致密砂岩储层毛管自吸微观分布特征

Microscopic distribution characteristics of spontaneous imbibition in tight sandstone reservoir

致密砂岩储层具有孔喉细小、强亲水、微裂缝发育等特征,在较大的毛细管力作用下,储层极易发生毛管自吸现象。为了揭示自吸水在孔隙网络中的微观分布,选取川西蓬莱镇组致密砂岩,开展模拟地层水条件下的垂直自吸实验,并运用核磁共振技术对致密砂岩自吸过程中的流体分布以及变化规律进行研究。结果表明:在自吸持续5 min时,自吸水主要集中在0~0.1μm的纳米级孔隙中,占比高达84%以上。随着自吸时间的延长,分布在0~0.1μm的纳米级孔隙中的自吸水占比逐渐下降,分布在0.1~1μm的亚微米级、1~10μm的微米级孔隙中的自吸水占比逐渐上升,其中0.1~1μm的亚微米级孔隙中自吸水的最大上升幅度从11%上升到25%;半径大于10μm的微米级孔隙数量较少,毛细管力作用极弱,导致该部分孔隙中的自吸水充满程度较低,最大占比仅为1.95%,自吸水占比没有明显的上升或下降趋势。在不考虑外部正压差作用时,毛管自吸现象会优先发生在纳米级孔隙中,孔隙半径、孔隙类型、不同孔隙半径占比、含水饱和度等是影响毛管自吸微观分布特征的主要因素。

The tight sandstone reservoirs are characterized by small pore throats,strong hydrophilicity,and well-developed micro-fractures,etc.The capillary imbibition phenomenon is prone to occur in the reservoir under the action of a large capillary force.In order to reveal the microscopic distribution of capillary imbibition of water in the pore network,the tight sandstones of Penglaizhen Formation in western Sichuan are selected to conduct the vertical spontaneous imbibition experiments under simulated formation water conditions,and the distribution and variation laws of the fluid during the imbibition process are studied by using the nuclear magnetic resonance(NMR)technology.The results show that when the spontaneous imbibition lasts for 5 minutes,the self-imbibition water is mainly concentrated in the nano-scale pores of 0-0.1μm,accounting for more than 84%.With the increase of spontaneous imbibition time,the proportion of self-imbibition water distributed in the nano-scale pores of 0-0.1μm gradually decreases,and the proportion of self-imbibition water distributed in the pores of submicron level of 0.1-1μm and micron level of 1-10μm gradually increases,in which the maximum increase of self-imbibition water in submicron pores of 0.1-1μm increases from 11%to 25%.The number of micron pores with a radius greater than 10μm is small,and the capillary force is extremely weak,resulting in a low degree of self-imbibition water.The largest proportion is only 1.95%.There is no obvious upward or downward trend in the proportion of self-imbibition water.When the external positive pressure difference is not considered,the capillary imbibition phenomenon will preferentially occur in nano-scale pores.Pore radius size,pore type,proportion of different pore radius,water saturation,etc.are the main factors that affect the microscopic distribution characteristics of capillary imbibition.