Pore-Scale Study on the Flow Dynamics of Coupled Low Salinity and Nanofluid Flooding in Carbonate Formations

Abstract

Abstract A significant volume of annual world oil production comes from carbonate reservoirs like the giant Middle East and Caspian Sea reservoirs. However, the production enhancement is complicated by geological heterogeneities of carbonate formations, such as a complicated network of natural fractures leading to highly permeable paths or shale streaks leading to discontinuous flow barriers. The primary objective of this paper is a feasibility study of coupled low salinity and nanofluid flooding for oil recovery enhancement from carbonate reservoirs. Accordingly, diluted seawater and two different types of nanoparticles (NPs) were exploited to prepare low-salinity nanosuspensions to understand the synergistic effects of low-salinity nanofluid (LSN) injection on oil droplet remobilization. As the multiphase flow experiments were performed using glass micromodels, surface wettability analysis was also conducted on flat glass plates to clarify the role of NPs at the interfaces. The fluid flow around shale barriers and fracture/matrix interactions were qualitatively scrutinized at the pore scale using multiphase flow tests on the oil-wet microfluidic chips inspired by the pore structures of rock samples of carbonate reservoirs. The results of contact angle experiments showed that the inclusion of NPs into low-salinity water can ameliorate the ability of the aqueous solution to reverse the surface wettability of the oil-wet samples to a more water-wet state due to the improved adsorption isotherm of NPs into the glass surface. Microscopic and macroscopic observations of the porous media flow tests also disclosed that the LSN injection could significantly improve breakthrough time as well as microscopic and macroscopic sweep efficiencies. In other words, a slight viscosity improvement of injected water due to the presence of NPs could relatively diminish the extension of fingering patterns in porous media and create a better displacement front, resulting in a higher breakthrough time of displacing fluid. Furthermore, due to surface wettability reversal, LSN injection reduced the amount of untouched oil behind the shale streaks and showed better intrusion into the matrix and a higher fluid exchange rate between the matrix and fractures. This study proves the effectiveness of LSN injection in improving the efficiency of enhanced oil recovery from carbonate formations. Besides, we highlighted the flow characteristics of LSN around the shale streaks and high permeable fractures.