A Study On The Mechanism Of Oil-water Two Phase Flow In Microfractured Reservoir Formation And Its Development Application

Reservoir rocks of low and ultra-low permeability are usually tight and brittle.As a result,various forces can cause the formation of microfractures during the processes of diagenesis and tectogenesis.These microfractures are crucial for effectively mobilizing oil reserves residing in the rock matrix of low and ultra-low permeability.Current studies are mainly focused on fractured reservoirs whose fractures are well connected,but there are few studies on microfractures that cannot form effective network because of poor connectivity.In this thesis,physical experiments,theoretical derivations and numerical simulations are combined together to investigate thoroughly the mechanism of oil-water flow in reservoirs with microfractures and their field development.Firstly,the mechanisms of microfracture formation in reservoirs are analyzed and a method of preparing cores with microfractures is established based on the analysis.By controlling the external pressure and compression time,microfractures with various sized and physical properties can be generated inside the core samples.Microfractures are strongly stress-sensitive and quantitative modelling of microfractures with given physical properties can be achieved by measuring microfractures' permeability-stress sensitivity curves in advance and controlling the effective stress interval in microscopic flow experiments,which lays the foundation for subsequent experiments of flow in microfractures.Next,the dynamic characteristics of flow in microfractures and their variation with respect to fracture aperture are quantitatively tested through contrast experiments.Similarities and differences between flow in microfractures,macrofractures and matrix pores are revealed.The experimental results show that there exits threshold pressure gradient for flow in microfractures,but the magnitude is less than that in matrix pores.90 ?m is the boundary of fracture aperture between microfractures and macrofractures in terms of oil displacement efficiency.While flow capacity of oil in microfractures is similar to that in matrix pores,flow capacity of water is obviously higher.Therefore,flow behavior in microfractured reservoir formations is obviously different from that in conventional non-fractured formation even if their physical properties are similar.Microfractures and matrix pores cannot be separated completely,not can they be confused with each other.Based on the microscopic flow experiment and considering the associated relationship between microfractures and matrix pores in microfractured reservoir formations,an equivalent continuum mode based on discrete microfractures is built.This model lays the foundation for evaluating microfractured reservoir formations and studying the flow behavior in them.Microscopically,the model treats microfractures as distributed discretely in the microfractured zones and fluid flow behavior in microfractures can be considered individually.Macroscopically,microfractures are developed within matrix formations and they change the physical properties of matrix formations as a whole and the flow behavior in the matrix.The model is used to analyze the effect of different parameters of microfractures on the physical properties and productivity of reservoir formations.The results show that compared to the characteristics of high-conductivity and low-storage for conventional fracture system and low-conductivity,high-storage for matrix system,microfractures lie in between and have the characteristics of medium-conductivity and medium-storage.The connectivity of microfractures is the most important factor affecting physical properties and productivity of the reservoir formation and it is also one of the most important differences between microfractures and conventional large and medium fractures.Finally,considering the stress-sensitivity of permeability and dynamic threshold permeability in microfractured reservoir formation,a nonlinear steady-state productivity model is built and analytical solution is obtained.Numerical simulation is used to simulate water flooding in microfractured reservoirs.The effect of microfractures on the mechanism of water flooding and its effectiveness for microfractured reservoirs is studied and suggestions for properly adjusting development plan is given.The results show that on the one hand,microfractures increase the liquid productivity and injectivity of the reservoir formation.On the other hand,they accelerate the rise of water cut and increase the residual oil saturation.Therefore,the new idea for developing the reservoir is to adjust the relationship between oil-water two-phase flow by taking advantage of the stress-sensitivity of microfractures.