Combinations of HCl and HF acids are commonly used in well stimulation in sandstone reservoirs containing substantial amounts of clay and feldspar. After reaction with reservoir minerals, the resulting spent acids arc produced through production tubing. The spent acids may be diluted with formation water and contain a variety of reaction products. Corrosion inhibitors may be depleted by adsorption on formation minerals. Laboratory tests were conducted to simulate the composition of "spent" HF-containing acids and inhibitor depletion in the formation. Test conditions were based on autoclave and coreflow experiments coupled with field data on acid flowback.

Acids containing HF remain corrosive even after extensive reaction with sandstone reservoirs. Inhibitor depletion and dilution are a significant factor in spent-acid corrosivity Field data indicate that about 80% of the injected acid is produced back diluted by about a factor of two. Guidelines have been developed for simulation of spent acid compositions with synthetic laboratory brines. Using these simulated brines, corrosion tests have been performed with 21 Cr duplex stainless steel and Super 13 Cr (SI3Cr) martensitic stainless steels in spent HCl/HF mixtures under conditions typically encountered in the deepwater Gulf of Mexico. Guidelines for acid formulation and inhibitor application have been developed.

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