Geothermal power and the oil and gas industry are distant cousins that seldom visit. But they really should have a family meeting because the latest advances in small-scale power generation and some new thinking around subsurface heat transfer could both benefit the petroleum industry by reducing its emissions footprint and provide western Canada an additional source of electricity.
Power generation from geologic hotspots close to the earth’s surface is not new. California has been doing it for decades, but the problem is there aren’t many suitable locations where the economics work.
CO2 plume geothermal (CPG) could change that. Swapping out water with supercritical CO2 in power generators and subsurface potentially improves the economics of geothermal and lowers the bar on the required temperature in reservoirs.
“Almost every existing geothermal field in the world requires temperatures of 160 degrees Celsius and up,” says Jimmy Randolph, inventor of the CPG technology and a researcher at the University of Minnesota. “Our system typically needs just 90 degrees Celsius. We can even go a little lower, but that’s where we typically want to be to make it easy.”
Randolph is also the founder and chief technical officer of TerraCOH, a Minneapolis-based company that has sublicensed CPG for application in the oil and gas industry. Led by John Griffin, the company’s president and chief executive officer, TerraCOH sees oil and gas producers benefiting from these power-generation advances by improving operational efficiencies, providing additional revenue and opening the door to lower-cost CO2 sequestration.
Tiers of deployment
Much has changed in electric power plants since Thomas Edison’s era, but what turns heat into electrons hasn’t changed since the age of steam. Whether burning coal, concentrating sunlight or splitting atoms, most thermal power plants heat water into steam to drive a turbine.
By replacing the steam with supercritical CO2, engineers have now unlocked up to 50 per cent greater thermal efficiency. Griffin says that GE and other companies are already building these super-efficient CO2 turbines, which are smaller, cheaper to build and easier to transport.
“Just to be clear, we don’t make the actual power systems,” Griffin says. “Our ideas are based on using other people’s power systems in a new application.”
Cogeneration
The simplest application in oil and gas is to add a CO2 generator as a form of small-scale cogeneration. The most elaborate application sees the injection of CO2 pumped into a hydrocarbon reservoir where it absorbs and reacts with the heat that then drives a CO2 generator at the surface.
A CO2 generator potentially allows most any oil and gas operation to use its production fluids to produce electricity, according to Randolph.
“Produced fluids bring heat to the surface that is then dissipated and not used for anything. We can use that heat to produce an additional revenue stream by creating electricity from it,” he says.
That power can be used for on-site requirements or sold into the grid. Since oil and gas operations run around the clock, this power is baseload energy that doesn’t fluctuate—unlike solar or wind.
Similar efficiency gains were realized in the oilsands well over a decade ago, when cogeneration was installed. The smaller size and efficiency of CO2 generators now makes it an option in the wider oil and gas industry.
The same scenario would also work in an oilfield using CO2 as an oil-recovery method, “but ultimately, we are looking to develop new sites where we put CO2 below ground explicitly for the purpose of harnessing geothermal heat,” Randolph says.
Plume
CO2 plume cogeneration, with its lower downhole temperature requirements, holds significant potential in western Canada because of the thousands of wells that have already been drilled.
“Many of these wells are good candidates for power generation,” Randolph says.
CO2, when injected into a reservoir with suitable temperatures and caprock, creates a large subsurface plume that absorbs the formation heat that can then be harnessed for power generation.
Despite the relatively low heat density of sedimentary basins, their large accessible volumes and the high mobility and thermal expansiveness of supercritical CO2 compared to water provides a strong thermo-siphon within the formation.
The thermo-siphon effect eliminates the need for parasitic pumping power requirements and significantly increases the electric power production efficiency of the geothermal system compared to water-based geothermal systems.
From a carbon emissions and carbon credits perspective, the injected CO2 can ultimately be geologically sequestered, resulting in a geothermal power plant with a negative carbon footprint.
Alternatively, if geologic CO2 storage is uneconomic, CPG systems could be operated with a limited, finite amount of CO2 re-circulated through the system. This kind of power operation could run with little or no makeup CO2.
“The combination of using non-water fluids in both the power generator and underground makes a lot of areas that weren’t viable for geothermal economically viable now,” Randolph says.
Compared to water-based geothermal power systems, this non-fluid variation produces two or three times as much power, according to Randolph.
“So even if you have to pay for the CO2, we think that between carbon credits as one source of revenue and a substantial increase in total electricity production efficiency, there is still better economics in this scenario,” he says.
Storage
Another variation of CO2 plume technology is energy storage.
“These deep subsurface formations can serve as large earth batteries where you store heat and pressure and then produce that energy when it is needed,” Griffin says.
TerraCOH is a second round semi-finalist in the NRG COSIA Carbon XPRIZE competition, a US$20-million global competition in search of a breakthrough technology to convert CO2 into high-value products. In June, the Clean Innovation Investor Forum and Carbon XPRIZE Summit heard Griffin’s investor pitch to build a demonstration plant for what he calls an earth battery solution.
Griffin says the idea was well-received because the technology also has the potential to store vast quantities of CO2 in the earth permanently. While the earth battery is geared to storing energy from low-carbon, intermittent power sources, such as solar and wind, it also targets the use of existing oil and gas wells and reservoirs.
“So we think there is a natural synergy with the oil and gas service industry. Those are the relationships that we are now trying to build,” Griffin says.
Depending on how quickly it can raise the capital and secure grants, TerraCOH expects to have a demonstration project running within 18 months.
