ABSTRACT

Produced water (PW) ponds are important facilities for supporting hydraulic fracturing in the Permian Basin. The control of H2S in these facilities is critical to safe and reliable frac and production operations. Effective microbial control strategies are required to mitigate sulfate-reducing bacteria (SRB) activity and fouling of production facilities with iron sulfide. Next generation sequencing (NGS) DNA analyses of Delaware Basin produced water (PW) samples highlighted a bacterial consortium dominated by putative halophilic fermentative and sulfate-reducing bacteria such as Halanaerobium spp. and Desulfohalobium spp. Anaerobic biodegradation of hydrocarbons can generate metabolites which serve as electron donors to support SRB activity. The novel application of calcium nitrate to produced water storage ponds for SRB control was piloted in Delaware Basin. DNA analysis demonstrated the impact of nitrate on the microbial consortium in the treated pond. Putative nitrate-reducing bacteria became dominant, with a greatly reduced abundance of SRB. Produced water bacterial growth experiments demonstrated the controls of redox potential and salinity on bacterial nitrate reduction, to aid in interpretation of the field data. The pilot was effective in preventing biogenic sulfidogenesis and has been adopted as a long-term mitigation strategy in PW storage ponds.

INTRODUCTION

Produced water recycling for hydraulic fracturing (fracking) operations has been an increasingly common practice to support oil and gas development in the Permian Basin. Aside from the economic benefits associated with reusing the water produced which is a byproduct of oil and gas operations, recycling reduces both the need for sourcing water (brackish or fresh) from the environment as well as the volume of produced water requiring disposal. Produced water ponds support successful recycling by providing temporary storage of recycled water and volume buffer for fracking. Raw produced water is usually treated in recycling facilities before being stored in the ponds. Although oxidizing agents such as bleach and peroxide are commonly used in these facilities to remove H2S and control bacteria, the biocidal effects are usually not persistent. The lack of effective microbial control strategies can result in sulfide generation due to activity of sulfate-reducing bacteria (SRB) in the produced water ponds and interruption to frac operations if the H2S is detected at the frac site.

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