In wells with relatively high levels of iron, the use of polyacrylamide friction reducers (FR) used for hydraulic fracturing can result in poor performance due to negative chemical interactions. Hazra, et.al. (URTeC 2487 July 2020) documented a problem with the chemical reaction of FR and iron (ferrous and ferric) during hydraulic fracturing. This chemical reaction can create an accumulation of a semi-solid mass referred to as "gummy bears" due to their rubbery texture (figure 1). These gummy bears can form in surface and subsurface equipment and inhibit well production. In addition to the formation of gummy bears, the performance of FR is significantly impaired when reacting with iron (figure 2).

Pyrite (FeS2) is a common mineral found in source rock. Ferrous iron (Fe2+) can be released by oxidative dissolution of pyrite minerals. In reservoirs with high concentrations of pyrite, iron can be released by dissolving reservoir rock during acid spearheading. Acid spearheading is a common industry practice during hydraulic fracturing operations. The process involves pumping a small quantity of acid pre-frac to dissolve rock material around the wellbore, cleaning up perforations, and reducing near wellbore entry friction. The focus of the acid spearhead is to lower breakdown pressures and improve injectivity during hydraulic fracturing. The problem that Hazra (2020) described was on a Woodford Shale project in Oklahoma. The Woodford is known to contain significant quantities of pyrite (observed at around 2%). One solution proposed was to eliminate the use of acid during the hydraulic fracturing operation. The potential downside was higher near wellbore frictions that would need to be addressed by higher hydraulic horsepower (HHP) and FR volumes.

Figure 1

Effect of Fe2+ and Fe3+ on friction reduction properties of polyacrylamide friction reducer Figure courtesy of Downhole Chemical Solutions

Figure 1

Effect of Fe2+ and Fe3+ on friction reduction properties of polyacrylamide friction reducer Figure courtesy of Downhole Chemical Solutions

Figure 2

Gummy Bears Photo courtesy of Downhole Chemical Solutions

Figure 2

Gummy Bears Photo courtesy of Downhole Chemical Solutions

The process of explosive perforating can create high near wellbore friction due to the perforation tunnel crushing that occurs during the perforation process. The acid spearhead is pumped to clean up this crushed zone and improve perforation tunnel performance. A new system of perforating was described by Albert (SPE 199274-MS 2019) that incorporated propellant and explosives perforating to eliminate the perforation tunnel crushed zone and reduce near wellbore friction. This paper will describe a Barnett Shale project that utilized this new composite perforating method to eliminate the use of acid.

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