Geochemical performance evaluation of pre-flushing of weak and strong acids during pH-triggered polymer flooding

摘要

Implementing pH-triggered polymer is an efficient way to improve the conformance control of hydrocarbon formations. pH-triggered polymer solutions have high mobility in low pH conditions, while due to their high swelling capacity in higher pH values, their viscosity increases with pH increment. Therefore, they can fill high permeability zones/strata and consequently improve the efficiency of enhanced oil recovery. To achieve the desired viscosity of a pH-triggered polymer, the pH of porous media is regulated by implementing an appropriate acid pre-flushing. In this study, we develop a mathematical model to predict the acid pre-flush behavior before the main pH-triggered polymer flooding. Convection, diffusion, and homogeneous/heterogeneous geochemical reactions for a slightly compressible fluid flow in porous media is formulated. This model consists of three non-linear conservation equations (i.e., mass balance, species concentration balance, and Darcy law), which are solved simultaneously in a global implicit approach. We discretize the transport and geochemical reaction terms with a finite difference method by implementing the benefit of the Newton-Raphson method. Moreover, the code vectorization programming approach of the Python language is applied to decrease the computation time. The introduced model is validated against the experimental data available in the literature. This EOR method is mostly implemented on sandstones and there are very limited investigations on the applicability of this method on carbonate rocks. Hence, we simulate the injection of several acids (strong and weak acids) into a carbonate rock under different flow conditions (i.e., different Damkohler numbers). The obtained pH values reveal that weak acids can provide better results for acid pre-flushing in carbonate formations. Therefore, implementing these acids can be a more efficient way to increase the acid residence time as well as to improve the conformance control during a pH-triggered polymer flooding project.