Abstract：

Coal is composed of porous matrix blocks bounded by a well-developed cleat network and is a dual porosity medium. While fluid mobility is mainly controlled by the developed cleat network, the matrix pore contribution to permeability can be ignored. For a typical coal seam, its permeability in-situ depends on the initial stress and temperature loadings. In addition, the stress loads may also change with production, further leading to permeability evolution. In this study, permeability tests were first conducted on two typical coal samples from the Eastern Ordos Basin. Both the stress and temperature loadings were implemented with the in-situ conditions at different coal seam depths and production stages, and the influences of stress and temperature on permeability were further examined. Combined with dimensional analysis, the sensitivity results then generate two empirical permeability models for the Shanxi and Taiyuan formations. The results show that coal deformation and permeability evolution are essentially the result of stress and temperature changes. The changes may generate three effects, a reservoir compaction effect, a matrix shrinkage effect, and a thermal expansion effect. Within the testing temperature range, the results show that coal permeability increases exponentially with the decrease of effective horizontal stress. However, coal permeability changes with temperature may be the opposite of those experienced with different stresses. With significant stresses, the matrix deformation is more pronounced and thus will occupy some space orginally occupied by the cleat, showing up as a narrowing down of the cleat and permeability decrease. That is, the permeability may increase with a decrease of temperature at significant stress loadings. As the stresses weaken, any two of the curves at different temperatures will meet with a specific stress loading. In other words, the permeability decrease due to thermal expansion is offset by matrix shrinkage at this point, and the permeability may increase with temperature with a lower stress loading. The curves for the 4# specimen are inverse in a range from 1.2 MPa to 1.9 MPa, while those of the 8# specimen have a range from 1.8 MPa to 2.5 MPa. Once the reservoir compaction is too weak to suppress the thermal expansion, the cleat will swell more rapidly than the matrix instead, and together with the dominant matrix shrinkage, further improve the permeability. The results also show that the empirical permeability models predict the coal seam permeability at different buried depths and different production stages accurately, with an average relative error of 28.5%.

Key words：permeability evolution; stress sensitivity; temperature sensitivity; prediction model

Received： 2019-09-19

Corresponding Authors：wellcompletion@126.com

Cite this article：ZENG Quanshu, WANG Zhiming. Stress and temperature sensitivity of coal permeability in the Eastern Ordos Basin. Petroleum Science Bulletin, 2020, 04: 512-519.

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