不同含水条件下黏土孔隙分布特征及甲烷吸附能力

选用常见的页岩黏土矿物伊利石和高岭石,开展了不同湿度平衡样品的N2吸附/脱附实验和高压CH4吸附实验,研究黏土矿物孔隙分布特征的变化,并从微观上量化评价含水饱和度,分析其对甲烷吸附的影响。研究结果表明:水分的存在主要影响黏土矿物微小孔隙的分布,高湿度条件下(RH=98%)的毛细凝聚作用导致微小孔隙(5.15 nm)在孔径分布曲线上消失及比表面积的大幅下降。同时研究表明:黏土吸水能力与微小孔隙发育程度密切相关,当RH=98%时,微孔更为发育的高岭石含水饱和度(S_w=71.43%)高于伊利石(S_w=46.15%),且在此条件下,由于小孔凝聚以及吸附特征的改变(气-固吸附转变为气-液界面吸附),样品甲烷吸附能力下降近85%。因此,干燥条件下的实验结果不能代表实际页岩储集特征。

Pore size distribution characteristic and methane sorption capacity of clay minerals under different water saturation

The common shale clay (illite and kaolinite) equilibrated in different relative (RH) humidity environment in advance are collected to conduct the N2 adsorption/desorption and high pressure methane sorption isotherms test.Furthermore,the pore size distribution with different water contents is analyzed,and the water saturation with various RH is quantitated with the view of pore volume and its influence on clay adsorption capacity is also studied.The results indicate that the pore characteristics considering the water content vary significantly comparing with the dry condition.The fine pores less than 5.15 nm will disappear on the pore size distribution curves because of the capillary condensation effect at the RH of 98%,and the significant decrease of specific surface area is also accompanied.It can be also concluded that the water adsorption for clay mineral depends on the fine pores,the Sw for kaolinite is 71.43% and higher than the value of 46.15% for illite while the RH is 98% due to the richer micropores of the former.In this condition,the adsorption capacity decreases by 85% which is caused by the blocked effect of condensed water and the change of adsorption mechanism (methane adsorbed on the water film rather than the pore wall).However,the experiments in dry condition mainly overstated the methane adsorption contribution of fine pores and cannot represent the actual shale reservoir characteristics.