Abstract

Our main result is the discovery of an optimum acid injection rate to obtain acid breakthrough in linear corefloods of carbonates using a minimum total acid volume. Low rates result in acid spending at the core surface while high rates result in the formation of multiple, highly ramified wormholes. At the optimum intermediate rate, a single, small wormhole penetrates the core.

The optimum acid rate is found to be a function of the rock composition and reaction temperature as well as the pore size distribution of the virgin formation rock. All of these factors are included in the theory developed here. This theory provides a quantitative prediction of the optimum rate. The practical ramifications of the results are also considered.

Introduction

Williams et al. recommend that carbonate acidizing treatments be carried out at the highest rate possible without fracturing the reservoir rock. This strategy is in complete accord with the findings reported by Paccaloni and Tambini based on their extensive field studies. Daccord et al. have, on the other hand, proposed a design procedure that requires low acid injection rates to achieve optimum stimulation with a given acid volume. Experiments reported to date do not, in fact, resolve this conflict. Hoefner and Fogler conducted acidizing experiments using both Indiana limestone and dolomite cores. For dolomite at room temperature, higher injection rates resulted in more total acid being required to achieve a given acid penetration in agreement with the concepts of Daccord et al. With Indiana limestone, smaller quantities of acid were found to be required to achieve a given penetration as the acid flux was increased. Thus, the reported laboratory experiments are ambiguous and do not support either one of the conflicting recommendations.

One purpose of this paper is to resolve this issue by showing experimentally the existence of an optimum acid injection rate. Thus, whenever the injection rate exceeds the optimum, a reduction in rate will improve performance. Similarly, rates that are less than the optimum must be increased. Furthermore, the optimum will be shown to be a complex function of the reservoir composition, temperature, and pore structure of the virgin rock, so that there can be no simple rules as to whether slow or fast rates are best.

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