The Effect of Acid Additives on Carbonate Rock Wettability and Spent Acid Recovery in Low Permeability Gas Carbonates

摘要

Spent acid retention in the near-wellbore region causes reduction of relative permeability to gas and eventually curtailed gas production. In low-permeability gas carbonate reservoirs, capillary forces are the key parameters that affect the trapping of spent acid in the formation. Capillarity is a function of surface tension at the gas-liquid interface and contact angle of the fluids in the rock. To weaken capillary forces, surface tension should be low and contact angle should be large. This work provides a comprehensive study on the effect of various common acid additives on carbonate rock wettability, and surface tension and contact angle, as the main parameters that control capillarity. Surface tension and contact angle experiments were conducted using Drop Shape Analysis (DSA) instrument at high temperature and pressure. Core flood experiments were also conducted to study the overall impact of the acid additives on wettability by analyzing irreducible fluid saturation in the rocks before and after spent acid exposure. Spontaneous water imbibition was conducted in each case to check for permanent or long-term wettability change as a result of using these additives.Acid additives such as methanol and corrosion inhibitors reduced both surface tension and contact angle. Iron control agents had no impact on surface tension, however, they decreased contact angle at the lower concentration used. Formic and acetic acids did not affect the surface tension, but they had a reducing impact on the contact angle. According to the core flood experiment results, formic acid decreased irreducible fluid saturation whereas methanol increased irreducible fluid saturation. On the other hand, the fluorochemical surfactant tested changed the rock wettability into more gas wetting. Use of this chemical would help in recovering spent acid. The results of the spontaneous water imbibition tests showed that organic acids and iron control chemicals did not have a permanent impact on wettability of the rocks. However, the wettability change as a result of using fluorochemical surfactant would persist for a long time as this chemical forms a film on the rock surface.