Study On Numerical Simulation Method For Multiphase Flow In Tight Oil Reservoir

Among the unconventional oil reservoirs,tight oil reservoirs occupy an important share which usually exist in tight rock layers in an adsorbed or free state.With low porosity and permeability,the tight reservoirs are generally difficult to be developed directly.It is usually necessary to use hydraulic technology to improve the oil product.In order to effectively predict the state of the reservoir after fracturing,and improve the oil product,numerical simulation technology is widely used in production practice.After fracturing,the reservoir usually contains a large number of fractures of different scales,including fractures with large scale and discrete distribution,as well as natural fractures with small scale and close connection.In view of different scale fractures,scholars put forward different models for research.The discrete fracture model(DFM)is widely used for large-scale diversion fractures.For porous media with more micro-fractures,the dual-porosity model(DP)is more widely applied.However,when simulating multiphase flow,the capillary pressures of fracture and matrix are quite different,which results in the rapid change of physical quantities between different media,and the accuracy and efficiency of simulation are challenged.In the discrete fracture model,due to the large difference of capillary pressure between the fracture and the matrix,the saturation gradient and pressure gradient in the matrix grid near the fracture may diverge or be discontinuous,which cause that the conventional discrete method can not accurately calculate the cross flow between the matrix and the fracture under the coarse grid.In the dual-porosity model,due to the high capillary pressure of the matrix,the imbibition is a dominant factor.The saturation changes rapidly in the matrix near the fracture,and there is obvious saturation distribution in the matrix block.The accuracy of the traditional model for the transfer flux between matrix and fracture is very low.In view of the above problems,this thesis carries out the following work:1.Starting from the steady flow of one-dimensional,the matrix-fracture interface flow is analyzed by mathematical method,and the analytical solution near the m-f interface is obtained.Mathematical analysis shows that when the fracture capillary pressure is 0,there will be three flow patterns near the interface between matrix and fracture:two-phase flow from matrix to fracture;only oil flow from matrix to fracture;and two-phase flow from fracture to matrix.(1)When the pressure difference between matrix and fracture can overcome the capillary end effec,the two-phase fluids flow from matrix to fracture,and the water saturation at the side of matrix near the interface rises rapidly tends to 1,which leads to the divergence of oil pressure gradient and saturation gradient.(2)When the matrix pressure is higher than the fracture pressure but the pressure difference cannot overcome the capillary end effect,the water phase pressure is discontinuous,the oil phase pressure is continuous,only the oil flows out of the matrix,and the water flux from the fracture into the matrix tends to 0.(3)When the fracture pressure is higher than the matrix pressure,two phase fluids flow from the fracture to the matrix.At this time,the oil phase pressure is continuous and the water phase pressure is discontinuous.2.A new discrete fracture model is proposed.In this model,the exchange flux between matrix and fracture is calculated based on the steady analytical solution near the m-f interface.The calculation scheme of matrix-fracture exchange fluxes is constructed based on the analytical solution of one-dimensional problem.The new discrete fracture model considers the influence of capillary pressure discontinuity between matrix and fracture,which can effectively describe the flow pattern near the matrix-fracture interface,so it can accurately calculate the exchange flux between matrix and fracture in a coarse grid,thus improving the calculation efficiency and accuracy of the discrete fracture model.In order to verify the efficiency and accuracy of the proposed discrete fracture model several numerical examples are used to verify and compare with the traditional DFM.The results of numerical examples show that the model in this paper has good accuracy under different conditions,which is consistent with the calculation results of the single porosity model which subdivides the matrix grid near the fracture and inside the fracture.However,the traditional DFM model overestimates the exchange flux,which leads to lower predicted watercut.3 A new analytical solution for imbibition process is proposed.For micro-scale fracture area with no obvious displacement effect,the transient effect will play a more dominant role.For the two-phase imbibition process,the distribution form of the solution before and after the water flooding frontier reaching the boundary is different.The oil production prediction of the whole process depends more on the empirical formula.In this thesis,the formula of the oil product in imbibiton process changing with time is obtained by mathematical analysis.In the early stage before the water flooding frontier reaches the boundary,the analytical similarity solution of one-dimensional dimensionless equation is adopted.A new approximate analytical solution is proposed in the late stage after the water flooding frontier reaches the boundary,in which the influence of the generalized viscosity ratio of two phases is considered,and the additional parameter g is used to describe it.The numerical tests show that the solution proposed in this thesis is more accurate than existing method.In practical application,the approximate analytical solution can be applied to calculate the transfer flux between matrix and fracture in dual-porosity model.