Surfactant retention in limestones

Chemical enhanced oil recovery processes are used to reduce the IFT between oil and water and mobilize the residual oil. To utilize this method, a surfactant slug is injected followed by a polymer drive. For the process to be economic, the surfactant retention must be low so a small surfactant slug can be injected without compromising the oil recovery. Much progress has been made in reducing surfactant retention in sandstones, but typically the retention in carbonates is high. Carbonate oil reservoirs represent a huge target for chemical EOR. Therefore, improving the efficiency of chemical floods in carbonate oil reservoirs was the focus of this research. Understanding the geochemical reactions of the injected fluid, in-situ fluid, and carbonate rocks is important for the chemical flood design. The waterflood brine typically contains divalent cations such as calcium and magnesium that will consume alkali and delay the propagation of the high pH front. A preflood can be used to prevent or mitigate adverse reactions and scaling. Most reservoir carbonates contain gypsum; therefore, it is important to use an alkali that does not react with gypsum or to suppress gypsum dissolution as was done in this study. Systematic experiments were done to examine the effect of pH on surfactant retention and the geochemical reactions in limestone by adding ammonia and sodium hydroxide to the chemical solutions. The addition of alkali significantly reduced the surfactant retention from 0.30 mg/g-rock without using alkali to 0.20 mg/g-rock using 0.75 wt% ammonia to 0.14 mg/g-rock using 0.3 wt% sodium hydroxide. Using sodium hydroxide to increase the pH reduced the surfactant retention by more than half, which directly translates to decreasing the surfactant cost by more than half. Since adding a low concentration of alkali to the chemical solutions is a minor additional expense, the chemical cost to produce an incremental barrel of oil is greatly reduced