裂缝性油藏注气开发分子扩散行为研究及数值模拟

【作者】 文玉莲；

【导师】 杜志敏；

【作者基本信息】 西南石油学院 ， 油气田开发工程， 2005， 博士

【摘要】 裂缝性油藏注气开采渗流机理复杂,分子扩散是其中重要的一种流体行为,尤其裂缝中注入气向基岩原油的扩散,是提高基岩原油采出程度的主要机理之一。然而在油藏开发实践和理论研究中,大多习惯性地将对常规油藏注气开采中分子扩散作用的认识沿用到裂缝性油藏注气开发中,即认为分子扩散可以忽略。近年来,裂缝性油藏注气开发技术应用越来越广泛,扩散对采油的作用也越来越受到关注,一些学者通过实验手段或理论方法对该种机理进行了研究,但他们的工作还存在很多不足。本文围绕气驱过程中的分子扩散行为,力图探讨油藏条件下油气分子扩散能力问题,建立考虑分子扩散的裂缝性油藏注气开采数学模型,分析扩散对裂缝性油藏气驱动态的影响。在研究中,综合运用了地质学、流体动力学、分子扩散物理学、渗流力学、油藏工程、数值模拟等理论和方法,主要取得了以下成果: 1.在分析裂缝性油藏基本地质特征的基础上,探讨了传统衰竭式、注水方式在裂缝性油藏开发中存在的局限性。以大量裂缝性油藏注气实例为重要依据,论证了注气开发裂缝性油藏的有效性和现实性,分析了裂缝性储层注气开采特征,展示了注气方式在开发裂缝性油藏中的优势。 2.分析了裂缝性油藏注气开采过程中的流体动力学行为,对各种驱动因素包括粘滞力、毛管力、重力及分子扩散等作用下的油气流动规律进行了描述。着重讨论了于常规油藏注气过程中作用很小而在裂缝性油藏注气过程中作用突出的分子扩散行为。 3.探讨了扩散数学描述方法及多孔介质中流体有效扩散系数的确定方法,研究了油藏条件下气相中和油相中的有效扩散系数取值范围及油气分子的扩散能力,建立了多孔介质中气在油相中的扩散流量计算模型。 4.建立了考虑分子扩散作用的裂缝性油藏注气开采三维三相黑油数学模型。建模过程中,根据考虑因素的不同,分别建立了三种数学模型。模型一,考虑裂缝与基质之间的流体质量交换包括来自压差和分子扩散两方面的贡献;模型二,在模型一的基础上,增加考虑了重力对裂缝-基质质量交换的贡献;模型三,以模型一为基础,增加考虑了扩散、汽化/凝析等传质作用对流体性质的影响,引入混合参数ω修正油气粘度和密度。 5.利用有限差分原理,建立了考虑分子扩散作用的裂缝性油藏注气开采数值模型;并在现有单重介质黑油计算机模型基础上,发展了模拟裂缝性油藏注气开采过程的考虑分子扩散影响的双重介质黑油计算机模型。 6.以所建数学模型为基础,建立了四分之一五点井网气驱机理模型,分析了裂缝性油藏气驱扩散效果。对比研究了考虑扩散和不考虑扩散之间的生产动态更多还原

【Abstract】 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.更多还原