Analysis of damage characteristics of coal body with supercriticalcarbon dioxide jet impact short time soaking
LIU Yong，ZHANG Dongxin，ZHANG Hongtu，WEI Jianping，SI Leilei
煤体力学和物理性质在超临界二氧化碳氛围下容易劣化，尤其在长时作用下，超临界二氧 化碳能够充分发挥其溶解和萃取能力，改变煤体力学强度和孔隙结构。 超临界二氧化碳作为极具 应用前景的钻井液，在钻井过程中与煤体接触时间极短。 在较短接触时间内，超临界二氧化碳是否 能够改变煤体力学强度和孔隙结构对超临界二氧化碳射流冲击破煤效率和钻井效率至关重要。 为 此，为进一步研究超临界二氧化碳短时浸泡对破煤效率的影响，开展了超临界二氧化碳冲击破碎短 时浸泡煤体研究，分析了不同浸泡条件下煤体冲击破碎特征，通过对比分析浸泡前后孔隙结构和力 学性质的变化，明确短时浸泡对煤体冲击破碎特征的影响。 结果表明:在短时浸泡条件下，超临界 二氧化碳射流对浸泡煤体的冲击破坏特征具有重要影响。 仅浸泡 3 min，超临界二氧化碳便可使煤 体发生吸附膨胀，影响煤体的孔隙结构和力学强度，使煤体累计孔体积降低了 5.54%，力学强度降 低 13.10%，使煤体冲击破碎粒径小于 1 mm 煤屑增加了 5.76%。 延长浸泡时间，煤体冲击破碎程度 持续提高。 浸泡 60 min 后，超临界二氧化碳对煤体虽仍以吸附膨胀为主，但煤体出现矿物溶蚀现 象，致使煤体累计孔体积降低了 30.19%，单轴抗压强度降低 33.41%，导致煤体的破碎程度发生了 大幅提高。 相同浸泡时间条件下，提高浸泡温度和压力，煤体吸附膨胀作用增强，孔体积增大，抗压 强度降低，使破碎程度规律性提高。 但当浸泡压力超过 16 MPa 后，抗压强度降低了 38.43%，冲击 破坏程度显著提升。 综上所述，超临界二氧化碳射流辅助钻井过程中，其吸附膨胀作用是影响煤体 物理和力学性质的主要形式，有效降低了煤体孔体积和抗压强度，提高了破碎程度，有利于钻井效 率的提高。
The mechanical and physical properties of coal are easily deteriorated under the supercritical CO2 atmosphere. Especially under a longtime action,the supercritical CO2 can fully utilize its dissolution and extraction ability to change the physical strength and pore structure of coal. As a highly promising drilling fluid,the supercritical carbon dioxide has a very short contact time with the coal body during the drilling process. Whether the supercritical carbon dioxide can change the physical strength and pore structure of coal within the short contact time is crucial to the efficiency of supercritical carbon dioxide jet impact on coal breaking and drilling efficiency. In order to further study the effect of shorttime immersion of coal body by supercritical CO2 on coal breaking efficiency,this paper carried out a study on the shorttime immersion of coal body by supercritical CO2 on the impact crushing of coal body. It analyzed the impact crushing characteristics of coal body under different immersion conditions and clarified the effect of shorttime immersion on the impact crushing characteristics of coal body by comparing the changes of pore structure and mechanical properties before and after immersion. The results show that the supercritical carbon dioxide jet has an important influence on the impact damage characteristics of the soaked coal body under shorttime soaking conditions. After soaking for only 3 minutes,the supercritical carbon dioxide caused an adsorption and expansion of the coal body,which affected the pore structure and mechanical strength of the coal body. Thus,it reduced the cumulative pore volume of the coal body by 5.54% and the mechanical strength by 13.10%,and increased the impact crushing particle size less than 1 mm coal chips by 5.76%. The degree of coal body impact crushing continued to increase by extending the soaking time. After soaking for 60 minutes,although the supercritical carbon dioxide still mainly adsorbed and swelled the coal body,the coal body showed a mineral dissolution phenomenon. It resulted in the cumulative pore volume of the coal body decreasing by 30.19% and the uniaxial compressive strength decreasing by 33.41%,which led to a significant increase in the degree of coal body fragmentation. Under the same soaking time condition,when the soaking temperature and pressure increased,the adsorption and swelling effect of the coal body was enhanced,the pore volume increased and the compressive strength decreased,which made the degree of crushing increase regularly. However,when the soaking pressure exceeded 16 MPa,the [JP2]compressive strength decreased by 38.43%,and the impact damage degree was significantly increased. In conclusion,the adsorption and swelling effect of the supercritical carbon dioxide jetassisted drilling process is the main form of influence on the physical and mechanical properties of coal body,which effectively reduces the pore volume and compressive strength of coal body and increases the degree of fragmentation. It is beneficial to the improvement of drilling efficiency.
supercritical carbon dioxide jet;shorttime immersion;damage characteristics;pore structure;mechanical strength