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考虑基质收缩效应的煤层气应力场-渗流场耦合作用分析

ISSN号：1000-7598
期刊名称：《岩土力学》
时间：0
分类：O357.3[理学—流体力学;理学—力学]
作者机构：[1]重庆大学西南资源开发与及环境灾害控制工程教育部重点实验室,重庆400030
相关基金：基金项目：国家自然科学基金创新研究群体科学基金资助项目（No.50621403）;教育部科学技术研究重点项目（No.109130）：重庆市院士基金资助项目（CSTS2005AB6007）.

作者：周军平[1], 鲜学福[1], 姜永东[1], 李晓红[1], 姜德义[1]

关键词：煤层气, 有效应力, 煤基质收缩, 渗透率, 渗流场:应力场, 耦合模型, coalbed methane, effective stress, coal matrix shrink, permeability, seepage field, stress field, coupled model

中文摘要：

在煤层气的初级生产过程中，为了获取较高的生产率，需要降低储层压力，储层压力下降对于煤层气的渗透率具有两个相反的效应：（1）储层压力下降，有效应力增加，煤层裂隙压缩闭合，渗透率降低；（2）煤层气解吸，煤基质收缩，煤层气流动路径张开，渗透率升高。Shi和Durucan、Palmer-Mansoori以及Gray等都建立了包含了基质收缩效应以及有效应力的影响的渗透率模型，其模型都基于以下两个关键假设：煤岩体处于单轴应变状态以及竖向应力恒定。为了检验上述两个假设的合理性，建立了一个考虑基质收缩效应以及渗流场一应力场耦合作用下的煤层气流动模型，对煤层气初级生产过程中渗透率的变化进行了耦合分析。分析结果表明：单轴应变的假设具有合理性，而竖向应力是随指向生产井的应变梯度的变化而变化的，其对于渗透率的变化具有重要影响，因此，竖向应力恒定的假设可能导致渗透率预测出现误差；上述渗透率模型都可能低估煤层气初级生产过程中渗透率的变化。

英文摘要：

To recover a large percentage of coalbed methane in coalbeds, reservoir pressure must be reduced significantly in the primary production. Two distinct effects are associated with the reduction of reservoir pressure during methane extraction：（1） the release of gas, and （2） an increase in effective stress. The increase in effective stress causes a decrease in the permeability of the coal owing to mechanical closure of flow paths. However, an opposing effect of coal-matrix shrinkage can occur, one that is attributed to a desorption phenomenon. This shrinkage widens the fractures that are primarily responsible for gas flow in coalbed reservoirs, and thus increases permeability. Some permeability models such as Shi-Durucan model, Palmer-Mansoori model and Gray model incorporating the effect of matrix shrink and effective stress influence were developed. These models were based on two key assumptions： coalbed is under uniaxial strain condition and constant vertical stress. According to this, a coupled seepage-stress coalbed methane flow model considering matrix shrinkage was used to examine the accuracy of these assumptions; then this model was used to analyze permeability change during primary production; the result suggested that the assumption of uniaxial strain appears valid, while constant vertical stress may introduce error; it varies as gradients in strain are induced towards the production well; and its contribution to permeability can be significant; then that existing coal permeability models could underestimate permeability change.

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