Depleted horizontal oil and gas wells could have a second life storing renewable energy, according to researchers at the U.S. Department of Energy’s National Renewable Energy Laboratory.
Because renewable forms of electricity generation like solar and wind require low-cost energy storage, the NREL researchers propose using depleted hydraulically fractured oil and gas wells to store electrical energy in the form of compressed natural gas to be released to spin an expander/generator when electrical demand is high.
“We started exploring the economics of the idea in 2016 through the Department of Energy-sponsored Lab-CORPS program,” said project spokesman David L. Young.
Young is senior scientist and group manager with the High Efficiency Crystalline Photovoltaics Group at the NREL in Golden, Colo. He has been with NREL about 20 years.
Lab-CORPS is a specialized technology accelerator and training curriculum that will enable lab-based teams to gain direct market feedback on their technologies and pursue the development of startup companies, industry partnerships, licensing agreements and other business opportunities.
Young explained that, following the 2016 DOE program, they secured funding to do a technical modeling analysis, which resulted in the publication of two papers:
- “Evaluation of Energy Storage Potential of Unconventional Shale Reservoirs Using Numerical Simulation of Cyclic Gas Injection”
- “Electrical Energy Storage Using Compressed Gas in Depleted Hydraulically Fractured Wells”
They dubbed this new technology “REFRAES,” for “REpurposed FRAc’ed wells for Energy Storage.”
“All told, the REFRAES idea allows oil/gas energy companies to turn underproducing wells from stranded liabilities to money-producing assets, with a very low carbon energy storage cycle.”
“We’ve had four researchers from NREL, three from Colorado School of Mines, and several from a private oil/gas company in Denver involved with the research,” Young said.
The Mechanics of REFRAES
As electrical grids diversify to renewable energy technologies to decrease costs or reduce carbon production, low-cost storage solutions will be needed to time-shift the energy, both daily and seasonally, to coincide with peak demands, the experts wrote.
In their research, the team noted that compressed air energy storage projects store electricity by using off-demand power to run compressors to inject air into man-made salt caverns in salt domes, but could also use hard rock or porous rock caverns or existing mines or saline aquifers.
Unfortunately, they said, CAES plants are geographically limited, owing to the need for geological salt dome formations or off-shore saline aquifers.
Young said they used natural gas in their model because it is readily available at most wells through the gathering line system. Also, natural gas is compatible with the chemistry in the well, whereas air might allow a well fire.
“We envision companies that own wells might be interested in using the technology to transition under-performing wells to REFRAES to not only enhance oil recovery, but to secure long-term profits from their assets through renewable energy storage rather than shutter or plug the wells,” Young said.
“Several companies have shown interest in REFRAES and we have identified a test well to do field tests. We are currently looking for field test partners,” he added.
REFRAES uses unconventional shale and tight sandstone dry gas wells that have been hydraulically fractured. These sites are numerous in the United States and most are conveniently located in areas with abundant wind and solar resources, especially in the West, the NREL team said.
According to the NREL team, REFRAES uses the existing infrastructure of frac’ed well sites, takes pressurized natural gas from gathering line pipes in an oil and gas field and compresses the gas back down the well hole using electrical energy from excess renewable sources. When electrical energy is called for, the gas is released from the well and sent through a turboexpander/generator, unburned, to produce electricity and then sent back to the gathering line manifold where it can be processed.
“In addition to the efficient, carbon-free storage cycle, the proposed concept has several other advantages over conventional CAES, including a larger number of available sites spread geographically across the U.S., lower risks in reservoir development, and lower capital costs,” said Young. “Few locations possess suitable geology for conventional CAES development, and mining subsurface storage reservoirs (such as salt dome caverns) carries inherent project risk and uncertainty that can derail projects.”
The Economics of REFRAES
In horizontal drilling, several wells are usually drilled from a single pad because these wells tend to decline in production in a relatively short time period.
The REFRAES concepts become increasingly feasible as more and more wells reach the end of their profitable lifetime.
“Additionally, these wells are already drilled, fractured and completed with a known production history, so they can be easily vetted for energy storage suitability prior to the start of any construction, greatly reducing geologic risk to the project,” Young said.
Transitioning the wells by adding a compressor and expander at the surface gives owners a financial incentive to make the switch, because they can earn revenue from generating electricity at peak use periods.
The NREL researchers said many projections anticipate using a mix of fossil fuel and renewables over the next few decades during the transition away from fossil fuels.
“Thus the natural gas infrastructure will likely remain during the global transition from fossil fuels to renewable energy. The use of gathering line natural gas should be considered a gateway gas to transition wells to REFRAES, as other gases could also be used within the closed cycle system,” Young said.
“All told, the REFRAES idea allows oil/gas energy companies to turn underproducing wells from stranded liabilities to money-producing assets, with a very low carbon energy storage cycle. It may also offer incentive for companies to claim abandoned wells that are typically a liability for area governments,” he added.