The Research Of The Capillary Effec On The Multi-Phase Flow In Tight Oil And Gas Reservoir

Tight oil and gas reservoir has great potential for development,and it is becoming more and more important in the oil and gas reservoir development industry.It is a complex porous coupled system composed of matrix and multi-scale fractures.Strong heterogeneity is one typical feature of the tight reservoir.Especially,the capillary pressure in matrix is usually very large due to its small porosity and low permeability.On the contrary,the capillary pressure in fracture is very small,or even can be ignored,compared with the matrix.The multiphase flow in this coupled system displays nonlinear characteristics caused by the heterogeneous capillary pressure,which differs from the Darcy's law.The capillary pressure may be discontinuous at the matrix-fracture(or matrix-wellbore)interface,and due to this,complex flow phenomenon across the interface will occur.Theoretical analysis shows that three flow patterns can be distinguished for the two phase steady flow across the matrix-fracture interface under the assumption of vanishing capillary pressure in fracture:(1)When the pressure difference between matrix and fracture can overcome the capillary end effect,both two phase will flow from matrix to fracture,and wetting phase saturation rapidly rises to 1 at the matrix side of interface,which causes the divergences of non-wetting phase pressure gradient and phase saturation gradients.(2)When matrix pressure is higher than fracture pressure,but the pressure difference cannot overcome the capillary end effect,only the non-wetting phase that can flow from matrix to fracture.(3)When fracture pressure is higher than matrix pressure,both two phases will flow from fracture to matrix.In this situation,the wetting phase pressure at the interface is discontinuous.The heterogeneity of capillary pressure will cause complex flow phenomenon across the interface,resulting in drastic changes or divergence of physical quantities near the matrix-fracture(or matrix-wellbore)interface,which makes it very difficult for high-precision reservoir numerical simulation.Based on the classification of the flow state and the corresponding interface conditions mentioned above,this thesis focuses on the effect of the heterogeneity of capillary pressure for the two-phase flow in tight reservoirs.The research contents are as follows:1.A new numerical well model for low permeability gas reservoir is constructed.The traditional numerical well model for multiphase flow is directly extended from the single-phase one.Since it does not fully consider the drastic changes of physical parameters near the matrix-wellbore interface,it brings significant errors.Based on the analytical equations of one-dimensional radial flow considering the capillary end effect,an alternative numerical well model for low permeability gas reservoir is constructed.Numerical examples show that the proposed model can reflect the dramatic change for saturation and gas-phase pressure in the vicinity of the production well both for two flow patterns,and therefore can predict the gas and water productions accurately at different grid scales.It means that the prediction results of the proposed well model are less dependent on the grid size.The calculation results also show that the capillary end effect will cause serious water-block phenomenon near the matrix-wellbore interface,which impedes the flow from matrix to the production well.2.Based on the capillary end effect,the nonlinear characteristics of the two-phase steady flow in core experiments are analyzed.In this thesis,a rigorous mathematical analysis is performed,and the following conclusions are drawn.If the pressure difference is small enough,the influence area of the capillary end effect covers the whole core,and the curve of the flow rate vs.the pressure difference presents a nonlinear relationship of concave function.With the gradual increase of the pressure difference,the function curve gradually tends to be linear,and a pseudo-threshold pressure difference can be obtained by extending the linear section.In this thesis,it is proved mathematically that the pseudo-threshold pressure difference is a finite value.By analyzing the influence of relevant parameters(core permeability,phase permeability curve,fluid viscosity,etc.)on the pseudo-threshold pressure difference,it is found that the lower the core permeability,the more significant the non-linearity of the flow rate curve and the larger the pseudo-threshold pressure difference.When the wet phase is neutral,the pseudo-threshold pressure difference will be quite small.It can be predicted that the hindrance of capillary pressure will be greatly reduced in the tight reservoir with neutral wetting phase.In addition,the influence of boundary layer effect for two-phase flow in tight reservoirs is also studied.The results show that even considering the boundary layer effect,the heterogeneity of capillary pressure is still an important cause of non-linear flow.The analysis based on the capillary pressure heterogeneity in this thesis provides a new perspective for the research of two-phase nonlinear flow in tight reservoirs.3.In the natural tight reservoirs,matrix and natural fractures coexist.There is still huge difference between the capillary pressure in matrix and in natural fractures.For the same reason,the heterogeneity of capillary pressure also have an important impact on the macroscopic flow in the coupled system,and the flow will present a significant nonlinear characteristics.In this thesis,the macroscopic nonlinear two-phase flow in the direction perpendicular to the fracture is discussed based on the assumption of the evenly distributed natural fractures.The results show that the wetting phase has a threshold pressure difference,which increases with the increase of fracture density.Under small pressure difference,the function curve of the non-wetting phase flow rate vs.pressure is a convex function.It will tends to be linear with the increase of the pressure difference,but the pseudo-threshold pressure difference still exists.In addition,the effects of the saturation,the permeability and the boundary layer effect on the function curve are analyzed.The results show that the water saturation and the permeability in matrix have great influence on the nonlinear characteristics of the flow.With the increase of the water saturation,the capillary pressure between oil and water phase decreases and the nonlinear characteristics is weaken.With the decrease of the matrix permeability,the heterogeneity of capillary pressure increases and the nonlinear characteristics of the flow also increases.When the boundary layer effect is taken into account,the nonlinear characteristics is significant even under the small pressure difference.