Naturally Fractured Reservoirs are important contributors to the world's oil and gas reserves representing around one-fifth of the world's reservoirs. They usually are of lower recovery and higher residual hydrocarbon saturation than the rest of reservoirs due to its mixed wettability, low matrix permeability, low fracture porosity, and the complicated fracture network. When alkaline-surfactant-polymer flooding is conducted, it enhances the recovery by altering the wettability, lowering the interfacial tension of the rock, reducing the mobility ratio, and forming a secondary in-situ surfactant, consequently reducing the residual hydrocarbon saturation. However, majority of ASP operations usually underestimate the complicated structure and description of reservoirs. As a result, this lacking causes a serious flood modeling issue on NFRs since they rely on the matrix system mainly being the storage of hydrocarbon, and the fracture system carrying out the production.

This paper presents a comprehensive study on modeling ASP flooding on NFRs. It mainly addresses the effects of wettability, IFT, capillary forces, fracture parameters along with fracture and matrix permeabilities on ASP using a dual porosity and dual permeability model. It also addresses the performance, recovery factor and the optimum slug size for an optimum EOR operation, as well as to give guidelines and theoretical justification for better flooding practices based on a modeling approach only.

Sensitivity analyses on the constructed model of slug size, initiation time, sequence of injection, and concentration are done. Results show the optimized slug sizes of the injectants (i.e. 200 stb/day: 1 year of pre-injected water, 2 years of AS slug and 2 years of ASP slug), optimum initiation time and sequence of injection (1 year of water pre-injection followed by the ASP slug then followed by water flooding), in addition to the optimum concentrations of chemical injectants (alkaline and polymer concentrations of 50 lb/stb each, and surfactant concentration of 30 lb/stb). Also sensitivity analyses on the mentioned parameters are conducted and the results show how they affect NFRs. The study introduces a non-conventional injection method named Cyclic Alkaline-Surfactant-Polymer flooding. The performance and effectiveness of C-ASP are presented in addition to a comparison between C-ASP and conventionally used methods.

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