Injected Gas Improves Acidification Job Quality of Sandstone Reservoirs

It has been noticed in well acidification operation that the acidification job quality is not satisfactory, the effective period of the acidification job is short, and the acidified area is limited. In well acidification operations, it has been found that the results of acidifying a gas well penetrating sandstone reservoirs are much better than the results of acidifying an oil well penetrating sandstone reservoirs. This observation has been used in designing oil well acidification program to improve the acidification job quality, that is, a gas can be injected with acid into the well in oil well acidification. The mechanisms and advantages of alternated injection of gas and acid are extensively studied in this paper. In laboratory experiment, the acidification results of undamaged cores and cores damaged by completion fluid under 3 gas injection models were studied, and the effects of different injected gases on the acidification results were analyzed. It is considered that gas injected in well acidification clearly improved the acidification job quality of damaged cores, especially when an acidification mode in which prepad gas plus alternated gas and acid injections was used during well acidification. For undamaged cores, the effectiveness of gas injection with acid in well acidification was not satisfactory. It was also found that the effectiveness of injecting CO₂ was better than that of injecting N₂. Alternated gas injection with acid in well acidification greatly improves the permeability of the reservoir rocks, and is able to enhance the effectiveness of conventional well acidification job. This technology has a broad development prospect.