It is estimated that more than 50% of original-oil-in-place remains unrecovered after the primary and secondary phases of oil recovery. Enhanced oil recovery (EOR) is a tertiary recovery phase that commonly uses gas injection or miscible flooding. One type of EOR, carbon dioxide-enhanced oil recovery (CO2-EOR), has attracted global attention due to the decline of oil reserves.
A key technology to help reduce greenhouse gas emissions, and by extension limit the effects of climate change, is carbon capture and storage. This process can be balanced with CO2-EOR as the process requires carbon to be injected into the rocks in order to force out any unrecovered oil reserves, allowing oil to be harvested but also capturing and safely storing the greenhouse gas.
Researchers from the Greek Institute of Nanoscience and Nanotechnology, NCSR “Demokritos” and Khalifa University, Abu Dhabi, have worked in collaboration to investigate the CO2-EOR process. The NIMROD instrument at the ISIS facility was used to perform neutron scattering measurements on prepared samples of reservoir rocks - candidate materials that could be used for geological sequestration - to create a methodology for monitoring the CO2-EOR process in real time at the nanoscale level. This was done by injecting supercritical CO2 into a limestone sample loaded with deuterated n-decane. Neutron scattering measurements of in situ supercritical CO2 injection into reservoir rocks provides novel insights into the oil displacement and the structural arrangement of CO2 molecules confined in the nanopores of the rocks. Porosity is one of the properties of reservoir rocks that determines to what extent CO2 can be stored during the CO2-EOR, and neutron scattering techniques can infer both open and closed porosity. In the present study, this provided important information about the pore accessibility to CO2 and therefore the ability to perform geological sequestration.
Their experimental results directly showed significant decane recovery following supercritical CO2 injection, but also revealed that some decane still remained inside the limestone sample, potentially trapped over the matrix surfaces or within “throats” of pores in the sample. It was also found out that only small fractions of the smaller mesopores, pores with diameters between 2-50 nm, were accessible to CO2, suggesting that such sites are not suitable for the geological sequestration of CO2. Most strikingly, the data suggested that CO2 molecules are prone to clustering within the pore matrix, raising potentially important implications for the efficacy of the EOR process. Structural analysis further showed that CO2 confined within other pores exhibited a densified state compared to the bulk supercritical fluid, indicating that enhanced packing of molecules could be promoted by the confinement.
Based on the data and results obtained in these experiments a new research direction is being planned to apply the methodology to correlate the CO2 structural properties in the presence of additives with the recovery enhancement of the CO2-EOR process.
“This work is a great example of applied science on NIMROD, using the instrument in more of an analytical capacity and focussing on, quite literally, stuff dug out of the ground! The experiment proved the ability of NIMROD and total scattering in offering insight into the details and mechanisms of enhanced oil recovery under relevant conditions, and should help to pave the way for more in-depth investigations into the field.” – Dr Tristan Youngs, Instrument Scientist responsible for Nimrod
"Neutron scattering techniques such as total scattering can be proved to be an essential tool for a better understanding of enhanced oil recovery mechanism at the nanoscale. The technique can also reveal the structural properties of CO2 confined within the pores of limestone. It is also worth mentioning that the ability of neutrons to detect the pores that are inaccessible to CO2 is important for the potential for geological sequestration." - Dr Konstantinos Stefanopoulos, Research Director, Institute of Nanoscience & Nanotechnology, NCSR “Demokritos”