致密储集层原油充注物理模拟——以准噶尔盆地吉木萨尔凹陷二叠系芦草沟组为例

Physical modeling of oil charging in tight reservoirs:A case study of Permian Lucaogou Formation in Jimsar Sag,Junggar Basin,NW China

选取准噶尔盆地吉木萨尔凹陷二叠系芦草沟组致密储集层岩心样品,进行原油充注物理模拟实验,研究致密储集层中原油的运移渗流规律及影响因素,分析致密油运聚特征和成藏下限条件。致密储集层内原油充注具有启动压力梯度与临界压力梯度两个具有界限特征的压力梯度点,存在低速非线性渗流和拟线性渗流两种特征;芦草沟组致密储集层原油充注过程中含油饱和度的增长过程可以分为跳跃增长型、快速增长型、平稳增长型3种类型,快速增长型样品所达到的最大含油饱和度最高,其次为平稳增长型,跳跃增长型最低;含油饱和度的增长受到孔隙度、渗透率、原油黏度、驱替压力梯度耦合控制,各要素相互影响、相互补偿。绘制的致密储集层原油聚集成藏判定图版显示,只有压力梯度突破临界压力梯度进入拟线性渗流区后,才能达到致密储集层含油饱和度为30%的下限值。稳定的致密储集层很难实现油气的先致密后成藏,而常规储集层油气充注后地层压实胶结与矿物次生加大胶结可能是形成致密油,并具有较高含油饱和度的原因。

Modeling experiments of oil charging were conducted to find out patterns and affecting factors of oil migration and seepage in tight reservoirs, and analyze oil migration and accumulation and low limit conditions of tight oil accumulation using core samples from tight reservoir beds of the Permian Lucaogou Formation in the Jimsar Sag of the Junggar Basin. Crude oil charging in tight reservoir beds has two pressure gradient points （start-up pressure gradient and critical pressure gradient, and has two features： low velocity non-Darcy seepage, quasi-linear seepage）. During crude oil charging in tight reservoir beds in the Lucaogou Formation, the process of oil saturation increase can be divided into three types： saltation increase, quick increase and stable increase. Samples of quick increase type reached the highest oil saturation, the second place is the stable increase type, and saltation increase type is the last. Oil saturation increase is controlled by the combined effect of porosity, permeability, oil viscosity and displacement pressure gradient. These factors interact and complement one another. By establishing template for oil accumulation in tight reservoir beds, it can be seen that only when pressure gradient breaks through the critical pressure gradient and the oil flow is quasi-linear, can oil saturation reaches the lower limit value （30%） in tight reservoir beds. It is hard for stable tight reservoir beds to become tight firstly and be charged with oil and gas later; while for conventional reservoir beds, after oil and gas charging, the formation compaction, cementation, and secondary mineral outgrowth may be the reasons for the formation of tight oil accumulation with high oil saturation.