The Acid-prepack method is a combination stimulation and sand-control procedure that helps yield high productivities from wells that require sand control. Acid-prepack is often the method of choice for external gravel pack placement and has proven to be a productive, reliable, and cost-effective treatment in wells all over the world. The acid-prepack method combines the stimulation benefits of a hydrofluoric (HF) acidizing treatment with the sand-control benefits of packing the perforations and the region external to the wellbore.

Alternating stages of acid and gravel slurry are pumped during the treatment. The acid dissolves the damage that is left in the formation from drilling fluids, perforating, completion fluids, and LCM. Of the types of damage that are removed from the perforations and formation, removal of LCM from the perforations is most critical. The perforations should be cleaned with HF and then packed with gravel to ensure that the external pack is connected to the internal pack. If the perforations are not cleaned and then packed with pack sand, formation sand can flow into the perforation tunnels when the gravel pack stabilizes during initial production.
Of the types of damage that are removed from the perforations and formation, removal of LCM from the perforations is most critical. The perforations should be cleaned with HF and then packed with gravel to ensure that the external pack is connected to the internal pack. If the perforations are not cleaned and then packed with pack sand, formation sand can flow into the perforation tunnels when the gravel pack stabilizes during initial production.

One of the most beneficial aspects of the acid-prepack method of sand control is the combination of damage removal or breakdown by the acid and the excellent sand control initiated by the external gravel pack. With damage removed from the formation face, the perforations readily accept carrier fluids that quickly leak off and allow the gravel to pack in the perforation tunnels. This acid and slurry process is repeated several times. As the perforations fill with gravel and the formation is penetrated by the gelled gravel pack fluids, the viscous gel causes flow resistance in the formation pores. This resistance diverts the subsequent stages of acid to other untreated areas of the same perforation, or other perforations, and more uniformly stimulates the entire interval of interest. Damage removal and perforation packing are then evenly distributed over the entire interval, rather than being confined to the first area penetrated by the acid. With the damage removed from the perforations and formation, the well is made more productive and exhibits lower pressure drop across the producing zone for a longer period of time in the well’s producing life.

Guar based gel’s crosslinking is broken when it contacts low-pH fluids such as acid, allowing a high degree of fluid leakoff and thus a better external pack. While fluid-loss control of the crosslinked gel is excellent, the fluid leakoff rate of the Guar gum's gel, once crosslinking is broken, equals that of a sheared and filtered hydroxyethyl cellulose (HEC) fluid. Combining the guar based linear gel with the acid-prepack treatment yields higher productivity with a high degree of sand control, because perforations are first opened and cleaned of LCM, allowing for a better gravel pack.

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