RIASSUNTO
Abstract
During the 1980's, studies initiated to resolve problems due to the activity of sulfate-reducing bacteria in oilfield systems were instrumental in the early recognition of the importance of biofilms in natural environments, the use of radiotracers to measure bacterial activity, the application of molecular techniques to study nonculturable bacteria and the detection of previously unknown Archaea in subsurface aquifers.
Over the past 15 years, however, oilfield microbiology practices have not kept pace with other fields of environmental microbiology research and now the ideas and practices applied in the oilfield lag significantly behind the most recent scientific advances. This is despite the fact that the oil industry is currently attempting to control very diverse and extensive sulfide producing microbial populations by the application of nitrate to bring about a shift in the population dynamics in a process of biological competitive exclusion.
Environmental microbiology is now in the midst of a revolution in the understanding of the marine and subsurface microbial world, much of which is resulting in completely new concepts of the interaction between microbes and the environment and vice versa. These advances must be recognized and wherever possible incorporated into oilfield microbiology technology.
This paper describes how the application of even a few of the recent advances in environmental microbiology offers a huge potential to improve our understanding of control and remediation of sour reservoirs using nitrate treatments.
Introduction
The application of nitrate treatments as an alternative to biocide for the control of the activity of sulfate-reducing bacteria and Archaea in seawater waterfloods has progressed faster than our ability to understand and effectively monitor the complex microbiological mechanisms stimulated within heterogeneous oil reservoirs and seawater injection systems. Whilst the increasing application of the treatment has been driven by case histories reporting success [1, 2] there are still a number of directions by which further optimization of the technology could be addressed:
Until recently, treatment recommendations were based upon historic application case histories (from other industries), laboratory tests and the stoichiometry of simplistic nutrient interactions.
The conditions within the systems that are being treated may be known (i.e. topside water injection facilities) but are mainly unknown (i.e. within the subsurface formation).
The mechanistic action of nitrate treatment is not defined and it may involve any one of, or a combination of, at least four processes.
There are no standardized methodologies for monitoring treatment effectiveness nor for defining Key Performance Indicators (KPI's) for benchmarking treatment effectiveness.
Lab studies employ planktonic batch bioreactors, whereas continuous biofilm mesocosms are far superior models.
There have been logistical issues with regard to the transportation of large volumes of liquid product.
Whilst there will be significantly more work required before the full mechanistic action of biocompetitive control of sulfate-reduction activity in a nitrate treated system is elucidated, there have already been advances which improve our understanding of treatment efficacy and treatment optimization which have not yet been fully exploited by those charged with dosing, monitoring and optimizing nitrate treatments in the field. Whilst guidance documents are available [3, 4], universally applied protocols have not yet been developed such that equivalent inter-field and inter-company generated data can be compared and interrogated to improve our overall understanding of the microbial mechanisms involved.