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.

Matrix acidizing is a well stimulation technique used to remove formation damage in the near wellbore region. But it comes with an associated set of challenges such as corrosion of the tubulars and iron precipitation in the formation. To counter these challenges, different chemicals, or additives, are added to the acid solution such as corrosion inhibitors and iron control agents. These additives may change the relative permeability of the spent acid, and formation wettability, and may either hinder or improve spent acid clean-up. Such effects of additives on the spent acid clean-up have not been documented. The aim of this research effort was to document the aforementioned change in the spent acid concentration (by using one additive at a time) before and after gas flowback. This was achieved by acidizing cores and creating wormholes halfway through them, then CT scanning them to observe the spent acid region. Later on, gas was flown through the core opposite to the direction of acid injection for 2 hours, and another CT scan was taken. The difference between the two CT scans was documented. Using a different additive each time, a series of such CT scans was obtained to develop an idea about whether the said additive was beneficial or detrimental to spent acid clean-up. It was found that the corrosion inhibitor FA-CI performed the best in terms of spent acid recovery after gas flowback for both Indiana Limestone and Texas Cream Chalk cores. Moreover, the corrosion inhibitor MI-CI was the worst for Indiana Limestone and the non-emulsifying agent M-NEA the worst for Texas Cream Chalk for spent acid recovery after gas flowback.