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<p>The technology of CO<sub>2</sub>-enhanced coalbed methane (CO<sub>2</sub>-ECBM) recovery offers multiple advantages, including improved clean energy utilization and long-term CO<sub>2</sub> sequestration. However, a threshold pressure gradient (TPG) often exists during CO<sub>2</sub> injection into low-permeability, water-bearing coal seams, resulting in nonlinear flow behavior. This phenomenon increases the resistance to CO<sub>2</sub> migration and limits the efficiency of the injection process. Accurately predicting the dynamic threshold pressure gradient (DTPG) in water-injected coal is therefore crucial for guiding the development of CO<sub>2</sub> injection strategies. Despite this, the influence of pore morphology and structural complexity on the TPG has not been fully investigated. The CO<sub>2</sub> injection threshold pressure gradient varies significantly with different pore structures and moisture conditions. In this study, a DTPG model is developed from a microscale perspective, incorporating water saturation, effective stress, and coal seam structural features, including seepage pores and adsorption pores. This model modifies the conventional two-phase gas–water flow model and enhances the accuracy of CO<sub>2</sub> flow prediction in coal seams. It does not rely on empirical constants, and each parameter has a clear physical interpretation. Furthermore, we propose a nuclear magnetic resonance (NMR)-based experimental method to quantify each parameter of the model and evaluate its applicability. Theoretical and experimental analyses reveal that the fractal characteristics of seepage and adsorption pores significantly affect the DTPG. Among the influencing factors, the pore throat ratio shows the strongest correlation with DTPG (correlation coefficient: 0.843), making it the most critical factor in determining DTPG variations in coal. Notably, when the pore throat ratio exceeds 8, the DTPG increases rapidly. Moreover, unlike static water-saturated reservoirs, the water saturation in water-injected coal seams does not exhibit a positive correlation with DTPG due to the coupled effects of coal structure and moisture content. These findings provide valuable guidance for understanding flow mechanisms and optimizing CO<sub>2</sub> injection strategies in the development of tight coalbed methane reservoirs.</p>

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Dynamic Threshold Pressure Gradient Model of CO2 Injection into Coal Seams Based on Fractal Theory

  • 期刊论文

  • 发表日期：

    2025年12月22日

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  • Jing Han,
  • Zhe Zhou,
  • Zhaolong Ge,
  • Jianming Shangguan,
  • Jinming Cui

摘要

The technology of CO2-enhanced coalbed methane (CO2-ECBM) recovery offers multiple advantages, including improved clean energy utilization and long-term CO2 sequestration. However, a threshold pressure gradient (TPG) often exists during CO2 injection into low-permeability, water-bearing coal seams, resulting in nonlinear flow behavior. This phenomenon increases the resistance to CO2 migration and limits the efficiency of the injection process. Accurately predicting the dynamic threshold pressure gradient (DTPG) in water-injected coal is therefore crucial for guiding the development of CO2 injection strategies. Despite this, the influence of pore morphology and structural complexity on the TPG has not been fully investigated. The CO2 injection threshold pressure gradient varies significantly with different pore structures and moisture conditions. In this study, a DTPG model is developed from a microscale perspective, incorporating water saturation, effective stress, and coal seam structural features, including seepage pores and adsorption pores. This model modifies the conventional two-phase gas–water flow model and enhances the accuracy of CO2 flow prediction in coal seams. It does not rely on empirical constants, and each parameter has a clear physical interpretation. Furthermore, we propose a nuclear magnetic resonance (NMR)-based experimental method to quantify each parameter of the model and evaluate its applicability. Theoretical and experimental analyses reveal that the fractal characteristics of seepage and adsorption pores significantly affect the DTPG. Among the influencing factors, the pore throat ratio shows the strongest correlation with DTPG (correlation coefficient: 0.843), making it the most critical factor in determining DTPG variations in coal. Notably, when the pore throat ratio exceeds 8, the DTPG increases rapidly. Moreover, unlike static water-saturated reservoirs, the water saturation in water-injected coal seams does not exhibit a positive correlation with DTPG due to the coupled effects of coal structure and moisture content. These findings provide valuable guidance for understanding flow mechanisms and optimizing CO2 injection strategies in the development of tight coalbed methane reservoirs.