For decades, reservoir characterization has played a crucial role in oil and gas projects – in identifying and extracting hydrocarbons from the subsurface. Now, some geologists and geophysicists are applying their industry expertise to the emerging fields of carbon storage and geothermal energy.
“We have recognized that our skillsets in evaluating and characterizing reservoirs is well suited for the energy transition,” said Patrick Elliott, co-founder and chief operating officer of Carbon Alpha, a Calgary-based company that facilitates decarbonization for industrial emitters. “This is a wonderful opportunity to grow geological and geophysical applications. The energy transition presents an opportunity for people to continue to work in the geosciences.”
Having co-founded two oil and gas companies in the past, Elliott and his business partner, Simon Bregazzi, co-founder and CEO, chose to change directions in February this year when they founded Carbon Alpha. Understanding that Canada has set a goal of decarbonizing roughly 300 megatons of annual CO₂ emissions (of the roughly 750 megatons it currently produces) by 2030, Elliott set his sights on being a part of that “massive” task.
“The reality is we will need lots of carbon sequestration, and it’s important we do a good job of characterizing reservoirs,” he said. “Developing credibility for successful CCS (carbon capture and storage) projects is necessary to facilitate and build out the CCS industry. The public needs to know that CO₂ is safely handled and will not adversely impact the health of people or the environment.”
Geologists and geophysicists also are using their knowledge and skills to bring geothermal energy into the new energy mix. In Denmark, for example, assessments show that where geothermal resources are present, geothermal energy could provide 20 to 50 percent of heating demands for hundreds of years, said Kenneth Bredesen, a geophysicist with the Geological Survey of Denmark and Greenland.
“Through recent research activities, we demonstrated how seismic reservoir characterization tools could be adopted from the oil and gas industry to de-risk geothermal prospects in a more quantitative manner,” he said. “Seismic reservoir characterization can play an important role to optimize site locations, which can be the tipping point between success and failure for future geothermal projects.”
The Right Reservoir
“All countries have come up with climate goals. To cut emissions, CO₂ has to be injected into formations,” said Satinder Chopra, editor of the AAPG EXPLORER’s Geophysical Corner and founder and president of SamiGeo, a geophysical company based in Calgary. “Many geologic formations have been identified worldwide that have the potential for large-scale CO₂ storage.”
Injection of CO₂in any reservoir is not a straightforward process and requires specific geological and geophysical data, Chopra explained.
When scoping out reservoirs for CO₂ storage, geoscientists must look at three fundamental requirements: containment capability, volume capacity and injection efficiency, Elliott said.
Containment ensures that injected CO₂ remains permanently in the reservoir. The reservoir’s capacity must be accurately calculated to avoid exceeding its limit and compromising containment. And, the ability to inject CO₂ at a high rate reduces the overall “horsepower” requirement for compression and can reduce the overall project cost.
“This is much in the same way that a high permeability reservoir is attractive from an extraction point of view,” Elliott added.
At present, the province of Alberta is not allowing depleted oil and gas fields to serve as carbon storage sites – though other provinces allow it. As a result, the focus is on deep saline reservoirs. Although they can be excellent candidates for CO₂ storage, they also have drawbacks.
“They are non-hydrocarbon bearing and were not a target for the oil and gas industry, so there is significantly less data available for assessment, which can increase risk,” Elliott said.
Saline reservoirs are also typically deeper, which can drive up project costs.
Yet, saline reservoirs have launched a renaissance in exploration. “They have reintroduced the concepts of exploration that have been absent for about two decades now because we’ve been focused on unconventional development in clastic systems,” Elliott explained. “The skillsets involved in carbonate rocks exploration have largely been ignored but will be required to assess many of the storage opportunities.”
From a risk and financial point of view, depleted oil and gas reservoirs are ideal for CO₂ storage.
“In many ways, we are completing the carbon cycle from the extraction of hydrocarbons for their useful purpose to the capture and storage of CO₂ into the same depleted reservoirs,” Elliott said.
From a surface operations standpoint, depleted oil and gas fields are likely to have in-place infrastructure, such as roads, and established pipeline rights of way and surface landholder agreements. From a reservoir standpoint, there is likely an abundance of data including well logs, cores, flow rates, initial reservoir pressures and volumetric certainty – all of which can lower a project’s risk and budget.
Such data aids in analysis to ensure that pressure from CO₂ injection doesn’t breach the integrity of the reservoir seal or create a situation in which well bores become leak points, Elliott added.
“Seismic will play a critical and familiar exploration role to assist in defining reservoir attributes, potential containment risk due to structural discontinuities and tectonism and so on,” he added.
It will also play a role during the storage and monitoring phase of a CCS project, as the CO₂ plume develops in the reservoir over time.
