Abstract

Cased Hole Frac Packs (CHFP) are among the preferred completion strategies for deepwater production wells due to sand production concerns and the high cost associated with subsea interventions. An acid stimulation treatment is usually conducted prior to the main frac operation in order to clean-up some of the perforation debris (including powdered metals from shaped charges) and residual fluid loss control (FLC) materials which can be a significant source of formation damage and plugging of screens and other wellbore jewelry. A few studies have also considered the technical and economical benefits of adding a scale inhibitor during the completion operation (Fitzgerald and Cowie, 2008; Lungwitz et al. 2007; Martins et al., 1992; Vetter et. al., 1988). The challenges associated with adding a scale inhibitor to the frac fluids are widely recognized, and some alternatives such as the use of encapsulated scale inhibitors and scale inhibitor-impregnated proppant have been proposed (Powell et al. 1995; Fitzgerald and Cowie, 2008). The addition of scale inhibitor to the early stages of the frac operation has also been explored (Maschio et al., 2007), but the temperature ranges evaluated have been rather limited. The purpose of this work is thus twofold: 1) To evaluate the effectiveness of two acid suites to clean-up Zn powder damage typically encountered during perforations with metal shaped charges, and 2) To qualify the incorporation of an acid pentaphosphonate scale inhibitor to the pre-frac acid in frac treatments targeting high temperature (up to 300°F) deepwater subsea wells. This paper describes in detail the laboratory qualification of the acid suite, and the acid / scale inhibitor combination treatment. Performance field data on the treatments is the subject of subsequent publications.

A HCl / organic acid suite with a novel, three-component corrosion inhibitor package was developed to clean-up Znrelated damage without compromising rock strength or corrosion protection at high temperatures (up to 300°F). In addition, the combination treatment consisting of scale inhibitor treated acid and overflush brine was found to be compatible with reservoir and wellbore fluids in the field of interest. The treatment was found to be non-damaging in high permeability (> 1000 md) Y core samples. Strong wettability changes and up to 50% reduction in permeability to oil were observed in low permeability X core samples. Further improvement of the acid / scale inhibitor combination treatment for low permeability reservoirs is also discussed. The results from this study contribute to a better understanding of scale management in high temperature subsea wells.

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