Mathematical Modeling of High-pH Chemical Flooding

Abstract

Summary This paper describes a generalized compositional reservoir simulator for high-pH chemical flooding processes. This simulator combines the reaction chemistry associated with these processes with the extensive physical and flow-property modeling schemes of an existing micellar/polymer flood simulator, UTCHEM. Application of the model is illustrated for cases from a simple alkaline preflush to surfactant-enhanced alkaline-polymer flooding. Introduction Use of alkaline chemicals to enhance oil recovery has been documented for several decades. Most of the work specifically to quantify these processes, however. has been performed during the past decade. The key features that control the alkaline processes and, hence, form the basis for quantification are (1) in-situ generation of surface-active agents by the reaction between alkali and acidic components of crude oil, (2) reactions with reservoir brine and minerals and consequent consumption of alkali and changes in ionic environments, and (3) effects of these reaction products and high pH on phase behavior, interfacial tension (IFT), surfactant adsorption, and transport properties. Many of these topics have been addressed in some detail as part of either alkaline flooding research or research in micellar/polymer flooding and reactive flow through permeable media. Use of alkaline chemicals along with surfactant and/or polymer has been suggested to improve the oil recovery performance of either process. In fact, most oil recovery mechanisms proposed for alkaline flooding are also present in micellar/polymer flooding. Alkaline flooding, therefore, may be considered a special case of surfactant flooding where the surfactant is generated in situ. However, no numerical reservoir simulator combines the detailed reaction chemistry and flow properties of the various combinations of high-pH chemical flooding processes. This paper presents a generalized model based on the local equilibrium assumption that takes into account the reaction chemistry associated with high-pH chemical floods. The model is coupled with an existing 3D compositional chemical flood simulator, UTCHEM, so that most of the physical processes and oil recovery mechanisms common to alkaline and micellar/polymer processes are retained. Assumptions We make the following assumptions about the reactive flow process. 1. Thermodynamic equilibrium is attained locally at every point in the medium. 2. Activity coefficients of all reactive species are unity. 3. The acidic components of crude oil can be represented collectively by a single pseudoacid component, HA. HA is highly soluble in oil and partitions between oil and water with a constant partition coefficient. 4. No redox reactions are present. 5. The reservoir is isothermal. 6. Temperature, pressure, and volume changes resulting from chemical reactions are negligibly small. 7. Water in any phase has the same composition as in the bulk aqueous phase and is in equilibrium with the matrix minerals.8. Supersaturation of aqueous species is not allowed. 9. Solid phases are stationary. 10. Precipitation/dissolution and cation-exchange reactions have a negligible effect on porosity and permeability.