Chopra explained that the ideal depth range of a storage formation should be between 800 and 3,000 meters, and that the reservoir rock should have high porosity and permeability for better CO₂ injectivity.
“For a reservoir rock with low permeability, the injection pressure does not have sufficient time for stabilization and could result in overpressure zones,” he said.
Reservoir characterization also will go hand-in-hand with building public confidence in carbon storage projects.
“Regulator and government credibility is critical because they have been entrusted by the public to provide technical oversight and assess operators’ plans and operations,” Elliott said. “And, carbon trading markets are reliant on assurances that CO₂ storage is measured and permanent because the financial instruments created by sequestration support carbon credit valuations. It will take time to get this industry going, but it’s going to be big, and it will be here for a long time.”
Full Steam Ahead
The Pacific Ring of Fire is a common term in the world of geothermal energy. Also called the Circum-Pacific Belt, it is a path along the Pacific Ocean characterized by active volcanoes and frequent earthquakes. In these locations throughout the globe, geothermal energy can most easily be accessed when groundwater at shallow levels is heated by molten rock from the Earth’s hot core. Geysers in Iceland and in California serve as examples of harnessing this energy.
Yet outside such locations, geothermal energy can still be a reliable energy source, in particular for heating homes and buildings, Chopra said. For example, cold water can be pumped through a hot sandstone reservoir, creating an upward flow of hot water through wells and ultimately be transferred into heat.
Reservoir characterization is crucial to identifying an ideal reservoir rock that allows for water percolation, in addition to the quality of the cap rock, and the thickness, temperature and velocity of the aquifer formation, Chopra said.
“Most of us are not situated on top of volcanic spreading ridges, but many countries have, for example, sedimentary basins at their disposal where hot water can percolate through porous reservoir rocks, faults or fractured zones,” Bredesen said. “If such subsurface conditions are situated close to densely populated areas, a geothermal plant can potentially contribute to a green energy mix, whether it be for heating or power generation.”
Denmark is in the process of establishing a regulatory and economic framework to move geothermal exploration and development forward to help reduce reliance on biomass for heating. The country has committed to reducing emissions by 70 percent below 1990 levels by 2030.
The Danish subsurface contains ideal geological conditions with widespread low-enthalpy geothermal aquifers that could be linked to the country’s district heating network – potentially reaching 60 percent of Danish households, Bredesen said.
Currently, three minor geothermal plants have been established and shown to produce hot water with temperatures between 45 to 70 degrees Celsius from 1- to 3-kilometer-deep sandstone aquifers.
Denmark is looking to the latest seismic technologies to “remove remaining geological, technical and commercial barriers” to further develop its geothermal resources, Bredesen added.
“Porosity and permeability are key geothermal reservoir parameters that we predicated in a case study based on seismic and well data located 30 kilometers north of the Danish capital,” he said. “The results improved our ability to resolve the spatial distribution and geothermal reservoir properties based on inversion and rock physics principles, as commonly used in the oil and gas industry.”
Seismic also plays a key role in mapping faults and fractures, which can help determine if permeability is conducive to geothermal extraction or if there could be a risk for accessing the required volume of hot water.
Many also are looking at sourcing geothermal energy from former oil and gas wells – a process that would require the wells to be injected with water should reservoir characteristics prove favorable, Chopra said.
The wells also would have to meet certain criteria for construction materials to avoid long-term problems with pipe corrosion and coating from circulating aggressive saline water, Bredesen added.
Because drilling new wells is expensive and geothermal budgets are often modest, it is crucial to fully utilize existing borehole data, such as log measurements and core samples, to de-risk geothermal projects from oil and gas wells.
However, drilling new geothermal wells will likely be inevitable to conform geothermal reservoirs built from remote sensing data in frontier areas, Bredesen added.
“Geothermal heat production has fairly low operational expenses, making it operationally competitive with, for example, air and seawater heat pumps,” Bredesen said. “And, excess geothermal heat can be stored in large, hot water pits and serve either as a buffer during the summer or be saved for winter use.”
Globally, 83 countries utilize geothermal energy, and its application for district heating purposes is growing fast in Europe. As a source of renewable energy for both power and heating, some predictions say that geothermal has the potential to cover 3 to 5 percent of the global demand by 2050, and even 10 percent by 2100.
“When we talk about exploiting the Earth’s heat, every intuition points at geoscience as one of the key competencies needed to unfold its full potential,” Bredesen said. “Many of the technical barriers as part of the green transition have some geoscientific component, whether it’s geothermal, CO₂ storage, wind farms or mineral exploration. So, let our geoscientific skills pave the way for sustainable energy and low-carbon solutions into the future.”