An analytical coal permeability model for tri-axial strain and stress conditions
An analytical coal permeability model for tri-axial strain and stress conditions
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DOI：
10.1016/j.coal.2010.08.011
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发表时间：
2010-11-01
影响因子：
5.6
通讯作者：
Pan, Zhejun
中科院分区：
文献类型：
--
作者：
Connell, Luke D.;Lu, Meng;Pan, Zhejun
煤的渗透率对有效应力敏感，因此与瓦斯运移过程中煤层的地质力学行为相耦合。由于煤收缩与气体解吸和膨胀与吸附，理解这种耦合地质力学行为是解释煤渗透性的核心。现有的煤渗透率模型，如Shi和Durucan（2004年）以及Palmer和Mansoori（1996年）提出的模型，通过假设单轴应变和恒定垂直应力简化了地质力学过程。然而，很难在实验室三轴渗透率测试和实验室岩心驱替测试期间复制这些条件以用于增强煤层气。通常实验室测试涉及静水应力状态，其中三轴单元内的限制流体中的压力均匀地施加到样品外部。在该实验布置中，允许样品经历三轴应变。本文提出了两个新的分析渗透率模型表示，来自一般的线性孔隙弹性本构关系，包括三轴应变和应力的影响，煤经历气体吸附诱导膨胀。提出了一种新的方法来表示煤吸附应变对割理孔隙度和渗透率的影响。这涉及区分煤基质、孔隙（或割理）和散装煤的吸附应变。所开发的模型表示适用于从一系列的实验室测试的结果，它表明，该模型可以预测的实验室渗透率数据。作为该表征的一部分，鉴定了各种吸附应变，并且表明孔应变显著大于（约50倍）本体吸附应变。该模型还提供了进一步了解煤的渗透率如何随煤的收缩和膨胀，以及渗透率回弹压力如何取决于所施加的有效应力。皇冠版权所有（C）2010由爱思唯尔B.V.出版保留所有权利。
Coal permeability is sensitive to the effective stress and is therefore coupled to the geomechanical behaviour of the seam during gas migration. As coal shrinks with gas desorption and swells with adsorption, understanding this coupling to geomechanical behaviour is central to interpreting coal permeability. Existing coal permeability models, such as those proposed by Shi and Durucan (2004) and Palmer and Mansoori (1996), simplify the geomechanical processes by assuming uni-axial strain and constant vertical stress. However it is difficult to replicate these conditions in laboratory tri-axial permeability testing and during laboratory core flooding tests for enhanced coal bed methane. Often laboratory tests involve a hydrostatic stress state where the pressure in the confining fluid within the tri-axial cell is uniformly applied to the sample exterior. In this experimental arrangement the sample is allowed to undergo tri-axial strain. This paper presents two new analytical permeability model representations, derived from the general linear poroelastic constitutive law, that include the effects of tri-axial strain and stress for coal undergoing gas adsorption induced swelling. A novel approach is presented to the representation of the effect of coal sorption strain on cleat porosity and thus permeability. This involves distinguishing between the sorption strain of the coal matrix, the pores (or cleats) and the bulk coal. The developed model representations are applied to the results from a series of laboratory tests and it is shown that the models can predict the laboratory permeability data. As part of this characterisation the various sorption strains are identified and it is shown that the pore strain is significantly larger than (approximately 50 times) the bulk sorption strain. The models also provide further insight into how coal permeability varies with coal shrinkage and swelling and how the permeability rebound pressure depends upon the effective stress applied. Crown Copyright (C) 2010 Published by Elsevier B.V. All rights reserved.