First Reuse of 100% Produced Water in Hybrid Treatments With Gelling Agents

Abstract

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 173783, “First 100% Reuse of Bakken Produced Water in Hybrid Treatments Using Inexpensive Polysaccharide Gelling Agents,” by Blake McMahon, SPE, Bruce MacKay, SPE, and Andrey Mirakyan, SPE, Schlumberger, prepared for the 2015 SPE International Symposium on Oilfield Chemistry, The Woodlands, Texas, USA, 13–15 April. The paper has not been peer reviewed. This paper reports the completion of a two-lateral well in the Williston basin where produced water (PW), filtered but otherwise untreated, was used throughout the slickwater and crosslinked components of approximately 60 hydraulic-fracturing stages. Proppant was placed successfully in all perforated zones by use of a hybrid design that used 7 million gal of water (of which 2.2 million gal was crosslinked). This paper will concentrate on the development and implementation of a metal-crosslinked fracturing fluid that showed excellent stability. Introduction The demand for an economical approach to reuse water from oilfield operations— namely produced, flowback, and nonpotable sources—is not a new concept. In slickwater fracturing, reuse of PW is a solved problem, thanks to the ready availability of synthetic friction reducers that are efficient even in heavy and unpredictable natural brines. Here, the physical chemistry of the system (a soluble polymer reducing pipe friction) is fairly straightforward and the key innovations driving successful friction reduction in PW were easy to identify and import from polymer science once the failure mechanisms for the parent polyacrylamides in PW were understood. Although the synthetic polymers used in PW are derived from more costly feedstocks than the parent polyacrylamide/ polyacrylate copolymers, the completion-cost increase is easily balanced out in most cases by various cost issues around water sourcing. In contrast, economical and operationally simple solutions that deliver crosslinked gels reliably have been more difficult to develop, largely because the chemistry of crosslinked gels is much more complicated. The industry’s preferred system, guar crosslinked by boron at high pH, has several shortcomings in highly saline PW, and most service companies advertise that water beyond 4 to 8% total dissolved solids (TDS) is highly undesirable in this system. Boron, calcium, and magnesium are specific ions that pose problems. Boron is common in PW, and the water used in this study had roughly three times the boron concentration that would be expected in a quality borate-crosslinked gel suitable for use in the same project. The presence of unpredictable and potentially variable amounts of adventitious crosslinker in the mix water is a major challenge to operational success. Calcium and magnesium ions have strong negative effects on the quality and durability of borate-crosslinked guar gels. The typical response has been to add stabilizers or chelants specific to hardness ions or to treat the water to remove hardness. Most water treatments are costly and capital intensive, and many generate waste streams. These increased costs can challenge operators seeking to extract value from PW who want crosslinked gels. Another approach is to dilute PW on the fly with fresh water to a salinity level that borate-crosslinked guar can accommodate. Although this practice negates reinjection of PW, it is not efficient because it requires fresh water and introduces inherent variability in water quality during fracturing operations because the blending levels fluctuate and the quality of input water changes.