The efficiency of matrix stimulation in carbonates is based on the generation of wormholes that penetrate the formation beyond the damaged near-wellbore region. Face dissolution, conical wormholes, dominant wormholes, ramified wormholes, and uniform dissolution are different dissolution patterns that can be formed during a carbonate matrix acid treatment. However, the process is most successful when dominant wormhole patterns are formed.
Studies have shown that there exists an optimum injection rate for the formation of dominant wormholes, and mathematical models have been developed to describe the propagation of wormholes and to obtain this rate for an efficient acid treatment. Most of these models give good predictions of the optimal flow rate or wormhole growth rate for laboratory experiments, but the challenge faced is in translating the results from laboratory to field scale.
This paper classifies the existing carbonate acidizing models in literature into seven broad categories and discusses the theory behind the models, major assumptions made, practicality in field scale applications, and technical shortcomings. The limitations of these models, such as the incorrect scaling of linear core scale results to radial field scale applications, formation heterogeneity, and non-inclusion of additives in reaction rate calculations are highlighted and a proposal for improvement is made to enhance the accuracy of one of the models under field conditions.
The recommendation has been made to inject the acidizing fluid at the maximum allowable injection pressure based on the fracture pressure limit due to the shortcomings in the available carbonate matrix-acidizing models in predicting the optimum injection rate at field conditions. To calculate the wormhole propagation rate for skin evolution monitoring in field treatments, the modified semi-empirical wormhole propagation model is recommended with the caveat of including the additives used in the field in the coreflood tests when obtaining the optimum rate parameters in the laboratory.