Extended Field Study Tracking the Performance of the Preservative Biocides, THNM and DMO, in Unconventional Wells

Abstract

Abstract Hydraulic fracturing is prone to microbial contamination due to the large volumes of water used to recover oil and gas from the reservoir. Biocides are frequently added during hydraulic fracturing to mitigate operational problems such as souring, corrosion, and biofouling. In this multi-year case study, the preservative biocides, 4,4-dimethyloxazolidine (DMO) and tris(hydroxymethyl) nitromethane (THNM), were applied during completions for the long-term control of microbial contamination capable of surviving harsh reservoir conditions and to optimize hydrocarbon recovery. A suite of high temperature, high pressure (HPHT) bioreactors were developed to investigate the ability of biocides to prevent souring in model hydraulic fracturing systems. The above-mentioned preservative biocides, known for their endurance, were selected for inoculation into the reactors. These results were compared to an identical biocide field application in the Vaca Muerta basin in Argentina. A multi-year study was conducted measuring efficacy of the biocide program in the produced fluids by measuring ATP, qPCR, most probable number (MPN) and NGS (Next Generation Sequencing) in selected samples. Concurrently, the operator measured H2S levels in the production fluids and calculated the estimated savings in H2S scavengers top side by implementing preservative biocides. Results indicated that both DMO and THNM were able to effectively control souring and microbial growth in treated reactors at 60°C compared to the untreated control. When comparing reactor results to field applications, both biocides were successful in maintaining control of the field microbial populations and produced H2S from flowback through a few years of production. Furthermore, observations of nearby wells treated with non-preservative biocides indicated those wells lost control of the microbial population in the reservoirs after approximately 10 months of production. This microbial control study allows the correlation of lab bioreactor performance to real-world field results and demonstrates the effectiveness of the preservative biocides, DMO and THNM, in the long-term control of microbial biofouling in an unconventional reservoir.