Matrix acidizing treatments are applied to gas wells to remove near-wellbore permeability impairment (formation damage). The acidizing process differs in gas wells compared with oil wells or water injection wells because of two-phase, gas-liquid flow during acid injection, and also because of the relatively low viscosity of the gas. This paper demonstrates how these differences affect the penetration of acid into the formation and the distribution of acid along the wellbore (acid placement).
When acid displaces gas in the formation around the wellbore, the favorable mobility ratio between the acid and the gas will lead to a piston-like displacement of the gas by the acid. However, behind the acid front, a residual gas saturation will remain. Our study shows that the impact of the residual gas saturation increases acid penetration into the reservoir, when compared with a single-phase displacement process.
The large contrast between the viscosity of the acid and the gas in the reservoir may be beneficial to a better distribution of acid along the wellbore. Because the acid is much more viscous than the gas, there will be an increased pressure drop wherever acid is placed in the formation. This can function as a natural viscous diversion - as acid enters the higher permeability zones, the increased pressure gradient can divert relatively more acid to lower permeability zones. We present modeling results that illustrate the magnitude of the viscous diversion effect in gas wells. This natural diversion precludes the use of gels or other viscous fluids for diversion in gas wells.