Digital Core Analysis Of Low-Permeability Reservoirs And Research On Microscopic Remaining Oil

China is rich in low-permeability oil and gas resources.Such resources have great potential for exploration and development.It is of great significance for the sustainable development of the oil and gas resources of our country to put a lot of effort into developing such resources.Many studies on petrophysical properties and distributions of the microscopic remaining oil in low-permeability reservoirs are the basic work for the efficient development of such resources.As an emerging numerical simulation method in recent years,digital core analysis technology is not only used for the characterization of pore structure,simulation of acoustic and electrical properties,and analysis of their impact factors,but also the simulation of diagenesis and seepage and study of the microscopic remaining oil.With the recent development,the digital core analysis and microscopic remaining oil research of high-porosity and high-permeability reservoirs have been increasingly perfect,but these studies of low-permeability reservoirs are facing new challenges since such reservoirs contain micrometer and nanometer pores.In order to construct digital cores with multiscale pore structures,two novel multiscale modeling methods were proposed.Based on the constructed multiscale digital cores,the comprehensive digital core analysis was carried out,including pore structure characterization,single-phase and two-phase flow simulation,electrical and acoustic properties simulation,and microscopic remaining oil analysis.Firstly,this thesis proposed two-hybrid modeling techniques of digital cores for lowpermeability reservoirs.The first one(XRCTST-QSGSM)was developed by combining the Xray CT scanning technique(XRCTST)and the quartet structure generation set method(QSGSM).XRCTST is responsible for capturing large pores whose sizes are above the experimental resolution of CT scanning in the rock and QSGSM is used to generate small pores the sizes of which are smaller than the CT resolution.The fusion of the two algorithms can construct digital cores with multiscale pore structures.Due to the expensive cost of the CT scanning experiment in the first hybrid modeling technique,the second hybrid modeling technique(DEM-QSGSM)that can construct a 3D digital core based on a few 2D pictures was put forward.DEM-QSGSM is formed by the combination of the discrete element method(DEM)and QSGSM.DEM-QSGSM generates digital cores by systematically simulating the formation process of sedimentary rocks.Based on the multiscale digital cores constructed by using the two-hybrid modeling methods,the pore networks of the models were extracted by using the medial axis algorithm,and their pore structures were comprehensively characterized.In addition,the finite volume method,pore network modeling method,and lattice Boltzmann method were used to simulate single-phase flow in porous media.Finally,the accuracy of the hybrid modeling methods was verified by comparing the physical properties obtained from the digital cores and those from physical experiments.The electrical and acoustic properties are important petrophysical properties.This thesis uses the finite element method to simulate the electrical and acoustic properties based on multicomponent digital cores and analyzes the main controlling factors of these physical properties.QSGSM is again used to construct multicomponent and multiscale digital cores.In order to analyze the controlling factors of electrical and acoustic properties,the volume fractions of minerals and pores in digital rocks were varied many times.By comparing the resistivity and elastic moduli of all models,it is concluded that the volume fractions of clay minerals and pores have greater effects on electrical and acoustic properties,while quartz,feldspar,and calcite only have slight impacts on elastic parameters.Complicated diagenesis is an important reason for making the reservoirs low-porosity and low-permeability.In order to investigate the influences of diagenesis event and diagenesis path on rock properties,based on the constructed multi-scale digital core,the QSGSM and morphological operation algorithms were used to simulate the two cementations and two dissolutions under the two diagenesis paths.By comparing the physical properties of digital rocks before and after diagenesis,it is found that cementation reduces the porosity,connectivity,fractal dimension,and absolute permeability,makes the overall pore/throat radius distribution and coordination number distribution smaller,and increases the tortuosity and resistivity.The effects of dissolution on rock properties are just the opposite of cementation.The diagenesis events of the same intensity have a weaker impact on rocks with better physical properties than the ones with worse physical properties.In order to analyze the distribution characteristics and formation mechanisms of the microscopic remaining oil in low-permeability reservoirs after water flooding,this thesis carries out two-phase flow experiments using XRCTST and the two-phase flow numerical simulation using the finite volume method on the water-wet sandstones.In the two-phase flow experiments of oil and water,several CT scans were performed to obtain 3D CT images of the rocks at different moments,and the analysis of the microscopic remaining oil was carried out based on these images.On the numerical simulation,the finite volume method and the fluid volume method were used to simulate the oil-water flow in the large-scale digital cores based on the Open FOAM platform.Before the simulation,a novel method of generating variable meshing is proposed.During the simulation,parallel operations and clusters were used to accelerate the simulation process.The results from the physical experiment and numerical simulation of oilwater flow in porous media show that the volume of the oil phase decreases,its number increases,its fractal dimension decreases,and the volume distribution curve of the oil phase moves toward a smaller value during the water flooding process.In order to analyze the oil occupation in a single pore,the Axis-Aligned Bounding Box algorithm was developed.With the aid of the algorithm,it is concluded that the volume of the remaining oil in the mediumvolume pores is the most at the end of water flooding,and the number of the remaining oil in the small-volume pores is large,but its volume proportion in the small-volume pores is small.According to the 3D shape factor and Euler number of the oil phase,the morphology of the oil phase is divided into five categories which are contiguous shape,network,multi-pore shape,film,and solitary drop.After water flooding,the oil phase mainly resides in the pores in the form of multi-pore shape and film,and the isolated oil drops are the most,but their volume is relatively small.According to the observations of 3D and 2D images,the residual locations of the remaining oil include the center of the pore,the wall of the pore,the throat,and the corner of the pore/throat.At the end of the water flooding,the volume proportions of the remaining oil in the center of the pores and corners of the pores/throats are higher.Finally,based on the seepage law of oil and water in porous media,it is analyzed that the snap-off phenomenon,nonpiston displacement,bypass flow phenomenon,and Jamin effect are the main reasons that cause the oil phase to reside in the rock.