The Effect of Evolved CO2 on Wormhole Propagation in Carbonate Acidizing

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 and 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 (i.e., the volume of acid injected in a coreflood for the wormhole to reach the exit end of the core, normalized by the initial pore volume of the core). Hydrochloric acid (HCl) is commonly used for carbonate acidizing treatments. The reaction between HCl and calcium carbonate (CaCO3) creates carbon dioxide (CO2). At laboratory conditions, 1,000-psi backpressure is commonly used for acidizing experiments to keep CO2 in solution. However, on the basis of the solubility of CO2 in water, 1,000 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. Because the properties of CO2 change with pressure and temperature, the evolution of CO2 and its effect on wormhole-propagation efficiency need to be investigated to evaluate optimal acid-injection conditions accurately. In this study, coreflooding experiments are conducted to examine the effects of evolved CO2 on wormholing behavior. The experiments are conducted at both room temperature and an elevated temperature. Different backpressures from 500 to 3,000 psi are applied in the experiments. The Buijse and Glasbergen (2005) model is used for curve fitting to obtain the optimal acid flux and optimal pore volume to breakthrough for each experiment. Computed-tomography (CT) -scan images are 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 wormhole propagation efficiency dramatically, and enlarged wormhole diameter is observed. At injection rates greater than the optimal rate, the effect of CO2 is less pronounced, particularly at higher temperatures.