SCCO_(2)作用下不同含水性煤孔裂隙结构变化机制

Effect of Supercritical Carbon Dioxide(SCCO_(2))on the mechanism of change in pore and fracture structure of coal with different moisture content

深部煤层封存CO_(2)增产CH_(4)产出过程中,处于超临界状态的CO_(2)(SCCO_(2))与煤中矿物质发生反应,改变煤的孔隙性,进而影响煤层封存CO_(2)的效果和甲烷增产效果。为发现SCCO_(2)–H_(2)O–煤岩作用对煤中孔隙的影响特征,以焦煤为研究对象,开展了不同含水条件下的超临界CO_(2)改造煤实验,基于矿物组成和孔隙性测定结果,对比煤中主要矿物质和不同尺度的孔裂隙变化的差异,探讨了不同含水状态下SCCO_(2)流体对孔裂隙性的作用机制。研究表明:①SCCO_(2)作用后,煤体表面粗糙、疏松,且由于矿物溶蚀使得一些裂隙得到贯通,微裂隙连通性增强。②SCCO_(2)流体对煤具有“扩孔”作用,表现为微、小孔含量下降,中、大孔占比升高,也即微、小孔隙向大孔隙的转化,且孔隙连通性改善;进一步发现,萃余煤吸附孔的分形维数稍微增加,粗糙度增大,而渗流孔的分形维数显著降低,复杂性和非均质性降低。③SCCO_(2)对煤中碳酸盐类矿物的溶解性最好,其次是黏土矿物,且随着含水率增加,萃余煤中的碳酸盐矿物占比先增加后减小。SCCO_(2)使干燥基态、饱和水态煤样中碳酸盐矿物显著溶解,有效改善了孔隙结构,且对饱和水态煤样作用效果更好。空气干燥基态煤样经SCCO_(2)作用后,新生成的白云石矿物聚集在孔喉中造成堵孔效应,缩小原有大孔隙尺寸,是引起不同含水性煤孔隙差异性变化的主要原因。

When the CO_(2) in a supercritical state(SCCO_(2))is released from deep coal seams,it reacts with minerals in coal,changing the porosity of coal,which in turn affects the effect of CO_(2) sequestration in coal seams and the effect of increas-ing methane production.In order to discover the influence characteristics of SCCO_(2)-H_(2)O-coal rock interaction on the porosity in coal,the experiments on the supercritical CO_(2) reforming coal under different water content conditions were carried out with coking coal as the research object.Based on the results of mineral composition and porosity measure-ments,the differences between the main minerals and the changes in pore cleavage at various scales in coal were com-pared,and the action mechanism of the SCCO_(2) fluid on the pore cleavage properties under various water content states was explored.The results show that:①After the SCCO_(2) treatment,the surface of the coal body is rough and loose,some fissures are penetrated as a result of mineral dissolution,and the connectivity of micro fissure is improved.②The SCCO_(2) fluid has a“pore-expanding”effect on coal,which is manifested by a decrease of the content of micro-and small pores,and an increase of the proportion of medium-and large pores,i.e.,the conversion of micro-and small-sized pores to large pores,as well as the improvement of pore connectivity.Furthermore,it is discovered that the fractal dimension of the ad-sorption pores of the coal samples increases slightly in size and roughness,while the fractal dimension of the seepage pores decreases significantly,and the complexity and non-homogeneity are reduced.③The SCCO_(2) fluid has the best sol-ubility on the carbonate minerals in coal,followed by clay minerals,and with the increase of water content,the proportion of carbonate minerals in the extracted coal first increases and then decreases.The SCCO_(2) fluid makes the carbonate miner-als in the coal samples of the dry basal state and saturated water state dissolve significantly,which effectively improves the pore structure,and the effect on the coal samples of the saturated water state is better.After the air-dry basal coal samples were subjected to the SCCO_(2),the newly formed dolomite minerals gathered in the pore throats to cause the plugging ef-fect,reducing the original large pore size,which is the main cause of the differential changes in the pore space of coals with different water content states.