Analysis of Bacterial Kill Versus Corrosion From Use of Common Oilfield Biocides

Bacterial proliferation is a severe problem in many oilfield systems, especially in aging systems with high water cuts. Depending on the types of microorganisms present, they can cause microbiologically influenced corrosion (MIC) or biofouling of filters, membranes, and metal surfaces. Common oilfield bacteria include sulfate-reducing bacteria (SRB) that can generate hydrogen sulfide (H2S) and iron sulfide (FeS) as a by-product (iron sulfide can occur in different structural forms), acid producing bacteria that can secrete organic acids that lower the pH within the microenvironment of a biofilm, as well as general heterotrophic bacteria that are often important in biofilm formation and maintenance, amongst others. To prevent corrosion or biofouling caused by these organisms, biocides are commonly added to the production fluids. Some concern has arisen that common oilfield biocides may be inherently corrosive at high end use concentrations and could cause general corrosion in the assets they are protecting from MIC. Accordingly, it is important to understand the risk of MIC, souring, and biofouling versus general corrosion from the biocides themselves. To examine the killing efficiency of oilfield biocides versus their corrosive potential, laboratory work was undertaken with five biocide products including: Tetrakis (hydroxymethyl) phosphonium sulfate (THPS), glutaraldehyde, glutaraldehyde / alkyldimethylbenzyl ammonium chloride (ADBAC) mixture, 5-chloro-2-methyl-4-isothiazolin-3-one/2-methyl-4-isothiazolin-3-one (CMIT/MIT), and a cocodiamine (quaternary amine). Each biocide was evaluated at four different concentrations ranging from 10–100,000 ppm of product. Killing efficiency was determined via bacterial kill studies, while wheelbox and bubble cell testing examined corrosion rates. Corrosion rates varied quite substantially from one biocide to the next, especially at high concentrations. Some biocides were found to be only mildly corrosive even at high dosages, while other biocides were much more corrosive at high concentrations. In general, it was observed that biocide corrosivity is directly related to the dosage of the biocide, with higher dosages correlating with higher corrosion rates. On the other hand, biocides were shown to be effective at killing common oilfield bacteria at relatively low dosages. This data suggests that biocides can be effective at killing bacteria at concentrations that do not cause significant amounts of general corrosion. Additionally, the common practice of batch treating biocides minimizes contact time between the biocide and the metal surface, which is in turn expected to minimize any corrosion that would otherwise be attributed to the biocides themselves. Taken together, this data would suggest that the benefit of biocide treatment to prevent MIC and biofouling substantially outweighs any potentially negative impact on corrosion.