Effect of pore structure and injection pressure on waterflooding in tight oil sandstone cores using NMR technique and pore network simulation

Abstract

As an economical and reusable method of enhancing oil recovery, waterflooding is widely used in the development of tight oil reservoirs, although it has often been found unsuccessful owing to the adverse properties of tight rocks. Accurate knowledge of the pore-scale distribution of oil in tight rocks during waterflooding is needed for devising strategies to maximize oil recovery. Here, we used a two-fold approach to identify the amount and spatial distribution of the residual oil as a function of injection pressure. We i) characterized the pore structure of two cores from a tight sandstone reservoir in the Ordos basin, China, and ii) conducted waterflooding experiments with different injection pressures using nuclear magnetic resonance measurements. We found that even small differences in the tight sandstones pore structures caused measurable variations of the amounts oil produced from different classes of pores. For example, in experiments using identical water injection pressures but samples with different permeabilities, a permeability increase by a factor of 3 corresponded to a 10% reduction in oil recovery. We also found that the increase in residual oil saturation caused by increasing the injection pressure was unfavorably dependent on the pore structure heterogeneity. This observation was supported by pore network simulations, which were additionally carried out. We therefore conclude that it is necessary to evaluate the pore structure heterogeneity of the reservoir rocks when attempting to enhance the oil recovery factor of a tight reservoir by means of the common strategy of raising the waterflood injection pressure.