Fractured Reservoirs By Gas Injection Development Of Molecular Diffusion Behavior Research And Numerical Simulation

Fluid seepage mechanisms are complex during gas injection in a fractured reservoir. Among them, molecular diffusion is important, especially, the diffusion of injected gas from the fractures to the matrix oil is one of the main mechanisms to enhance matrix oil recovery. But in practice or theoretical studies, most people follow a wrong opinion that for a fractured reservoir molecular diffusion is negligible during gas injection as for a non-fractured reservoir. As gas injection technology is applied to more fractured reservoirs in these years, molecular diffusion is getting more and more consideration. Several researchers investigated this mechanism, but there are limitations in their work. In order to exactly understand the diffusion behavior and seepage laws during gas injection in a fractured reservoir, this paper makes a theoretical study on this process, based on geology, fluid dynamics, diffusion physics, reservoir engineering and numerical simulation theories. The following achievements are obtained:1. Following the first analysis on geology characteristics of fractured reservoirs, limitations of depletion and water flooding are discussed. Availability and advantages of gas injection are showed by offering a great number of real examples of gas injection application to fractured reservoirs and by analyzing the production characteristics of them.2. Fluid dynamical behaviors, induced mainly by viscous forces, capillary forces, gravity and diffusion, are analyzed for the gas injection process in a fractured reservoir. Especially, molecular diffusion is focused on.3. How to describe the diffusion behavior by mathematic means is discussed, and methods are offered to determine the effective diffusion coefficients. The general values of the coefficients under reservoir conditions are estimated, and a model is given to calculate gas diffusion flux in an oil phase in porous media.4. Three-dimensional, three-phase, black-oil mathematical models are established for the gas injection process in a fractured reservoir. Regarding different factors, three models are given. The first is to regard contributions from both viscous pressure and molecular diffusion when describing mass transfer between the fracture and the matrix. Based on the first model, the second is to add contribution from gravity to fracture-matrix mass transfer, while the third is to take into account the effect of such factors as diffusion and vaporizing/condensing on fluid properties by introducing a mixing parameter w to modify the fluid viscosity and density.5. According to the first mathematical model, a numerical simulation model is established by finite-difference methods. And a dual-porosity black-oil simulator for the gas injection process in fractured reservoirs is developed by mending the existing single-porosity black-oil computer procedure.6. With a quarter five-spot grid model, the effect of molecular diffusion on gas displacement efficiency is studied. The difference in production performance is analyzed between with diffusion and without diffusion. The influence of diffusion is investigated when changing the value of each of the following parameters: effective diffusion coefficient, fracture density, ratio of fracture permeability to matrix permeability, ratio of fracture porosity to matrix porosity, production rate and injection rate.7. An example is given to study the effect of diffusion, which further demonstrates that a model able to exactly describe molecular diffusion is needed when performance prediction is in progress for an optimized development scenario.