Numerical studies and analysis on reaction characteristics of limestone and dolomite in carbonate matrix acidizing

Carbonate reservoirs with abundant oil and gas resources are mainly composed of limestone and dolomite. As the acid-rock reaction progresses, limestone is gradually consumed and more acid gradually reacts with the slower dolomite. However, the existing acid rock-reaction models do not consider the competition between limestone and dolomite with acid, and cannot objectively describe the degree of contribution of limestone and dolomite to wormholes at each stage of acidizing. Here, an acidizing model was developed to study and analyze the effect of the reaction characteristics of limestone and dolomite on the carbonate acidizing. In this study, the probability of limestone and dolomite contact with acid is assumed to be related to the content of limestone and dolomite, and a new acid-rock reaction model is proposed by considering the reaction rate and reaction sequence of limestone and dolomite. The acid-rock reaction model is coupled with the porous medium heat transfer model and the twoscale continuum model, and a comprehensive carbonate acidizing model is established. The model can respond to the competition between limestone and dolomite in each stage of acidizing at different temperatures and quantitatively characterize the dynamic change of different mineral contents. Based on the model, this work studies and analyzes the effects of the temperature, dolomite characteristics (uniform, blocky, and columnar) and distribution orientation on wormholes propagation and acidizing efficiency. The simulation results show that the comprehensive carbonate acidizing model can accurately reveal the higher contribution of limestone to the wormhole when the temperature is less than 338 K. As the temperature increases, the difference of reaction rate between limestone and dolomite gradually decreases, and the contribution of dolomite to the wormholes increases. When the temperature is 418 K, the dissolution of limestone and dolomite in the wormholes and the area around the wormholes is almost the same. Moreover, when the temperature is 298 K, the smaller size and density of blocky and columnar dolomite can make the wormholes tortuous and complicated, thus reducing the acidification efficiency. When the size and density are larger, the presence of dolomite can hinder the generation of branching wormholes and guide the propagation of wormholes, enhancing the acidizing efficiency. In addition, the orientation of dolomite does not change the propagation distance of wormholes, but it changes the propagation orientation of wormholes. This study privates a guide for the acidizing treatment of carbonate reservoirs with the different mineral compositions of limestone and dolomite.