超临界二氧化碳短时作用煤体裂隙演化CT/XRD实验研究

CT experimental study on the fracture evolution of coal body under supercritical CO_(2)short duration action

ECBM可以在提高煤层气开采效率的同时实现CO_(2)封存,推动“双碳”目标实现,在深部煤层气的开发中具有广泛的应用前景。煤层裂隙结构和分布规律决定了超临界二氧化碳(SC-CO_(2))的压裂效果,是提高煤层气开采效率的关键。长时作用下SC-CO_(2)与煤体接触产生的吸附膨胀和溶解萃取作用能够改变煤体裂隙形态,但压裂过程中SC-CO_(2)与煤体接触时间较短,短时作用下SC-CO_(2)的吸附膨胀及溶解萃取作用对煤体裂隙的影响尚未明确。因此开展了SC-CO_(2)短时作用下煤体的裂隙演化研究,通过CT扫描研究不同变质煤体裂隙随累计浸泡时间的变化规律;建立CT二维扫描图像的灰度分布函数并构建浸泡时间与裂隙演化规律的关系,定量表征煤体裂隙的变化。结合XRD实验研究浸泡时间对煤体物质成分变化的影响,明确SC-CO_(2)短时作用煤体裂隙演化规律的主要原因。结果表明:SC-CO_(2)短时作用下吸附膨胀作用会导致煤体裂隙收缩,溶解萃取作用使裂隙发生扩展;浸泡煤体过程中吸附膨胀和溶解萃取作用同时发生且强度随时间发生变化,在不同时间段内分别交替占据对煤体裂隙的主导作用;不同变质煤会影响吸附膨胀和溶解萃取作用的强度和主导时间。褐煤浸泡30 min时吸附膨胀占据主导作用,浸泡90~240 min溶解萃取作用增强并占据主导。烟煤浸泡30~90 min时吸附膨胀作用较强,浸泡90~240 min时溶解萃取占据主导。无烟煤浸泡30~150 min时吸附膨胀作用占据主导,浸泡150~240 min时溶解萃取占据主导。

The enhanced coalbed methane(ECBM)technology has the potential to enhance coal seam gas extraction efficiency,while also achieving CO_(2)storage,thereby promoting the dual carbon reduction goals.It holds broad prospects for its application in unconventional natural gas development.The fracture structure and distribution within coal seams determine the effectiveness of supercritical carbon dioxide(SC-CO_(2))fracturing,which is pivotal in enhancing coal seam gas extraction efficiency.Under prolonged exposure,the adsorption swelling and dissolution-extraction effects generated by SC-CO_(2)interacting with coal can alter the morphology of coal fractures.However,during the fracturing process,the contact time between SC-CO_(2)and coal is relatively short,thus the impact of the adsorption swelling and dissolution-extraction effects of supercritical carbon dioxide on coal fractures under short-term conditions remains unclear.Therefore,this study investigated the evolution of coal fractures under a short-term SC-CO_(2)exposure.Different metamorphic coal samples were studied through CT scans to observe the variations in fractures with accumulated immersion time.A grayscale distribution function of CT two-dimensional scan images was established,and the relationship between immersion time and fracture evolution patterns was constructed.This was combined with the XRD experiments to examine the impact of immersion time on the changes in coal composition.The primary factors driving the evolution of coal fractures under a short-term SC-CO_(2)exposure were thus clarified.The results indicate that under a short-term SC-CO_(2)exposure,the adsorption-induced swelling leads to the contraction of coal fractures,while the dissolution-extraction effect causes the fractures to expand.During the coal immersion process,both the adsorption-induced swelling and the dissolution-extraction effect occur simultaneously,with their intensities changing over time.Within different time intervals,they alternately dominate the evolution of coal fractures.Different metamorphic coals affect the intensity and dominant time of adsorption expansion and dissolution extraction.The adsorption and expansion dominated when lignite was immersed for 30 min,and the dissolution and extraction increased and dominated when it was immersed for 90−240 min.The adsorption and expansion were stronger when bituminous coal was immersed for 30−90 min,and the dissolution and extraction were dominant when it was immersed for 90−240 min.Adsorption and swelling dominated when anthracite was immersed for 30−150 min,and dissolution and extraction dominated when it was immersed for 150−240 min.