Existence of an optimum injection rate in carbonate acidizing and the effect of rock heterogeneity on wormhole patterns

The existence of an optimum rate and its variation with acid concentration, rock mineralogy, and temperature were studied experimentally. Three distinct modes of acid attack in carbonate were observed in the experiments. They were found to be functions of competing parameters which control the reaction of acid and rock.;At comparatively low injection rates, acid tends to create a compact dissolution pattern which means that the inlet face of the core is relatively uniformly dissolved. For this to occur, the overall reaction rate is limited by convection of acid to the core face. At moderate injection rates, diffusion of the acid to the pore walls controls the overall rate of consumption of acid and the portion of fluid loss is not large enough to limit channel growth. In this instance, a major wormhole forms. When this mode of acid attack occurs, wormhole penetration rate increases with increasing injection rate. At high injection rates, wormholes become highly branched and fractal in nature; in the limit, acid consumption is surface reaction controlled, as occurs in sandstone acidizing with HF. The data show that for this mode of attack, acid penetration depth decreases with increasing injection rate for a given volume of acid. Thus, the experimental results exhibit an optimum rate corresponding to the transition from the wormhole pattern to the uniform dissolution pattern. The optimum rate shifts to a higher value if the overall reaction rate increases due to increase of temperature or acid concentration.;The optimum rate is a function of the rock composition and reaction temperature as well as the pore size distribution of the original rock. All of these factors are included in the model developed. The model is based on determination of the injection rate for which the reaction in the largest pores at the tip of the wormhole will be mass transfer controlled. It provides a quantitative prediction of an optimum rate. The correct trends are predicted as rock composition, acid concentration, and formation temperature are all changed. The predicted results agree with the experimental results reasonably well.;Effect of heterogeneity in the permeability field on wormhole pattern was studied by acidizing experiments with a CT scanner. The results show that acid tends to flow in the most permeable path. Tracer flow simulation by UTCHEM, a three-dimensional chemical flooding simulator, in the core was run to characterize the effect of heterogeneity in the permeability field. Heterogeneity factor, f...