Pore-scale analysis of residual oil occurrence from water flooding in sandstone reservoirs

Abstract

In response to the problem of highly dispersed remaining oil and the difficulty in accurately assessing the development potential during the high water cut period of Daqing Oilfield, this paper takes a typical sandstone reservoir as the research object. Using micro-micron CT imaging technology, a digital core model with real pore scale was constructed, and combined with the computational fluid dynamics method, dynamic water displacement simulations under different pressure gradients were carried out. The research results show that at low displacement pressure, due to the “capillary barrier” effect, the injected water is unable to overcome the throat breakthrough pressure, resulting in a non-flowing zone. As the pressure increases to 1600 Pa, the system establishes stable flow, but due to the heterogeneity of the pore throats, there are obvious “priority channels” selectivity and bypass flow phenomena. Through quantitative characterization of the topological structure of the remaining oil space, the residual oil after water flooding is classified into five types: agglomerated, porosity-like, membranous, columnar and droplet-like, and the formation mechanisms of each type of remaining oil are revealed. Combined with the evolution characteristics of reservoir “large pore connectivity” caused by more than 60 years of water injection development in Daqing Oilfield, this paper proposes that for agglomerated remaining oil, deep well profile adjustment and polymer flooding should be adopted to improve the macroscopic sweep efficiency, while for membranous and columnar residual oil, the super-low interfacial tension characteristic of ternary composite flooding (ASP) should be utilized. The research results provide microscopic theoretical support for formulating differentiated enhanced recovery measures for high water cut oilfields.