Acid Dissolution Mechanism in Offshore Carbonate Reservoirs with Mineral Layers During Acidizing
Authors: Su Xuhang , Qi Ning , Jiang Ping , Zhou Fujian 
Published: October 2024

Abstract
During the formation of offshore carbonate reservoirs, the interaction between magnesium-rich seawater and calcium carbonate sediments leads to carbonate reservoirs with heterogeneous mineral distributions, especially mineral layers. The mineral layers tend to affect the dissolution patterns in the whole region to a larger extent during acidizing, due to the large difference between the acid-rock reaction rates of dolomite and calcite. It is necessary to study the law of the influence of mineral layers in carbonate rocks on the dissolution patterns to avoid under- and over-acid dissolution. Currently, there is extensive research on the acid dissolution mechanism in limestone reservoirs with homogenous mineral distribution. However, studies on carbonate reservoirs with mineral layers are considerably fewer. This work aims to investigate and evaluate the acid dissolution mechanism in offshore carbonate reservoirs with mineral layers during acidizing.
The traditional acid dissolution model is improved to consider the acid flow and acid-rock reaction in the presence of mineral layers. By conducting parameter sensitivity studies using this model, the effect of the different mineral layers on the dissolution results is studied at different flow rates. These results, i.e., both dissolution patterns and breakthrough volume, can be used to evaluate the acidizing effect. Further analysis of the simulation results reveals the acid dissolution mechanism in carbonate reservoirs with mineral layers, clarifying the optimization principles for acidizing operations in such reservoirs.
Based on the acid dissolution model in this study, the specific results of numerical simulation can be obtained to analyze the specific characteristics of acid dissolution. The mineral layers in this study are mainly manifested in the regional variations in the contents of calcite and dolomite. Compared to dolomite, calcite is a fast-reacting mineral, thus increasing the breakthrough speed of the acid, whereas dolomite has the opposite effect. For reservoirs with a higher average content of calcite, it is recommended to achieve higher acidizing efficiency through high pump rates to remove as much contamination around the wellbore as possible. Conversely, for reservoirs with a higher average content of dolomite, it is recommended to lower the acid pump rate to ensure thorough removal of contamination. The effect of the calcite layer on the acidizing effect can be ignored in dolomite reservoirs, while the effect of the dolomite layer on the acidizing effect must be considered in limestone reservoirs. The width of the mineral layer affects the breakthrough volume more than the position. When the position of the mineral layer is constant, the effect on breakthrough volume increases as the width of the mineral layer grows. When the width of the mineral layer is constant, the closer the mineral layer is to the inlet, the smaller the breakthrough volume difference rate.
This work analyzes and evaluates the acid dissolution mechanism in offshore carbonate reservoirs with mineral layers during acidizing. In addition, the improved model can be used for acid dissolution simulation studies of non-uniformly distributed carbonate rocks with various types of minerals.