The Effect of Evolved CO2 on Wormhole Propagation in Carbonate Acidizing

Abstract

Abstract Matrix acidizing is a commonly used stimulation technique for oil and gas wells. In carbonate reservoirs, acid is injected into the formation to dissolve carbonate rock in order to create highly permeable channels called wormholes. For a constant volume of acid injection, different injection rates create wormholes with different lengths. The injection rate that creates the deepest wormhole is called the optimal acid injection rate. Optimal injection rate is determined in laboratory experiments by measuring the pore volumes to breakthrough – the volume of acid injected in a core flood for the wormhole to reach the exit end of the core, normalized by the initial pore volume of the core. HCl is commonly used for carbonate acidizing treatments. The reaction between HCl and CaCO3 createsCO2. At laboratory conditions, 1000 psi back pressure is commonly used for acidizing experiment to keep CO2 in solution. However, based on the solubility of CO2 in water, 1000 psi may not be enough to keep CO2 in solution. In such a case, undissolved CO2 can be present as a gaseous phase in the system. As the properties of CO2 change with pressure and temperature, in order to accurately evaluate optimal acid injection conditions, the evolution of CO2 and its effect on wormhole propagation efficiency need to be investigated. In this study, we conducted core flooding experiments to examine the effects of evolved CO2 onwormholing behavior. The experiments were conducted at both room temperature and at an elevated temperature. Different back pressures from 500 psi to 3000 psi were applied in the experiments. The Buijse-Glasbergen model was usedfor curve fitting to obtain the optimal acid flux and optimal pore volume to break through for each experiment. Computerized Tomography (CT) scan images were taken for each core sample after acid injection to evaluate the structures of the wormholes. The test results show that the effect of CO2 on wormholing depends on the temperature, pressure, and injection rate. For low injection rate, CO2 present as a gaseous phase slows down wormhole propagation efficiency dramatically and enlarged wormhole diameter is observed. At injection rates above the optimal rate, the effect of CO2 is less pronounced, particularly at higher temperatures.