Reviewing CO2 dynamics in acidizing carbonate reservoirs: Mechanisms, impacts, and models

Acidizing is a stimulation technique used in underground reservoirs to enhance well productivity by increasing the permeability of the rock matrix. During the reaction between acid and carbonates, carbon dioxide (CO₂) is produced, and factors such as its quantity and physical state significantly influence the efficiency of the acidizing process. This review explores the impact of CO₂ on acidizing through four primary mechanisms: relative permeability reduction, surface area reduction, diffusivity modification, and oil viscosity reduction. Each mechanism can either positively or negatively influence the efficiency of wormhole propagation, which is crucial for the success of acidizing treatments. Experimental studies reveal that the production of non-aqueous CO₂ leads to a reduction in relative permeability. The reduction in available surface area caused by CO₂ leads to enhanced acid propagation. The effect of CO₂ on diffusion is complex, as it can either decrease or increase the diffusion coefficient depending on its phase—aqueous, gaseous, liquid, or supercritical—and whether it promotes enhanced mixing. Additionally, oil viscosity reduction in the presence of an additional phase can improve acid propagation under certain conditions. This review also highlights key research gaps. The threshold backpressure required to maintain CO₂ in the aqueous phase remains poorly defined, with studies indicating that even pressures exceeding 6.90 MPa (1000 psi) may not suffice in certain cases. The combined and individual effects of aqueous and non-aqueous CO₂ under diverse reservoir conditions remain poorly understood. Additionally, while multiphase pore-scale numerical models have shown promise in simulating CO₂ behavior during acidizing, core-scale models often fail to capture the intricate interplay of mechanisms, particularly when multiple phases coexist. Addressing these gaps requires future experimental and numerical studies to focus on the porous media implications of CO₂ interactions. Specifically, research should aim to identify the critical parameters and develop robust methodologies to quantify the effects of CO₂-related mechanisms. By doing so, this work can guide future research toward improving the predictability and effectiveness of acidizing treatments while ensuring practical applicability across diverse reservoir conditions.