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Abstract

Hydraulic fracturing of oil and gas wells requires high volumes of water. Often these waters originate from rivers, lakes, ponds, and recovered water from previous fracturing treatments. These waters are often infested with aerobic and anaerobic bacteria that can cause multiple problems. These include degradation of fracturing chemicals, down-hole corrosion, biological-based H₂S generation, and down-hole flow-impairment due to slime producing bacteria. Historically, these waters have been treated with biocides such as tetrakis (hydroxymethyl) phosphonium sulfate (THPS), glutaraldhyde or quaternary ammonium-based surfactants.  Lately, oxidizing biocides such as peracetic acid, chlorine dioxide and hypochlorous acid have been used as environmental alternatives.

Recently, tetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione (Dazomet) has been used as a biocide with good results in the Barnett, Marcellus, Eagle Ford and Haynesville gas shale basins in the US. This biocide is effective against most aerobic and anaerobic bacteria encountered in water sourced for fracturing applications. Although not a fast-killing biocide, it is effective for long-term maintenance of the formation and well with loadings normally about 0.4 L/m³ of 24% active solutions.

Further investigation has now discovered that the combination of this biocide with others has significantly reduced necessary loadings of these biocides to values less than 0.2 L/m³. These Dazomet-combinations also tend to show much higher degrees of efficacy, showing superior bacteria management over the single or oxidizing biocides. This enhanced performance also allows for reduced chemical exposure to the environment. The performance of these Dazomet biocide-combinations and field trials will be presented and discussed.

Introduction

Multi-stage fracturing treatments of horizontal wells in shale formations require very large volumes of water, often exceeding 38,000 m³. This fluid volume is needed to create the fracture network for optimum reservoir contact. The source of this water is rarely municipal treated water; rather this resource is taken from surface waters such as rivers, lakes, reservoirs and frac ponds. Recently, mine-back waters, well flow-back water and produced water have become other viable sources of water, either to minimize disposal costs or to overcome water shortages. Blends of fresh water and produced water have become common practice to reduce the consumption of fresh, surface water.