The Bakken formation has emerged as one of the major oil and gas sources in the United States primarily because of technological advancements in hydraulic fracturing and horizontal drilling. Because of the high level of dissolved iron and calcium, water in the Bakken formation can be challenging in terms of scale control compared to other types of formation water with low levels of dissolved iron and calcium. Generally, for controlling carbonate scale, the high level of dissolved iron present in water significantly reduces the performance of commonly used scale inhibitors, such as polycarboxylic acids and amino tri(methylene phosphonic) acids etc. Most studies attribute such adverse impacts to the additional consumption of scale inhibitors by iron carbonate. However, evidence has shown the mechanism can be more complicated because of potential crystal distortion and scale seed effects during the coexistence of the two scale forms (Smith et al. 2008). In addition, compatibility issues caused by high concentrations of calcium in water can also help vitiate the performance of scale inhibitors.

This paper presents a series of field and laboratory studies conducted to better understand and address such problems. A discussion of acquired results and their implications is presented. Using Bakken formation water with more than 200 mg/L offerrous ions and 18,000 mg/L calcium ions, the inhibition performance of some conventional scale inhibitors and a few innovative solutions under anaerobic static and dynamic test conditions are evaluated and compared. Modeling analyses of Bakken water helped identify calcium carbonates and iron carbonates as primary potential scales. The compatibility of scale inhibitors in Bakken water with and without crosslinked gel fracturing fluid was also investigated. The results revealed two new chemicals are compatible with the fracturing fluid, both exhibiting high iron and calcium tolerance. These new solutions enabled effective control of calcium carbonate and iron carbonate scales in Bakken formation water.

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