Microscopic Flow Mechanism Of Shale Oil Based On Digital Cores With Multi-mineral Phases

The huge amount of shale oil resources in China is an important resource with the most potential for succession after the shale gas revolution.The pores of shale oil reservoirs are mainly micro-nano pores,and the pore walls have the characteristics of multi-mineral phases.The mineral properties of the pore walls at the micro-nano scale have a great influence on the flow.Therefore,the microscopic simulation of shale oil should consider multi-mineral phase pores.This thesis proposes a method for construction of shale reservoir multi-mineral digital core based on pore size distribution or 2D images.Based on the N-S equation,mathematical model and numerical solution methods of both single-phase and two-phase flow simulation considering different mineral pores are established.The microscopic flow mechanism of shale oil is revealed based on regular pores and digital cores of real shale oil 3D mineral facies.The thesis mainly conducts research for three aspects:Firstly,a digital core construction method for shale oil with multi-mineral phases is established.Based on the Markov chain Monte Carlo method,combined with the pore size distribution or 2D images for numerical reconstruction,two methods for constructing multimineral digital cores are proposed and verified for correctness.The pore network model method is used to analyze the pore structure distribution characteristics of shale digital cores,which shows that the flow space in shale cores is limited,but has multi-scale characteristics from nanometers to micrometers.The microstructure is complex,and shale cores have poor connectivity.Secondly,a mathematical model of shale oil single-phase micro-flow considering the adsorption layer and the corresponding numerical solution method are established.The mathematical model is solved based on the finite volume method and the SIMPLE algorithm,and the relevant solver is developed.Its correctness is verified by combining the experimental results.Based on this model,the flow simulation of shale oil regular pores and digital cores reveals the microscopic flow characteristics of shale oil: the adsorption layer on the pore walls of shale oil organic matter has a non-negligible influence on the flow,and the boundary slippage is the main reason why the permeability estimated by Darcy’s law is underestimated;the pore size is the dominant factor in shale oil flow.Thirdly,a mathematical model of microscopic shale oil/water flow considering the different slip lengths of the two phases and the corresponding numerical solution method were established.The mathematical model is solved based on the finite volume method,VOF model and PIMPLE algorithm,and the correlative solver is developed.Its correctness is verified by the results of classic calculation examples.Through the simulation results of shale oil regular pores and digital core flow,it can be seen that the breakthrough speed of the oil-water front in the inorganic pores is faster,but the displacement effect of organic pores near the wall boundary is better;as the proportion of organic pores increases,the isotonic point in the phase permeability diagram shifts to the left,the relative permeability of shale oil increases,and the relative permeability of water decreases on the whole.The thesis establishes a new method system from the construction of shale multi-mineral digital core to microscopic flow simulation,and applies it to regular pores and real shale oil 3D digital cores,revealing the microscopic seepage mechanism of shale reservoir fluids and providing a theoretical basis for efficient exploitation of shale oil reservoirs.