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

Abstract

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. Key words: tight reservoir, oil charging physical modeling, non-linear seepage, hydrocarbon accumulation threshold, Jimsar Sag, Permian Lucaogou Formation