裂缝发育的特低渗透砂岩储层特征

Characterization of fractured ultra-low permeability sandstone reservoirs

裂缝的发育对特低渗透砂岩储层有重要影响。基于室内实验数据,从孔渗关系、微观孔隙结构、应力敏感性及油水两相渗流等角度对裂缝发育储层的特征进行了分析,结果表明：与裂缝不发育的储层相比,相同孔隙度下裂缝发育储层的渗透率更高,孔隙度10%~20%时,裂缝发育储层渗透率约为裂缝不发育储层的3~5倍;渗透率相同时,裂缝发育储层喉道半径分布范围更宽,大尺寸的喉道比例更高,主流喉道半径更大,渗透率主要由大喉道贡献;裂缝发育储层应力敏感性更强,有效应力增大引起的渗透率损失约为裂缝不发育储层的2~3倍;裂缝发育储层束缚水饱和度和残余油饱和度均较高,两相共渗区范围窄,随着含水饱和度的增大,油相曲线急剧下降,水相曲线上凸型快速抬升,且幅度很大,无水采油期很短且期内采出程度低,见水后含水率急剧上升,最终采收率很低。

Ultra-low permeability sandstone reservoirs are significantly influenced by development of natural fractures. Based on laboratory experiments, fractured uhra-low permeability sandstone reservoirs were detaiiedly characterized in terms of porosity-permeability relationship, micro-pore structure, stress sensitivity, and oil-water two-phase flow. The results show that permeability of fractured reservoir was higher than that of non-fractured one under equal reservoir porosity. When the reservoir porosity ranged from 10% to 20%, the permeability of fractured reservoir was approximately 2-4 times higher than that of non-fractured one. Compared with experiments on non-fractured reservoir, fractured reservoir is featured by larger throat radius span, higher proportion of largeradius throats, larger radius of dominant throats, and major contribution of large-radius throats to permeability under equal permeability. Fractured reservoir is more stress-sensitive, and the corresponding permeability loss of effective stress increase was 1-2 times higher than that of non-fractured one. Oil-water two-phase flow in fractured reservoir is featured by higher bound water and residual oil saturations, shorter common-flow interval, rapid oil relative permeability decline and water relative permeability increase with the increase of water saturation, short water-free production period, rapid water-cut increase after water break-through and low ultimate recovery factor.