Shifts in Bacterial and Archaeal Community Composition in Low-Permeability Oil Reservoirs by a Nutrient Stimulation for Enhancing Oil Recovery

Indigenous microbial enhanced oil recovery technology by selective nutrient injection is a potential alternative that leads to oil production improvement in low-permeability oil reservoirs. Nutrient flooding in oil reservoirs can shift the balance of microorganisms within a population; an in-depth exploration of this phenomenon can enable us to selectively activate particularly beneficial microbial species for enhancing oil recovery. In this study, high-throughput sequencing was employed to analyse indigenous microorganisms (e.g., archaea and bacteria) in an oil production well (W226), compared to a control well (W202), in the Xingzichuan Oil Recovery Plant (Ansai, Shaanxi, China). According to alpha diversity analysis and community composition, the nutrient injection exhibited a significant impact on indigenous archaea at the genus level. The predominant archaeal genus Methanolobus (more than 66%) in the control well shifted to Methanocalculus (50.8%) and Methanothermococcus (30.6%) genera in the oil production well. Conversely, the activators increased bacterial community richness but reduced its evenness. Bacterial community analysis at the genus level revealed that nutrient injections significantly increased specific populations with the potential to emulsify, lower interfacial tension, and lower oil viscosity, including the genera Arcobacter, Halomonas, and Thalassolituus. At the same time, some microbial species that are harmful for the oil recovery process (e.g., the sulphate-reducing bacteria Desulfovibrus, Desulfocurvus, Desulfocarbo, and Desulfoglaeba), were inhibited. In conclusion, nutrient flooding reduced the abundance of harmful microorganisms and increased beneficial functional microbial populations linked to beneficial functions, contributing to the enhancement of oil recovery in low-permeability oil reservoirs.