The horizontal direction resistivity is significantly higher than the vertical direction resistivity.The pore structure analysis results show that the micro-cracks generated in the rock are mainly in the vertical direction due to the pressure,and the vertical pore connectivity is significantly better than the horizontal direction.(3)An increase in temperature promotes the rate of movement of ions in the solution,which in turn increases the conductivity of the rock.Therefore,as the temperature increases,the rock resistivity will decrease.