A Study of the Chemical Interactions Between Brine Solutions and Dolomite

Abstract

Summary Application of gelled polymer treatments to change the flow characteristics of a reservoir is a viable improved oil recovery technique. Many gel systems are affected by the solution pH in that gel time is pH dependent. The treatment of carbonate reservoir rock is of particular concern because (1) fluid-rock interaction can alter the pH of the injected solution from the design value and (2) dissolution of carbonate can increase the divalent ion concentration, which can also affect gelation behavior. In this study, the interaction between injected potassium chloride brine and dolomite was investigated through displacement experiments in dolomite cores and mathematical modeling based on equilibrium thermodynamics. Parameters varied were pH of the injected solution, flow rate, and the common ion effect through variation of the calcium and magnesium ion concentrations in the injected solution. Core effluent values of pH and concentrations were measured. Experiments at different flow rates established conditions in which equilibrium was achieved in the core. Equilibrium values of pH were almost constant at a value of 10 when the injected pH was varied between 4 and 10. Results indicate that equilibrium conditions exist for most field conditions. A geochemical model was used to predict equilibrium pH and concentrations, as well as the amount of dolomite dissolved. The model accurately predicted effluent pH of experimental displacement data for the conditions wherein equilibrium was achieved. All model parameters were obtained from literature equilibrium data and were not dependent on curve fitting of the experimental data.