How Downhole Conditions Influence Acidizing Performance in Carbonate Reservoirs: An Experimental Study

Summary Carbonate reservoirs, composed primarily of calcite, dolomite, or a mixture of both, are commonly stimulated using hydrochloric acid (HCl) to enhance productivity. At temperatures exceeding 200°F, the reaction between HCl and carbonate becomes extremely rapid, often resulting in shallow acid penetration and face dissolution. This reaction is further complicated by the phase behavior of the carbon dioxide (CO2) produced. Laboratory experiments typically use a system backpressure of 1,000 psi to retain CO2 in solution; however, under these conditions, CO2 may partially exist in the gaseous phase, inducing forced convection and artificially increasing the apparent reaction rate or the overall dissolution rate of rock in acid. In contrast, most subsurface reservoirs operate at pressures well above 1,000 psi, often exceeding CO2’s supercritical pressure, where such forced convection does not occur. This discrepancy leads to an overestimation of overall dissolution rates and underestimation of acid penetration in laboratory settings. To address this, coreflood experiments were conducted across a broader range of pressures, temperatures, and injection rates to better simulate realistic reservoir conditions. Micro-computed tomography (CT) scanning was used to evaluate wormhole propagation, while CO2 solubility and phase transitions were analyzed as functions of pressure and temperature. Results showed that at 300°F, CO2 solubility in water increased fourfold when pressure rose from 1,500 psi to 8,000 psi. At 250°F, increasing pressure from 1,500 psi to 7,000 psi reduced the acid volume needed for breakthrough by 50%. At an injection rate of 5 cm3/min and under a pressure of 5,000 psi, 15 wt% HCl was able to break through calcite rock samples even at temperatures as high as 350°F without causing any face dissolution. These findings highlight the importance of conducting reaction kinetics measurements under realistic downhole conditions to optimize acidizing treatments effectively.