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Evolution of Coal Permeability under Constant Effective Stresses: Direct Measurements and Numerical Modeling
Energy & Fuels ( IF 5.2 ) Pub Date : 2021-09-21 , DOI: 10.1021/acs.energyfuels.1c01425 Yaoyao Zhao 1, 2 , Dongxue Cui 1 , Jishan Liu 3 , Mingyao Wei 4 , Yingke Liu 2, 5
Energy & Fuels ( IF 5.2 ) Pub Date : 2021-09-21 , DOI: 10.1021/acs.energyfuels.1c01425 Yaoyao Zhao 1, 2 , Dongxue Cui 1 , Jishan Liu 3 , Mingyao Wei 4 , Yingke Liu 2, 5
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Coal permeability is normally measured under the assumed condition of local equilibrium, i.e., the fracture pressure is equalized with the matrix pressure. When the local equilibrium condition is not met, the coal is in a transient state. In this study, we define coal permeability at a transient state as nonequilibrium permeability. A gas expansion-based experimental technique is developed to directly measure both the overall strain and the fracture strain of coal while the matrix strain is calculated based on the volumetric relation. This technique was applied to the case of gas injection into a coal sample. Carbon dioxide was injected into a coal sample at a range of pressures from 0.925 to 5.876 MPa and the confining pressures from 3.925 to 8.876 MPa, while the effective stress is maintained as a constant of 3 MPa. After continuous injection of carbon dioxide into the sample for 10 h, the experimental permeability is measured by the transient method, the overall strain is measured by the strain analyzer, and the fracture strain is measured by the gas expansion method. These strain measurements are used to directly calculate the fracture permeability, the matrix permeability, and the resultant permeability, respectively. The calculated permeability and the resultant permeability match with the experimental observations reasonably well. All three permeability profiles are similar, but their magnitudes are slightly different. These permeability profiles are used to benchmark a numerical model that includes the fracture-matrix interaction. The benchmarked numerical model is used to create a permeability map for the coal under the constant effective stress condition. The map represents a complete characterization of coal permeability from initial to ultimate equilibrium. The results demonstrate that the transient effects are significant and cannot be ignored for coal during coal seam gas extraction.
中文翻译：
恒定有效应力下煤渗透率的演变：直接测量和数值模拟
煤渗透率通常在假设的局部平衡条件下测量，即破裂压力与基质压力相等。当不满足局部平衡条件时，煤处于过渡状态。在本研究中，我们将瞬态下的煤渗透率定义为非平衡渗透率。开发了一种基于气体膨胀的实验技术，可以直接测量煤的整体应变和断裂应变，同时根据体积关系计算基体应变。该技术应用于将气体注入煤样的情况。在 0.925 至 5.876 MPa 的压力范围和 3.925 至 8.876 MPa 的围压范围内将二氧化碳注入煤样，同时将有效应力保持为 3 MPa 的常数。向样品中连续注入二氧化碳10 h后，采用瞬态法测量实验渗透率，应变分析仪测量总应变，气体膨胀法测量裂缝应变。这些应变测量分别用于直接计算裂缝渗透率、基质渗透率和合成渗透率。计算的渗透率和合成的渗透率与实验观察结果相当吻合。所有三个渗透率剖面都相似，但它们的大小略有不同。这些渗透率剖面用于对包含裂缝-基质相互作用的数值模型进行基准测试。基准数值模型用于在恒定有效应力条件下创建煤的渗透率图。该图代表了从初始到最终平衡的煤渗透率的完整特征。结果表明，煤层气开采过程中瞬态效应显着，不可忽视。
更新日期：2021-10-07
中文翻译：
恒定有效应力下煤渗透率的演变：直接测量和数值模拟
煤渗透率通常在假设的局部平衡条件下测量，即破裂压力与基质压力相等。当不满足局部平衡条件时，煤处于过渡状态。在本研究中，我们将瞬态下的煤渗透率定义为非平衡渗透率。开发了一种基于气体膨胀的实验技术，可以直接测量煤的整体应变和断裂应变，同时根据体积关系计算基体应变。该技术应用于将气体注入煤样的情况。在 0.925 至 5.876 MPa 的压力范围和 3.925 至 8.876 MPa 的围压范围内将二氧化碳注入煤样，同时将有效应力保持为 3 MPa 的常数。向样品中连续注入二氧化碳10 h后，采用瞬态法测量实验渗透率，应变分析仪测量总应变，气体膨胀法测量裂缝应变。这些应变测量分别用于直接计算裂缝渗透率、基质渗透率和合成渗透率。计算的渗透率和合成的渗透率与实验观察结果相当吻合。所有三个渗透率剖面都相似，但它们的大小略有不同。这些渗透率剖面用于对包含裂缝-基质相互作用的数值模型进行基准测试。基准数值模型用于在恒定有效应力条件下创建煤的渗透率图。该图代表了从初始到最终平衡的煤渗透率的完整特征。结果表明，煤层气开采过程中瞬态效应显着，不可忽视。
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