The effect of different CO2 phases on wormhole development in carbonate rocks

Acid stimulation is commonly used for carbonate reservoirs to enhance wells’ productivity by creating highly conductive channels called wormholes. The success of the stimulation depends on how deep these channels penetrate the formation. Hydrochloric acid (HCl) is commonly used for the carbonate stimulation process with carbon dioxide (CO₂) as a byproduct of the reaction between HCl and calcium carbonate (CaCO₃). Depending on the operating temperature and pressure, CO₂ can form a gaseous phase (bubbles) or be dissolved completely in the fluid. To achieve an understanding of the effect of CO₂ bubble formation on wormhole development, we used a low acid concentration (not more than 1 wt% HCl) at a range of flow rates. In this study, an elevated back pressure of 8.2 MPa is applied to keep the CO₂ dissolved in the solution and then compared with another set of experiments where no back pressure is applied. Sensitivity runs on various back pressures (while keeping all other parameters constant) are conducted to acquire a detailed understanding of the wormhole behaviour at a range of back pressures (0.1, 2.7, 5.5 and 8.2 MPa). We test the results in the dissolution phase space of Peclet and Damköhler dimensionless numbers. Although we show that for constant flow rate conditions, the existence of gaseous CO₂ significantly increases the pressure prior to the wormhole breakthrough, surprisingly no noticeable effect on the wormhole initiation process itself was found.