| Experience has shown that the response of oil wells to sandstone matrix acidizing is different from the response of gas wells. For oil wells, the improvement in permeability resulting from the stimulation treatment peaks at a certain acid volume and then decreases as the acid volume increases. For gas wells, however, the improvement is roughly proportional to the acid volume. It has been reported that stimulation of oil wells could be improved by displacing the oil in the zone to be acidized by gas. Gas preconditioning is sought to prevent the formation of emulsions or sludges resulting from the reaction between the oil and spent acid products.; The present research investigates the effect of gas preconditioning of the damaged zone on permeability improvement. Experiments were conducted on Berea sandstone cores saturated with 29.2° API oil at selected reservoir conditions of 180°F and 3,000 psi. CO2 and N2 gases were used alternatively for preconditioning. It was found that with gas (CO 2 or N2) preconditioning, improvement in permeability increased with increasing acid volume. Further, using gas preconditioning with small volume of acid, that would be ineffective with regular stimulation, would result in improvement of up to 200% of the original permeability. At an acid volume that would just restore the original permeability, the gas preconditioning would yield about 300% improvement. It was also found that CO2 is more effective than N2, which is expected to be due to the difference miscibilities of the two gases. |
