Numerical Simulation Of Acoustic And Electrical Properties Of Natural Gas Reservoir Rocks Based On Digital Cores

As a cutting-edge numerical simulation method, digital core technology has been playinga critical role gradually regarding to the research of rock physics properties. It can be appliedto study the effects of micro-factors on the acoustic and electrical properties of natural gasreservoir rocks and compensate for the deficiencies of traditional rock physics experiments.By observing the specific role and impact of the law of various micro-factors, a solidfoundation can be layed on the research of new methods and theories.In this dissertation,3-D digital cores are constructed by X-ray CT and process-basedmethod respectively. We proposed a new method to analysis the representative elementaryvolume (REV) of the digital core by combining the porosity and autocorrelation function. Thedistribution of gas-water in the pore space of reservoir rocks is determined through numericalsimulation of gas-water two-phase separation based on digital cores using lattice Boltzmannmethod. Basic interfacial phenomena such as surface tension, wettability et. al. can bere-produced validly. By comparison with the experimental results obtained by X-ray CT fromDigital rock physics Lab in Australian National University, results show that the law ofgas-water distribution drawn from numerical simulation is consistent with the experimentalresults, which lay the foundation for carrying out digital rock physics experiments of naturalgas reservoir rocks.The effects of grain size, grain sorting, diagenesis and fluid properties on the elasticproperties of natural gas reservoir rocks are studied using the finite element method (FEM)based on digital cores. The results show that the grain size, grain sorting and diagenesis havenotable effect on rock physics properties and pore structure, which will cause the variation ofelastic moduli and velocities. The effect of the grain size on the elastic properties ofwater-saturated rock is weaker than that of dry rock. Moreover, the bulk moduli are more sensitive to fluid change when the grain size is very small. The worse the sorting is, thegreater the elastic moduli and velocities of natural gas reservoir rocks will be. According todiagenetic simulation algorithm, three digital diagenetic models are proposed based on digitalcores. The results show that diagenetic trends for cementation model Ⅲ are the stiffestcompared to those of cementation model I and Ⅱ. For the same cement volume, cementationmodel Ⅱhas the minimum cements at contacts compared to the others. For the same porosity,the calcite cements are stiffest for bulk moduli, followed by quartz cementation and claymineral cementation. The realization of the digital diagenetic will further promote thedevelopment of digital rock physics experiments. After comparing the change ratios of theelastic parameters with the variation of gas properties, it is found the Lamé constant is themost sensitive to the variation of gas properties, followed by Poisson ratio. The finite elementsimulation algorithm is extended by the introduction of the six orthogonal basis vectors sothat it can be applied to study the anisotropy of natural gas reservoir rocks. The3-D digitalcores with penny shape cracks are constructed using digital image processing technology, theelastic parameters of which are calculated by the extended finite element method. Thenumerical results are consistent with the results calculated by anisotropic Gassmann theory,which lay the foundation for studying the anisotropy of natural gas reservoir rocks usingdigital core technology.The effects of micro-factors (such as grain size, connectivity, wettability et. al.) on theelectrical properties of natural gas reservoir and oil reservoir rocks are comparatively studiedutilizing digital core technology, which reveals micro-difference between natural gasreservoirs and oil reservoirs. Similar to the oil reservoirs, the electrical properties of naturalgas reservoirs are affected by many factors. The trend and the extent are different underdifferent factors. Due to the divergences in physical properties between natural gas and crudeoil, the effects of the same factors on the electrical properties of reservoir rocks are distinct.For the same pore structure, porosity, water saturation and salinity of formation waters, theresistivity of natural gas reservoirs is higher than that of oil reservoirs mainly due to the waterfilm thickness and the solubility of natural gas in water. After analyzing the reason why Non-Archie phenomenon generates, the relations between saturation exponent andmicro-factors (such as water saturation, pore structure, wettability and the solubility of naturalgas in water) are investigated from a mathematical point of view. Combining with rockphysics experiments, the saturation model for natural gas reservoir evaluation is advancedbased on Lambert W-function. After processing the experimental and numerical simulationdata using the new model, the degree of fitting is considerably well.