东海西湖凹陷致密砂岩气藏高含水特征及产水机理探讨

High watercut characteristics and water-producing mechanisms in tight sandstone gas reservoirs, Xihu sag, East China Sea

探井测试及开发井生产阶段大量产水严重制约西湖凹陷致密砂岩气藏的经济有效开发。利用岩芯核磁共振实验及"双饱和度"测井评价方法,分析气藏高含水特征及高含水地质成因,并探讨气藏产水机理,为气藏开发阶段有效防水治水提供依据。结果表明:西湖凹陷致密砂岩气藏高含水特征主要表现为"双高"特征,即高束缚水饱和度和高可动水饱和度;储层微细孔喉发育造成气藏束缚水饱和度高,而成藏时天然气充注程度较低则造成气藏可动水饱和度高;气藏含水饱和度高于临界水饱和度,在压差的驱动下可动水随天然气一起流动是气藏产水主要原因,同时如果生产压差过大,会导致部分束缚水转化为可动水,加剧产水程度。控制合理的生产压差是降低西湖凹陷高含水致密砂岩气藏产水风险和延长气井稳产期的重要手段。

Developing economically and effectively tight sandstone gas reservoirs in Xihu sag has been severely restricted by considerable water production in one stage of exploration-well testing and another stage of development-well production.So,both characteristics and geological origin of high watercut were analyzed by means of core NMR experiments and"dual saturation"logging evaluation.Furthermore,some water-producing mechanisms were summarized to provide the basis for effective water prevention and control.Results show that,in these gas reservoirs,(1)high watercut mainly appears the characteristics of"two highs",i.e.,high irreducible-water saturation and high movable-water saturation;(2)high irreducible-water saturation is caused by the development of micro pore throat whereas high movable-water saturation caused by low gas charging degree during gas accumulation;and(3)the main reason to produce water is that movable water flows together with gas under the driving of differential pressure when water saturation is higher than critical value.Meanwhile,if the difference of production pressure is too large,irreducible water will transfer into movable water,and consequently water production may increase.Therefore,for these tight sandstone gas reservoirs with high watercut,Xihu sag,to maintain a reasonable difference of production pressure is an important measure against water-producing risk to prolong a stable production period.