Abstract

Waterflood oil recovery in many carbonate oil reservoirs is low due to both high residual oil saturations and low sweep efficiency because of high heterogeneity. An example is the Sabriyah Mauddud reservoir in Kuwait. Alkaline-surfactant polymer flooding (ASP) has great potential for enhanced oil recovery both because ASP flooding reduces residual oil saturation and because of the polymer improves sweep efficiency. Unfortunately, the initial ASP coreflood experiments using conventional alkali showed unacceptably high surfactant retention in the reservoir cores. Several approaches to reducing surfactant retention were tested. Numerous strategies such as the use of chelating agents, sacrificial agents and chemical gradients were tested to reduce retention. The most effective strategy used a hybrid-alkali (NaOH + Na2CO3) in addition to a hydrophilic polymer drive containing a novel co-solvent. In this approach injection pH was increased to 12.5, compared to 10.5 using only Na2CO3. Such high pH is undesirable in sandstones because of reactions with silica minerals, but theexperimental results described here suggest the process is suitable for carbonate reservoirs. With this approach, both low surfactant retention and high oil recovery were achieved in very tight reservoir cores (8-35 mD). This novel approach was validated in a live oil coreflood using preserved cores to represent the reservoir material in the most rigorous way possible. This significant decrease in surfactant retention makes ASP flooding in the Sabriyah Mauddud reservoir viable.

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