A Research And Application On Nonlinear Seepage Numerical Simulation Of Waterflooding Heavy Oil Reservoir

With the reduction of conventional oil reserves, the heavy oil resources are considered more and more important.As the exploitation of heavy oil is gradually expanding the scale, more and more people research on it, and part of heavy oil has been listed as a key development project by the government, heavy oil exploitation will increasingly show the important strategic position. But because of heavy oil’s characteristics, there is difference in underground seepage characteristic between conventional crude oil and heavy oil, which has great influence on reservoir development, choice of technology and development indexs. So it is necessary for heavy oil reservoir’s effective development to correctly understand the characteristics of heavy oil seepage.Heavy oil is a complex, multicomponent homogeneous organic compounds. Heavy oil consists of a series of large molecules and has some form of super molecular structure, which makes heavy oil have the characteristics of high viscosity, high density and non-newtonian property. The main characteristics of heavy oil’s non-newtonian property is start-up pressure gradient,which means that oil does not flow when the driving pressure gradient is less than the threshold pressure gradient,and When the driving pressure gradient is greater than the threshold pressure gradient heavy oil will follow the law of quasi-linear seepage curve. Heavy oil’s starting pressure gradient can be mainly explained by the complexity of porous media channel, the non-Newtonian property of fluid, and the abnormal properties of fluid boundary layer.It is widely proved that Darcy’s law only describes the small part of the entire process flow,many seepage processes does not meet Darcy’s law, it’s usually called non-Newtonian flow or nonlinear fluid flow. Nonlinear seepage generally includes these types in the following:(1) non-Newtonian Fluid Flow,and the fluid viscosity is a function of shear rate; (2) the high speed of nonlinear turbulence, then the force of inertia can not be ignored, and the additional turbulence resistance has significant impact on the flow; (3) deformable porous media flow, then porous media flow parameters is a function of flow conditions; (4) non-isothermal flow; (5) low-flow, then the physical parameters of porous media is a function of flow conditions. Waterflooding heavy oil reservoir is a typical example for nonlinear seepage.Currently, the numerical simulation technology has become a powerful tool to understand and describe the development and production of oil and gas reservoirs law. However, the numerical simulation of nonlinear flow is still in its infancy, there is no systematic methods、techniques and simulation systems so far. At present, the research on nonlinear seepage is mostly for single-phase flow, there are three areas of consensus in the following:(1) The influence of crude oil’s properties on seepage:heavy oil’s high viscosity is the main reason for its non-newtonian property, which is mainly viscoelastic or pseudoplastic. (2) The influence of the medium’s permeability on seepage:the interaction of medium and fluid is a serious impediment to the fluid flow, which lead to heavy oil’s start-up pressure gradient. (3) the inelastic media:in the process of oilfield’s development, the porosity and permeability will change with the formation pressure, and this process is often irreversible. These three aspects will make the flow of oil in the reservoir exhibit nonlinear flow properties.Firstly this paper widely investigates the formation mechanism of heavy oil’s starting pressure gradient, the research course of starting pressure gradient, experiment’s measurement method of starting pressure gradient, nonlinear seepage of mathematical model and numerical model. Then based on nonlinear seepage experiments law of QHD32-6 and SZ36-l’s heavy oil, and considering the shape of seepage curve、 the formation mechanism of heavy oil’s starting pressure gradient、the relationship of mobility and threshold pressure gradient, this paper will build the motion equation with variable starting pressure gradient. In addtion, the mathematical model of variable starting pressure gradient will be derived, and the numerical model method will be established by IMPES method.What’s more, we’ll compile the numerical simulator of nonlinear flow. On the condition of ensuring the simulator reliable by extensive testing, we’ll use the nonlinear seepage simulator to calculate mechanism model of 5-spot well pattern、9-spot well pattern and the typical well pattern of QHD32-6 oil field.The simulation results show that starting pressure gradient has three kinds of impacts on oil field development:(A) The starting pressure gradient can accelerate the formation of a minimum resistance’s flow passage, since then most or all of the injected water will flow through the channel, which may reduce the sweep area; (B) The starting pressure gradient can exacerbate the differences of water and oil’s mobility and intensify fingering phenomenon, which will result in more serious phenomenon of non-piston and greatly reduce oil displacement efficiency; (C) The starting pressure gradient can intensify the flow resistance difference between layers which will increase the contradiction of layers. Through digging these laws of the starting pressure gradient phenomenon in heavy oil field development, we’ll provide a theoretical and technical support for the rational development of heavy oil.