Scientists from Cairo University explored the possibility of using the depleted oil field of Abu Sannan in the Western Desert of Egypt as an underground storage facility for CO₂. For that, they took one of its main reservoirs Abu Rawash and made a detailed assessment of its structure, geology and rock properties defining if it was capable of safe retaining billions of tons of carbon dioxide for millennia.
The interest to such methods is quite understandable: the depleted oil-and-gas formations always were viewed as the best candidates for underground storage of CO₂. They already proved their ability to capture fluids for millions of years, and they are thoroughly studied due to drilling and producing. The infrastructure built for their development (wells, pipelines, sites) is ready for use. For Egypt, that is active in developing the climate agenda, this is an opportunity to create efficient and relatively cheap system of technological emissions reduction.
For that, the researchers performed extensive work combining the traditional methods of oil exploration with the tasks of CO₂ storage.
Subsurface structure studies were the first step. The scientists used seismic data and restored a 3D picture of Abu Rawash reservoir deep down. It turned out that the field was split by a net of normal faults creating an alternation of elevations and depressions – pockets capable of retaining the pumped-in CO₂. The South-Eastern block of the field looks especially attractive: the natural geometry of the formation forms a stable closed trap. Light fluids (like carbon dioxide) can easily move and accumulate in such zones.
After that the researchers moved to assessing the physical properties of the rock. Based on the logging data (gamma-ray logging, density logging, acoustic logging, neutron logging and electrical logging) they calculated the key parameters: porosity, water saturation, clay content and permeability. Abu Rawash rocks turned out exceptionally fit for storing gas: their porosity reaches 24-28%, clay content – only about 5%, and natural fracture intensity of carbonates provides for good permeability. It means there will be enough space for gas to be distributed within the reservoir and there will be no need for high pressure during pumping the gas in. Water saturation of 40% also tells us that the reservoir is partially released after decades of production and can accept significant volumes of CO₂.
Another important factor is a thick mass of dense rock under the formation – it is a natural isolating “lid”. It prevents gases from migrating upward and makes the system stable within geological time. It would be impossible to talk about safe underground storage facility without such a reliable overlap.
Combining the structural data and petrophysical calculations, the researchers built a detailed 3D reservoir model. It allowed for calculating its potential void volume and for defining, which part of this volume CO₂ molecules can occupy. Not only the volume of voids was taken into account, but also the CO₂ density in the pumping-in process, the residual water saturation and the filling efficiency factor. It turned out that Abu Rawash is capable of safe storage of approximately 72.6 bn tons of CO₂ – the value compatible with decades of industrial emissions of big regions. This makes this reservoir one of the best targets for a major carbon dioxide storage project in Egypt.
