The potential for the use of existing energy infrastructure will likely influence the direction in which Louisiana's energy system is developed. The nearly 5,000 abandoned or nonproducing (depleted) oil wells currently spread throughout the state pose a significant economic and environmental hazard.
The Louisiana Department of Natural Resources has estimated that over 4,700 of these orphan wells need to be monitored and managed with the eventual goal of being plugged (Louisiana DNR Report). Although the abandoned wells are generally viewed as liabilities left behind by the previous generation of energy production, what if they could be utilized as productive assets for the next generation of energy development?
Petroleum Engineering Students May Find Opportunity in Microbial Hydrogen Production
When considering the use of depleted oil reservoirs for microbial hydrogen production, rather than abandoning the wells completely, there is potential for the use of the wells as subsurface platforms for producing clean energy. Petroleum engineering students and young professionals may find this option especially appealing since it provides the potential to combine their knowledge of subsurface processes with one of the most talked about transition fuels: hydrogen.
One of the primary bacteria used to produce hydrogen is the hyperthermophilic Thermotoga maritima. When grown under high-temperature conditions, T. maritima produces hydrogen gas (Pradhan et al., 2015). As a result of the elevated temperatures present in some Louisiana formations, such as the Tuscaloosa Formation (80 to 100°C), these formations may support microbial hydrogen production. Fig. 2 shows an example of an injection wellsite in Vermilion Parish, Louisiana.
In general, the basic premise of the concept is simple. Depleted reservoirs contain heat, flow paths, residual hydrocarbons, and wells. If the proper subsurface geochemical conditions are present, microorganisms can be introduced or stimulated to convert available substrates into hydrogen. Thus, instead of being nothing more than a relic of past energy production activities, the well becomes part of a new low-carbon energy system.
Reservoir Characterization Is the Basis of the Concept
Before initiating any type of downhole biological activity in the reservoir, engineers must have a thorough understanding of the chemistry and physics of the reservoir. Engineers will take brine samples for analysis of salinity, pH, and concentration of trace minerals and nutrients and will map temperature gradients and pressure conditions (Pradhan et al., 2015). Reservoir characterization is important in a reservoir such as the Tuscaloosa since microbial performance is highly dependent on the local reservoir conditions.
Once the reservoir characterization phase is completed, the second stage of the process will be the microbial optimization stage. While T. maritima has demonstrated an ability to produce hydrogen efficiently in laboratory studies, a reservoir is clearly not a laboratory flask. The microorganism must function at elevated pressures, interact with complex fluid chemistries, and maintain viability in environments that have a heterogeneous distribution of nutrients and hydrocarbons. It is expected that the interaction between petroleum engineering and microbiology will play a critical role in determining whether microbial hydrogen production can progress from concept to field implementation. Pilot projects, such as the installation of downhole biofilm carriers, aim to test these concepts in the field. However, as Fig. 3 shows, aging infrastructure presents challenges that must be addressed.
An example of a possible pilot project design includes the placement of a downhole biofilm carrier or bioreactor support system in an existing well. The system would serve as a substrate for microbial growth and provide the operator with the ability to monitor several parameters including pressure, temperature, amount of gas generated, and produced fluid composition (Southern States Energy Board, 2021). This approach is still at the R&D stage; however, it represents a trend that is growing in the energy sector: utilize the subsurface infrastructure for purposes other than hydrocarbon extraction.
Microbial hydrogen production is a promising area of research, but it is currently far from being a commercially viable option. There are several technical and financial challenges associated with developing this concept. For instance, while hydrogen production has been demonstrated on small scales in laboratories and pilot plants, large-scale field demonstration has not occurred.
Additionally, there are many requirements for the separation, handling, and purification of gases prior to their use as hydrogen, which may complicate the deployment of this technology. Differences in reservoir heterogeneity among different locations may cause variability in performance, and regulations governing the conversion of older oil fields to biological energy systems are still being developed. Due to these factors, microbial hydrogen production should not yet be viewed as a commercial-ready solution, but rather as a promising research path that deserves additional focus by the engineering community.
The interest in hydrogen is increasing globally due to its value in refineries, the manufacture of ammonia, the production of methanol, and potentially in heavy-duty transportation. At the same time, there is an increasing recognition of the role that the subsurface can play in terms of energy storage, carbon management, geothermal development, and other emerging systems. Therefore, microbial hydrogen production in depleted oil fields represents a natural fit in the larger paradigm for viewing the subsurface as a platform for facilitating cleaner energy pathways. This paradigm represents a vision for the future in which the subsurface is no longer viewed as primarily a source of fossil energy resources, but instead as a resource base that can be used to develop cleaner energy options.
This transition to a new paradigm for viewing the subsurface is significant for the next generation of petroleum engineers. In the very near term, the majority of petroleum engineers will be working at the intersection of conventional oilfield expertise and newer energy technologies. Their work will require them to think about not only producing hydrocarbons, but also about how to repurpose existing subsurface assets for newer energy applications. This requires thinking outside the box and viewing legacy oilfield infrastructure in ways that were not possible in the past. What was once thought to be a liability (i.e., the presence of orphaned wells) could now potentially be viewed as a valuable asset as part of a future hydrogen economy if the technical viability and economic feasibility of microbial hydrogen production can be demonstrated.
In Louisiana, the orphaned well topic is typically discussed as a legacy issue. However, for young professionals and students who are interested in energy issues, orphaned wells can be viewed as an opportunity to influence the direction of the energy industry going forward. If the technical viability and economic feasibility of microbial hydrogen production can be demonstrated, then what was once viewed as a sign of decline can be viewed as an example of creative energy innovation. Even the possibility of using the subsurface to produce hydrogen is sufficient to motivate further exploration of this concept.
For those now pursuing a degree in petroleum engineering and for the next generation of petroleum engineers, the concept presents an important message. The energy transition has led to the perception that petroleum engineering has been relegated to the "past." However, the reality is that many of the transition technologies require the same technical skills that petroleum engineers have developed. Examples include well integrity, reservoir management, fluid systems, risk assessment, and subsurface monitoring that are required in emerging applications such as geothermal energy, carbon sequestration, underground hydrogen storage, and potentially microbial hydrogen production. The energy transition may broaden the scope in which petroleum engineers can apply their expertise, rather than making their expertise obsolete.
The energy transition may not replace petroleum engineering skills—it may simply redirect them toward new subsurface applications such as hydrogen, geothermal energy, and carbon storage.
For Further Reading
Progress Report: State Efforts To Address Orphaned Oil and Gas Wells, Office of Conservation—Department of Energy and Natural Resources.
Hydrogen Production by the Thermophilic Bacterium Thermotoga Neapolitana by N. Pradhan, University of Cassino and Southern Lazio; L. Dipasquale, Istituto di Chimica Biomolecolare; G. d’Ippolito, Telematic University Pegaso; et al.
Gulf Coast Stacked Storage Project, Southeast Regional Carbon Sequestration Partnership
Hydrogen Shot: An Introduction, US Department of Energy
