Oil Recovery From Fractured Carbonates by Surfactant-Aided Gravity Drainage: Laboratory Experiments and Mechanistic Simulations

Abstract

Abstract Waterflooding recovers little oil from naturally fractured carbonate reservoirs if the matrix is oil-wet and fracture intensity is high. Laboratory experiments and mechanistic simulations have been conducted to understand the injection of dilute anionic surfactant solutions into oil-wet, fractured reservoirs. In this process, surfactant diffuses into the matrix, lowers IFT and contact angle, which decrease capillary pressure and increase oil relative permeability enabling gravity to drain the oil up. The rate of oil recovery increases with an increase in matrix permeability, a decrease in initial gas saturation, a decrease of fracture height or spacing, and an increase in wettability altering capabilities of the surfactant. Increasing the surfactant concentration does not necessarily enhance the oil recovery rate, because IFT and wettability alterations are not related to surfactant concentration linearly. Adsorption of anionic surfactants on calcite can be suppressed with an increase in pH and a decrease in salinity. Introduction About 60% of the world's oil is found in carbonate reservoirs.[1] Recovery from reservoirs depends on reservoir heterogeneity, oil quality, drive mechanisms, and reservoir management. Many carbonate reservoirs are naturally fractured and oil-wet / mixed-wet.[2,3] Such reservoirs are difficult to produce after the primary production if the fractures form a connected network.[4] Waterflooding is effective only if the formation is water-wet. Flooding processes do not work in general, because large viscous gradients cannot be imposed. Gravity drainage (surfactant, gas, and thermal) techniques can be applied, but the recovery is slow. Surfactant-enhanced gravity drainage process is being developed[5–15] to improve oil recovery from oil-wet/mixed-wet, fractured carbonate formations and is the subject of this study. Cationic surfactants of the type alkyl trimethyl ammonium bromide, CnTAB, are effective (recovery ∼70% original oil in place, OOIP) in imbibing water into originally oil-wet chalks at concentrations greater than their CMC (∼1 wt%).{6–9} Cationic surfactants form ion-pairs with adsorbed organic carboxylates of the crude oil, and solubilize them into the oil, thereby changing the rock surface to be water-wet. This wettability alteration can lead to countercurrent imbibition of brine and thus to oil recovery. The interfacial tension between the surfactant solution and oil are not low (> 0.1 mN/m). Several cheaper cationic surfactants of the form C10NH2 and bioderivatives from the coconut palm, termed Arquad and Dodigen (priced at 3 US$ per kg), have been identified.[10-12] The two key problems with this method are still the high concentration and the high surfactant cost. Nonionic and anionic surfactants have been considered for fractured reservoirs.[13–15] Dilute anionic surfactants have been considered in the presence of a low concentration potential determining ion (∼0.3 M Na2CO3). Interfacial tension (IFT) can be lowered to low levels (∼10–3 mN/m), wettability can be changed to intermediate wettability, and imbibition can be improved (>50% OOIP).[14–15]