Various models and simulators offer insight into the likely souring potential of global oilfield assets. However, more recent research has demonstrated the necessity of high-quality, laboratory-based data to correctly calibrate the biological activity coefficients in the souring model. Extreme environment testing is essential to simulate downhole field conditions under controlled laboratory conditions. High-pressure, flowing bioreactors are best placed to mimic the hydrostatic pressure and thermal conditions which the microbial communities are exposed to under water-flooding conditions. The world’s largest souring database from extreme environment testing is considered to be held by Rawwater, standing at over 600 years’ of data at the time of writing and generated in the organisation’s world-leading high-pressure bioreactor suite. Managing the microbes Despite the systemic nature of SRP in the O&G industry, there are just a few tried-and-tested methods which have shown to be beneficial in mitigating and/or controlling the production of sour fluid topsides. Supplementing injection water with competitive exclusion chemicals such nitrate has been shown to reduce sour gas production. The presence of nitrate in the downhole formation stimulates the proliferation of nitrate-reducing bacteria (NRB) as the biological reduction of nitrate is more energetically favourable when compared with sulfate reduction. Complete nitrate reduction to inert nitrogen is clearly beneficial to operators as opposed to the production of sulfide from sulfate reduction. A further benefit of nitrate injection is that, under certain environmental conditions, incomplete nitrate reduction occurs, resulting in the production of nitrite. These nitrite compounds block the enzymatic reduction of sulfite to sulfide; therefore, blocking sulfate respiration.
During prolonged periods of water-flooding, this volume of cooled rock can extend into the reservoir, often supporting the growth of SRP along fracture faces of the downhole matrix. Supporting the SRP Outside of the standard requirements for anaerobic, sessile microbial life to flourish [water, anoxic conditions, chemical compounds for reduction and oxidation (redox) reactions, physical surfaces for attachment], the rate and extent of subsurface sulfide generation is often dictated by other physical and chemical conditions in the formation. Physical parameters such as pressure and temperature can either restrict or facilitate survival and growth of the SRP, whereas chemical parameters such as salinity and pH often determine the degree of microbiological sulfide generation. Generally speaking, SRP can be active at pH ranges from 4.0 to 10.0, and are most productive at temperatures up to 80 °C and at hydrostatic pressures below 10,000 psig (69 MPa). Modelling the microbes In order to inform material selection and field development, oilfield reservoir souring forecasting models are used to predict the future souring propensity of assets around the world. Using operational, planning and survey data from the field (namely outputs from reservoir simulators), good souring models describe (i) the reservoir cooling as a result of water injection, (ii) the growth of SRP and generation of H 2 S in the subsurface, and (iii) the transit of H 2 S through the formation to the production facility. Further calculations are often conducted to partition the mass of sulfide produced at predetermined pressure, volume and temperature (PVT) and pH conditions (e.g. at the first stage separator) to generate gas phase H 2 S ppmv concentration profiles.
17 Microbiology Today May 2023 | microbiologysociety.org
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