Study On The Electrical Simulation And Conductive Model Of Shaly Sandstone Based On Digital Rock

For pure sandstone reservoirs dominated by intergranular pores,Archie's equation is usually used to correlate rock conductivity with water saturation.When sandstone contains clay minerals,the relationship between rock conductivity and water saturation of shaly sandstones becomes complicated because of the additional conductivity of clay.The interpretation of shaly sanstones is always a difficult problem in well logging.Various methods have been proposed to evaluate the effects of clay minerals on reservoir properties.However,we are still searching for a common and reliable saturation model of shaly sandstoneds.Firstly,based on the two-phase 3D digital rock of Berea sandstone(matrix,pore),three-phase(matrix,wet clay,free water)digital rockss of shaly sandstone were constructed through random method.The formation water conductivity with different salinity was set,and the finite-element method was used to simulate the electrical characteristics of sandstones and shaly sandstones digital cores in order to study the relationship between the conductivity of core saturated with brine C₀ and the brine conductivity C_w(C₀-C_w curves).The influence of different clay minerals,clay minerals content and porosity were taken into account.Secondly,the open operation of a morphology algorithm was applied to simulate the drainage processes based on three-dimensional digital rocks,then the fluid distributions under different water saturation conditions can be obtained.The finite-element method was used to calculate the conductivity of pure sandstones and shaly sandstones in different saturation,then the relationship between resistivity index I and water saturation Sw(I-S_w curve)was studied.The effects of clay minerals,clay mineral content,formation water conductivity on the rock electrical propertie were fully considered in the simulation.Finally,the classical shaly sandstone conductivity models were evaluated and a new power function conductivity model was established.The simulation results of core saturated with brine show that the clay content of 7%is a critical point.When the clay content is lower than 7%,the C₀-C_w curve is approximately a straight line,and the non-linear relationship is not obvious in the low salinity section.When the clay content is more than 7%,the low salinity section presents obvious non-linear.The additional conductivity(C_ex)is related to the clay minerals,and montmorillonite has the highest cation exchange capacity(CEC),resulting in the largest additional conductivity C_ex.The additional conductivity of illite,chlorite,and kaolinite decreases orderly.The increase of clay content in shaly sandstones increase C₀.At the same time,clay fills pores decrease the porosity,and causes the decrease of C₀.These are two opposing factors of conductivity coexist in shaly sandstone.Under different circumstances,the two factors have different influence on the conductivity of shaly sandstones.Therefore,the influence of these factors should be considered in the establishment of shaly sandstone conductive model.The simulation results of rock conductivity with different saturations indicated that the I-S_w curve of shaly sandstone in log-log coordinates was approximately linear in the high water saturation,but the I-S_w curve presents obvious nonlinear characteristics in the low water saturation.The I-S_w curves of different shaly sandstones basically coincide in the high saturation,and appear obvious bifurcation in the low saturation.decrease with the increase of clay content,decrease with the increase of clay additional conductivity,and increase with the increase of formation water conductivity.In order to verify the above simulation results and the conductivity model,experimental measurements of rock conductivity were made on 9 samples of tight sandstone with clay in Sulige area of China.The experimental measurement results show that the trend of core C₀-C_w and I-Sw curves are consistent with the simulation results simulated from digital core,and the electric resistivity results calculated by power function model are in good agreement with the resistivity measurement results.The new saturation model was used to calculate the water saturation of sandstone interval in Sulige area,the calculated results are in good agreement with the water saturation of closed core sample.The study of the conductive properties of shaly sandstone is not only helpful to understand the properties of shaly sandstones,but also of great significance to the well logging evaluation and reservoir xploration.