Scientists use 4D microtomography to discover how gas and foam extract oil trapped in rock
Researchers have been able to document changes occurring at the micro level in carbonate rock using enhanced oil recovery methods.
“One of the main problems with mature oil fields is that after long-term development, a significant portion of the oil remains ‘locked’ in the rock. Conventional water or gas injection often works unevenly: the fluid passes through the most permeable channels, leaving smaller, less accessible pores largely untouched. As a result, some reserves remain unclaimed because extracting them is difficult and expensive,” says Rail Kadyrov, Associate Professor of the Department of Regional Geology and Mineral Resources.
One effective solution to this problem, he said, is foam injection – a mixture of gas and a surfactant solution.
“Such foam can partially block ‘fast’ channels through which gas escapes too easily, and redirect the flow to zones where oil still remains. This is especially important for carbonate reservoirs, which often have a complex and heterogeneous pore structure,” the geologist emphasizes.
To study the changes occurring in the rock during foam injection, the researchers used a unique 4D microtomography method.
“We scanned the rock sample at different stages of displacement to see how oil, gas, and foam are redistributed within the pore space,” explains Kadyrov. “The experiments were conducted on limestone mini-cores 5 millimeters in diameter at a temperature of 50 degrees Celsius.”
The scientists conducted experiments under two scenarios: the first used nitrogen to displace the oil, and the second used carbon dioxide. After applying the gas, the samples were treated with foam.
“When moving through a denser sample, nitrogen quickly formed a preferred channel. This means that the gas began to follow the most favorable path, leaving some oil behind in the main flow. After transitioning to nitrogen foam, flow resistance increased sharply, and oil was further displaced from previously bypassed areas,” adds the Associate Professor. “The picture was different for carbon dioxide. When injected into a carbonate sample, CO₂ moved differently than nitrogen: it didn’t form a single, early, dominant channel, but spread more dispersedly, gradually displacing oil from large and medium-sized elements of the pore network. This is due to the specific interactions between CO₂, oil, and pore structure. At supercritical pressure, the foam didn’t reach the sample in a stable form, whereas when the pressure dropped to subcritical conditions, it began to effectively enter the core and further mobilize oil, displacing it from the pores. Experiments with CO₂ foam showed that its effectiveness depends on the pressure and conditions of delivery to the sample.”
Experts found that, under both scenarios, foam was most effective at extracting oil from medium-sized pores. The smallest pores remained the most stable safe haven for residual oil.
“Using a unique research method, 4D microtomography, we were able to not only measure the resulting increase in oil recovery but also see how it occurs: where exactly the flow passes through the rock, which zones are bypassed, and under what conditions the foam begins to act as a flow redistribution tool,” concludes Rail Kadyrov.
The results are presented in a paper published in the Arabian Journal for Science and Engineering, allowed the researchers to determine what exactly needs to be done to enhance oil recovery from mature fields. Specifically, it was shown that the effectiveness of gas-foam treatment depends on the foam composition, pore size, and rock permeability, as well as the pressure and conditions of reagent delivery to the reservoir.
The work was carried out using funds from a subsidy allocated to Kazan Federal University for the implementation of a state assignment in science (project No. FZSM-2023-0014).
