By Adam Smeltz | UNIVERSITY PARK, Pa. – Oil produced from shale reservoirs drove record crude output in the U.S. over the past decade, but inefficiencies in extraction often leave as much as 90% of the oil behind, according to the federal Energy Information Administration.
To help maximize production from the tightly packed formations of shale rock, a team of researchers at Penn State developed a new oil extraction workflow that may improve shale oil recovery up to an additional 15% while providing long-term storage for carbon dioxide (CO2) emissions.
The workflow was successfully implemented for the Texas Eagle Ford Shale, where it demonstrated improvement in oil extraction, the researchers said, explaining that it can be expanded for application in other shale reservoirs. They reported their findings in the journal Fuel.
The innovation centers on improving cyclic CO2 injection, a process in which CO2 is pumped into the reservoir to enhance oil production. Also known as “CO2 huff-n-puff,” the decades-old injection method bolsters extraction from naturally occurring rock formations. These rocks contain microscopic pores, called nanopores, where significant portions of hydrocarbons – the main component of oil — accumulate, according to the researchers.
Hamid Emami-Meybodi, associate professor of petroleum and natural gas engineering, faculty associate in Penn State’s EMS Energy Institute and lead author, compared the underground shale environment to a sponge. Similar to the tiny openings in a sponge that fill with water, nanopores effectively soak up and retain hydrocarbons until the surface is disrupted.
“I would call this one of the best recycling systems in the industry,” Emami-Meybodi said of the improved injection process. “Leveraging CO2 to bolster oil production eases environmental impacts, helps fulfill growing energy demand and contributes to the U.S.’ energy independence and security.”
During injection, CO2 is fed into the reservoir through a well. Then the well is shut to allow the injected gas to soak for a sufficient period. The gas mixes with the oil, altering its properties and improving oil mobility and extraction, according to the researchers.
By introducing CO2 in oil mixtures at different pressures, the injection process helps force hydrocarbons out of nanopores and to the surface. But the method’s effectiveness varies widely with changing operational conditions, depths and oil types, the researchers noted.
“Optimizing injection is challenging due to numerous variables – including the oil properties and makeup of the shale environment – that can complicate extraction,” Emami-Meybodi said.
Cyclic carbon dioxide injection, also known as “CO2 huff-n-puff,” is a multi-phase process in which CO2 is pumped into a reservoir to enhance oil production from naturally occurring rock formations. Credit: Hamid Emami-Meybodi. All Rights Reserved.
He and his team collaborated with industry partners to explore improvements in efficiency and productivity, as even small percentage increases in hydrocarbon output can become large boosts in oil production. The largest domestic shale reservoirs are estimated to hold billions of gallons of oil apiece, with the Eagle Ford Shale alone spanning about 20,000 square miles.
The research group also emphasized the opportunity to repurpose shale wells as long-term storage hubs for the injected CO2. Other industries may supply the gas as a byproduct of their operations. Instead of releasing it to the atmosphere, where it would add to pollution, energy companies can leave much of it trapped beneath the surface for years, the researchers said.
“CO2 has a high attraction to attach to organic matter surfaces in shales,” Emami-Meybodi said. “If a goal is to sequester CO2 for the long term, the injection cycles can adjust to push the gas deeper into underground formations and optimize the storage.”
For their study, the researchers focused on the 400-mile-long Eagle Ford Shale, which stretches from south to east Texas and is among five major shale plays in the United States. Current extraction efforts, including hydraulic fracturing, typically capture less than 10% of oil in the Eagle Ford Shale, the researchers said.
They revised the injection methodology by intensifying the hydrocarbons’ exposure to CO2. In their optimization workflow, the researchers covered more surface area with the CO2 and adjusted the number of cycles, pressure, amount of injected CO2 and duration of the injections.
“We determined the modifications may let the injection method draw around 15% more of Eagle Ford’s oil hydrocarbons,” said Emami-Meybodi, who holds the Dr. Charles H. Bowman and Lynn A. Holleran Early Career Professorship in Petroleum and Natural Gas Engineering. “Injecting more CO2 generally enables greater reach into reservoirs and more effective mixing with embedded crude, helping to release more of the oil.”
Further, repurposing shale oil wells for CO2 storage could be a cost-effective way to temper greenhouse gases, said Ming Ma, co-author on the paper and a postdoctoral fellow in the John and Willie Leone Family Department of Energy and Mineral Engineering (EME) at Penn State. His contributions to the research included the development of an in-house numerical model and writing code to simulate oil extraction under different approaches to injection.
“Our big hope is not only to see progress in the efficiency of hydrocarbon recovery from shale plays but also widespread utilization of abandoned shale wells to keep more CO2 out of the atmosphere,” Ma said. “Related work will include applying the new simulation to additional field data and more thoroughly assessing prospects for hydrocarbon recovery.”
Following the boom in shale well development — shale oil production is poised to peak in 2027, according to the EIA — the U.S. faces a surge in so-called “mature” shale plays with diminished production, said Emami-Meybodi, who directs the Subsurface Energy Recovery and Storage Joint Industry Partnership (SERS JIP) at Penn State. Mature and abandoned shale wells represent promising prospects for CO2 storage, he said.
“Every day a well is closed, you’re losing money by not producing oil, but if you’re storing CO2 there, it can generate revenue,” Emami-Meybodi added. “While carbon dioxide can pose a leak concern in most conventional wells, that’s less of an issue in the structure of shale wells.”
Qian Zhang, a doctoral student in EME at Penn State, also contributed to the paper. The research was supported by the American Chemical Society Petroleum Research Fund and the member companies of SERS JIP.
