Throughout the globe, fractured reservoirs are found in sedimentary basins of varying sizes. The world’s remaining conventional hydrocarbon resources are estimated to be contained in severely fractured carbonate reservoirs, with more than half of these resources located in the United States. These reservoirs, however, have received little consideration as prospective options for carbon dioxide storage. Despite this, they have a great potential for integrating CO2 storage with enhanced oil recovery (EOR).
This article looks at fractured bedrocks in length to determine how these structures act as a natural source of CO2.
What are fractured Bedrocks?
Solid bedrock is not covered by soil. Between the soil and the solid rock is a transition zone of cracked and weathered bedrock exists. Any split in a geologic formation, such as a joint or a fault, that separates the rock into two or more pieces is referred to as a fracture.
In some instances, a fracture will result in a deep fissure or crevice in the rock. Stresses that exceed the rock’s strength often create fractures, leading the rock to lose cohesion along its weakest plane.
Fracture in rock is a three-dimensional process in which fractures propagate in all directions. In addition, it is essential to note that when the fracture develops in size, the microcracks in the brittle process zone are left behind, resulting in a weak area of rock.
This weaker portion is more sensitive to changes in pore pressure, dilatation, and compaction than the rest of the structure. Note that the temperatures and forces near the Earth’s surface are considered in this explanation of the formation and propagation of fractures.
Deep beneath the Earth’s crust, rocks are subjected to very high temperatures and pressures. This leads them to behave in the semi-brittle and plastic regimes, resulting in fracture processes that are substantially distinct from one another.
The stress at the crack tips is distributed between two processes, one of which will promote the fracture’s propagation and the other which will blunt the crack tip.
The material in the brittle-ductile transition zone will show both brittle and plastic characteristics, with the gradual beginning of plasticity in the polycrystalline rock occurring in the transition zone.
Cataclastic flow is the most common kind of deformation. It is responsible for the failure and propagation of fractures due to a combination of brittle-frictional and plastic deformations.
Hydraulic fluids, such as water may pass through fractures and into surrounding areas. Owing to their substantial permeability and fracture porosity, highly fractured rocks may serve as excellent aquifers or hydrocarbon reservoirs.
Due to the difficulty in sampling this altered rock, the experts rely on a unique sampling instrument buried in the hillside. It is about 44 feet long, extending from the top of the cracked bedrock to its base.
This tool rapidly determines whether a certain area is an active site of CO2 generation.
Fractured Bedrock as A Natural Source of Carbon
The University of Texas at Austin conducted research that found CO2 may be generated further down in bedrock cracks. This source can account for up to 29 percent of the daily average CO2 released by the land, depending on the season.
This study is supported by The Department of Energy, the Geological Society of America, and the National Science Foundation.
The researchers discovered a link between CO2 production in the rock and the seasonal uptake of water by deep tree roots located many meters below the surface of the Earth. This finding suggests that tree roots and the microbial communities surrounding them are the sources of the CO2 — and that bedrock fractures are an ideal environment for life to flourish.
By examining hundreds of samples taken between 2017 and 2019, the researchers found that the CO2 did not remain in the cracks for long periods. During the dry season, CO2 is mainly transported upward into the soil, then released back into the atmosphere.
Since almost half of the CO2 dissolved into the water during the rainy season when groundwater rose to fill the cracks, nearly 50 percent of it ultimately flowed into streams and rivers.
This discovery does not imply that landscapes release more CO2 into the atmosphere, but it does call into question the traditional knowledge about where CO2 is generated. It may also aid in the improvement of climate change models since knowing how and where CO2 is generated is critical to producing accurate predictions.
This research adds to a growing body of evidence that fractured bedrock is an ecologically significant area. Rempe and colleagues, for example, discovered evidence of rock moisture in cracked rock supporting plants during droughts in a 2018 research.
The study is significant because it sheds light on a landscape section that is thought to be a “black box” between the soil and groundwater.
Fractured bedrock is quite prevalent in Texas, where the soil is shallow and has many deep roots.
It may be an essential component of the carbon cycle in these ecosystems, and understanding it will be crucial as time goes on and the climate changes.
A Natural Resource of CO2 For enhanced oil Recovery
With the evidence of fractures in the bedrock, these features indeed play a significant role in enhanced oil recovery projects where CO2 is injected into the oil reservoirs to push out residual oil.
A combination of oil, natural gas, and a part of the injected CO2 flows into adjacent wells and is generated at the surface throughout this process.
CO2 is collected from this production stream and re-injected in a closed-loop process, resulting in further oil recovery. Almost all the CO2 injected into a field gets trapped underground over time, filling the pore space remaining after the oil and accompanying gas is extracted.
CO2 EOR, which is applicable to a wide range of appropriate oil and gas reservoirs, may improve ultimate oil and associated gas recovery by 10 to 25 percent in areas where it is used.
However, it’s important to consult professionals before implementing this strategy in your oil field. Professional consultancy companies like Melzer Consulting have the ability to assess your project area and advise accordingly on the feasibility of carbon capture and storage in your project.