Oil-Gas-Water Three Pahse Darcy-Stokes Model And Application In Tahe Resevoir

The storage space of carbonate fractured-vuggy reservoirs includes caves,fractures and matrix pore,of which porosity and permeability own strong heterogeneity.The pore size vairies from micrometers to several hundred meters,leading to the fact the continuous condition can not be applied to describe the carbonate fractured-vuggy reservoirs.These characteristics of the reservoir contribute to the complex the flow in fractured-vuggy reservoirs,which is totally different from the conventional study.In the matrix pore,micro fractures and Karst pore,the fluid follows the DARCY flow.In the caves and huge fractures,the fluids follow Stokes flow.Therefore,there are limitations for the application of conventional reservoir simulation on the carbonate fractured-vuggy reservoirsThe Tahe carbonate fractured-vuggy reservoirs have experience the primary and secondary production and have entered the gas injection stage to enhance the oil recovery.Currently,the 2-phase(oil and water)model coupling the flow in porous media and free space has been well investigated.However,few study documents the 3-phase(gas,oil and water)model coupling the flow in porous media and free space in carbonate fractured-vuggy reservoirs.In this study,the 3-phase flow model is presented in carbonate fractured-vuggy reservoirs to explore the flow regulation,optimize the gas injection production strategies,enhance the oil recovery and reduce the financial cost.Firstly,a fractured-cavity reservoir is divided into continuous medium and discrete medium.frature and tiny caverns as continuous medium,and large-scale cracks and caverns as discrete mediums.The continuous medium includes the fractures,matrix pore,micro caves and filled caves.The flow in the continuous medium is studied using DARCY model.The discrete medium includes huge fractures and caves.The flow in the discrete medium is studied using STOKES model.Meanwhile,the 2-phase model is generalized to 3-phase model in DARCY-STOKES flow.The main steps of generalization include four parts as follows.(1)In the discrete medium area,large-scale fractured-cavity are divided into filled-cavity and unfilled ones according to the filling conditions.The filled is described by the three-phase DARCY equation of oil-gas-water.(2)The fluid flow in the unfilled caves belongs to cavity flow and needs to be described by the Stokes equation.(3)Because the DARCY-STOKES equations for oil-gas-water three-phase systems have not been established currently.the Navier-Stokes equations are extended from oil-water two-phase flow to oil-gas-water three-phase flow;Secondly BJS boundary condition are extended from the oil-water two-phase flow to the oil-gas-water three-phase flow two.(3)In the the normal direction of the continuous interface,the conventional infterface condition,CVPM,was replaced by a new interface condition,JPVCM.Thus,a complete three-phase Darcy-Stokes flow model for oil-gas-water in a fracture-cavity reservoir was constructed.Then,the commercial software COMSOL is employed for solving partial differential equations,the oil-water-water three-phase Darcy-Stokes model is used to study the flow behavior of nitrogen injection in the reservoir.The results show that the oil and gas distribution in the cavity is mainly affected by gravity.In top there is gas,in the middle there is oil,and on the bottom there is water.Because the flow transmissibility of the cavity is much higher than that in the small vugs and fractures and the fluid flow velocity in the ground is usually also small,the calculation efficiency is not toleratable.To improve the calculation efficiency,the DARCY-EPC model is proposed.The pressure potential across all areas of the cavity is very close to the same value.The entire fracture-cavity reservoir is called the DARCY-IPOS model it has the following advantages:It does not need to calculate the pressure gradient in the process of solving the fluids flow equations,thus avoiding the technical challenge to solve the three phase Darcy-Stokes equations.Furthermore,the independent variables relate to each cavity are only three,significantly reducing the variables and grid cells.Application examples prove the IPOS model can also reflect the fluids distribution in the cavities in the same way of the DARCY-STOKES model.And different from the spherical equipotential models,with sphere indicates the cavities and tubes represent the fractures,DARCY-EPC model retains the shape of the cavity and the features of microscopic fracture-caves flow.This study established the 3-phase model coupling the flow in porous media and free space,generalized the BJS condition on the section between DARCY flow and Stokes flow,improved the normal boundary condition using JPVCM conditions and proposed the DARCY-EPC model to fast simulate the model coupling flow in porous media and free space.