Diagnosis and Removal of Microbial/Fines Plugging in Water Injection Wells

Abstract

SUMMARY Because bacteria are naturally present in the water12 used in water injection for secondary oil recovery, these organisms adhere to available surfaces and produce biofilms3 just as they do in all aquatic ecosystems examined to date4. The bacterial biofilms in individual water injection wells develop to an extent dependent on the nutrient supply in the injection water. Organic contaminants are converted to bacterial biomass, and to bacterial exopolysaccharide slimes. Some organic compounds (notably lactate and acetate) are oxidized by anaerobic bacteria that use sulphur compounds as hydrogen acceptors and produce H2S. This souring process is dependent on the input of organic nutrients and it is the function of a heterotrophic bacterial population that is concentrated very close (<3 meters) to the well bore. The generation of H2S in the bulk of the water-flooded formation is severely limited by the very low levels of available bacterial nutrients in these areas and most H2S production is limited to the immediate viscinity of water injection and production wells. Injection well back flow data12 and injection well cleaning data2 both demonstrate, unequivocally, that heterotrophic bacterial growth in a waterflood operation is limited to areas very close to injection and production well bore holes. These areas are readily accessible to biocide treatment. Bacterial biofilms develop very rapidly on the extensive surfaces of artificial and natural rock cores14, and these slime-enclosed adherent bacterial populations can reduce permeability by direct plugging. Treatment with 5-12% sodium hypochlorite (bleach) is sufficient to remove these bacterial accretions in both field2 and laboratory5 experiments, and treatment with sufficient bleach (3-4 tubing volumes) is regularly used to clean injection wells plugged by bacterial growth. We have recently shown6 that the bleach concentration used to remove bacterial biofilms can be reduced to ±1% without loss of efficacy. Bacterial biofilms trap particulate material8 and injection wells in systems with large amounts of particulate material in the injection water become plugged with biofilms impregnated with these particles. Bleach alone is not effective in cleaning these microbial/fines plugged wells but a combination of bleach and acid (in that order, with a water "spacer") is very effective5 because bleach removes the biofilm and allows access of acid to acid-soluble fines. Our developing general understanding of the nature of injection well plugging allows us to custom-design a cleaning protocol to remove this damaging material and restore original levels of water injectivity.