Numerical Investigation on Low Salinity Augmented Microbial Flooding LSAMF within a Sandstone Core for Enhanced Oil Recovery Under Nonisothermal and Fluctuating pH Conditions

Abstract

In an era of increasing energy demand, declining oil fields and fluctuating crude oil prices globally, most oil companies are looking forward to implementing cost effective and environmentally sustainable enhanced oil recovery (EOR) techniques such as low salinity waterflooding (LSWF) and microbial EOR (MEOR). The present study numerically investigates the combined influence of simultaneous LSWF and microbial flooding for in-Situ MEOR in tertiary mode within a sandstone core under spatiotemporally fluctuating pH and temperature conditions. The developed black oil model consists of five major coupled submodels: nonlinear heat transport model; ion transport coupled with multiple ion exchange (MIE) involving uncomplexed cations and anions; pH variation with salinity and temperature; coupled reactive transport of injected substrates, Pseudomonas putida and produced biosurfactants with microbial maximum specific growth rate varying with temperature, salinity and pH; relative permeability and fractional flow curve variations due to interfacial tension reduction and wettability alteration (WA) by LSWF and biofilm deposition. The governing equations are solved using finite difference technique. Operator splitting and bisection methods are adopted to solve the MIE-transport model. The present model is found to be numerically stable and agree well with previously published experimental and analytical results. In the proposed MIE-transport mechanism, decreasing injection water salinity (IWS) from 2.52 to 0.32 M causes enhanced Ca2+ desorption rendering rock surface towards more water wet. Consequently, oil relative permeability (kro) increases with >55% reduction in water fractional flow (fw) at water saturation of 0.5 from the initial oil-wet condition. Further reducing IWS to 0.03 M causes Ca2+ adsorption shifting the surface wettability towards more oil-wet thus increasing fw by 52%. Formation water salinity (FWS) showed minor impact on WA with <5% decrease in fw when FWS is reduced from 3.15 to 1.05 M. During LSAMF, biosurfactant production is enhanced by >63% on reducing IWS from 2.52 to 0.32 M with negligible increase on further reducing IWS and FWS. This might be due to limiting nonisothermal (40 to 55 °C) and nutrient availability conditions. LSAMF caused significant WA, increase in kro with fw reduction by >84%. Though pH increased from 8.0 to 8.9, it showed minor impact on microbial metabolism. Formation damage due to bioplugging observed near injection point is compensated by effective migration of biosurfactants deep within sandstone core. The present study is a novel attempt to show synergistic effect of LSAMF over LSWF in enhancing oil mobility and recovery at core scale by simultaneously addressing complex crude oil-rock-brine chemistry and critical thermodynamic parameters that govern MEOR efficiency within a typical sandstone formation. The present model with relatively lower computational cost and running time improves the predictive capability to pre-select potential field candidates for successful LSAMF implementation.