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Role of Alkali Type in Chemical Loss and ASP-Flooding Enhanced Oil Recovery in Sandstone Formations


 Chemical combination flooding techniques, particularly alkali/surfactant/polymer (ASP) flooding, have proved to be effective in enhanced oil recovery (EOR). The development of this flooding technique in the Daqing Oil Field (China) shows that it can prevent production declines and help oil companies increase profits. However, ASP chemical loss and the resulting chromatographic separation in sandstone formations remain as limitations in the practice of ASP flooding.
 Laboratory investigations have analyzed the behavior and characteristics of chemical loss in sandstone reservoirs recently subjected to strong-base [sodium hydroxide (NaOH)] and weak-base [sodium carbonate (Na2CO3)] ASP flooding. A set of experiments were reasonably designed to study how formulation compositions, slug combination patterns, and heterogeneity affected the chromatographic separation and consumption loss characteristics of chemicals in sandstone reservoirs subjected to ASP flooding. Our investigations determined chemical-loss ratios through various experiments, described the underlying mechanism behind the discovered consumption loss characteristics, and discussed the effects of comparative ASP flooding processes. Furthermore, the incremental oil recovery factor and degree of permeability damage in heterogeneous sandstone reservoirs subjected to strong-base and weak-base ASP flooding processes were assessed and compared. Then, the role of alkali type in chemical loss, EOR efficiency, and reservoir flow assurance in sandstone formations were ascertained for the first time.
 The results indicated that chemical-loss behaviors and chemical chromatographic separation could be alleviated in a weak-base ASP flooding. In particular, in heterogeneous sandstone reservoirs, the average loss ratios of alkalis and surfactants could be reduced by 9.61% and 15.67%, respectively, compared to the strong-base ASP flooding. A profitable EOR of 20% or more could also be obtained with a weak-base ASP flooding. Moreover, a reduction of approximately 15% in the permeability-damage ratio could be realized in weak-base ASP flooding compared with the strong-base system, and the reservoir flow-assurance issues related to chemical loss could be addressed. The optimal designs for ASP formulations and slug combination patterns could technically and sustainably achieve high oil recovery in sandstone reservoirs with a weak-base ASP flooding.
 The results help illustrate the chemical combination flooding mechanism and can contribute to the existing knowledge regarding the additive effects of chemicals during the EOR process. Moreover, they are significant for further improving oil displacement efficiency and reducing the injection cost in heterogeneous sandstone reservoirs subjected to the weak-base ASP-flooding process.