Abstract In situ coalbeds are commonly inclined to one or multiple directions. The principal permeability tensor of an inclined coalbed may thus be non-coaxial to the uniaxial strain conditions, which are composed of two underlying assumptions: invariant vertical stress and confined horizontal boundaries. This paper developed a mathematical model to represent the principal permeability tensor of inclined coalbeds during pore pressure depletion under uniaxial strain conditions. This model adopted two geological properties to correlate the orientations of principal permeabilities and uniaxial strain conditions. One is the dip angle of inclined coalbeds, and the other is the pitch angle of cleats. The developed model was verified by field permeability data of the coalbeds in the San Juan Basin. Dip angle and pitch angle are the featured parameters of the developed model. This paper thus evaluated how the two angles influenced the magnitudes and evolution behaviors of principal permeabilities during pore pressure depletion under uniaxial strain conditions. The influences of dip angle and pitch angle on principal permeabilities can be attributed to their influences on the effective stresses normal to principal permeabilities. In order to upscale the principal permeability tensor for CBM recovery, this paper formulated a tensor-based index system for coal permeability evaluation. The tensor-based index system is composed of three indexes: the principal permeability tensor, the equivalent permeability tensors coaxial to engineering issues, and the geometric mean (GM) permeability of the three principal permeabilities. The principal permeability tensor can be used to optimize the pattern of vertical CBM wells, to determine the optimal directions of lateral wells, and to predict the propagating orientation of hydraulic fractures. The equivalent permeability tensors coaxial to engineering issues can be implemented into numerical simulators to improve the precision when evaluating and predicting the production performance of CBM wells. The GM permeability represents the ensemble transport capacity of coalbeds and can be used as an indicator to represent the productivity of coalbed reservoirs. This paper also preliminarily discussed how to use the tensor-based index system in CBM recovery.

中文翻译：

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单轴应变条件下孔隙压力耗竭期间倾斜煤层的主渗透率张量：建立数学模型，评估特征参数的影响，并扩大煤层气采收率

摘要 原位煤层通常向一个或多个方向倾斜。因此，倾斜煤层的主渗透率张量可能与单轴应变条件非同轴，单轴应变条件由两个基本假设组成：不变的垂直应力和受限的水平边界。本文开发了一个数学模型来表示单轴应变条件下孔隙压力耗竭期间倾斜煤层的主渗透率张量。该模型采用了两种地质特性来关联主渗透率的方向和单轴应变条件。一个是倾斜煤层的倾角，另一个是割理的俯仰角。开发的模型得到了圣胡安盆地煤层的现场渗透率数据的验证。倾角和俯仰角是所开发模型的特征参数。因此，本文评估了两个角度如何影响单轴应变条件下孔隙压力耗竭过程中主渗透率的大小和演化行为。倾角和俯仰角对主渗透率的影响可归因于它们对垂直于主渗透率的有效应力的影响。为了提高煤层气采收率的主渗透率张量，本文建立了基于张量的煤层渗透率评价指标体系。基于张量的指标体系由三个指标组成：主渗透率张量、与工程问题同轴的等效渗透率张量和三个主渗透率的几何平均（GM）渗透率。主渗透率张量可用于煤层气直井井网优化、侧井优化方向的确定、水力裂缝扩展方向的预测等。与工程问题同轴的等效渗透率张量可以在数值模拟器中实现，以提高评估和预测煤层气井生产性能时的精度。GM渗透率代表煤层的整体输运能力，可作为表征煤层储层产能的指标。本文还初步探讨了如何在煤层气采收中使用基于张量的指标体系。并预测水力裂缝的扩展方向。与工程问题同轴的等效渗透率张量可以在数值模拟器中实现，以提高评估和预测煤层气井生产性能时的精度。GM渗透率代表煤层的整体输运能力，可作为表征煤层储层产能的指标。本文还初步探讨了如何在煤层气采收中使用基于张量的指标体系。并预测水力裂缝的扩展方向。与工程问题同轴的等效渗透率张量可以在数值模拟器中实现，以提高评估和预测煤层气井生产性能时的精度。GM渗透率代表煤层的整体输运能力，可作为表征煤层储层产能的指标。本文还初步探讨了如何在煤层气采收中使用基于张量的指标体系。GM渗透率代表煤层的整体输运能力，可作为表征煤层储层产能的指标。本文还初步探讨了如何在煤层气采收中使用基于张量的指标体系。GM渗透率代表煤层的整体输运能力，可作为表征煤层储层产能的指标。本文还初步探讨了如何在煤层气采收中使用基于张量的指标体系。