Enhancing CO2 Trapping Mechanism in Ultra Deep and Highly Dipping Saline Aquifers in The Western Desert Of Egypt
Authors: Mansour K. , Merghany I. A. , ElSayed R. , Ismail A. , Ellamey C. , Agam A. , Taqey A. , Gomes R. , Stojic S. , Bouchet F. , Nasiri A. , Nasreldin G. , Abdi H. Ashtiani , Eleoni M. , Dredge J. , De Boer Y. 
Published: November 2024

Abstract
In this case study we present a best practice for site screening & optimizing safe CO2 sequestration through simulation in a challenging ultra deep saline aquifers at depth ranging from 3 to 4 Km within the western desert of Egypt. In General, CO2 storage sites are typically just deeper than 800 m to make sure that CO2 is at supercritical state, these are typically depleted reservoirs or shallow saline aquifers with gentle formation dipping and moderate salinity, However in the western desert the screened storage sites within these zones are characterized by high dip angles of more than 3 degrees, surrounded by major faults and with very high salinity around 150,000 ppm. There are four main challenges with those ultra deep thick saline aquifers (Fig. 1 & Fig.2): (1) high salinity & density of water; most of the supercritical CO2 only invades upper part of the thickness due to density difference between the supercritical CO2 and saline water, (2) high dip angle that causes the CO2 plume to move rapidly up dip towards major faults and existing wells causing accumulation of CO2 on faults and increasing risk of fault reactivation and leakage of CO2 to upper producing zones, (3) high salinity reduces the solubility of CO2 in water consequently promoting the movement of CO2 plume towards up dip and reducing the safe trapping and containment of CO2, and (4) geomechanics integrity of the faults and caprock due to CO2 pressure injection (i.e., pressure of the formation is virgin and high: 6000 psi)). Figure 1Unconventional CCS storage sites within the Western Desert of Egypt due to very deep saline aquifers with high dip angle, faulted structure and extremely high salinity.Figure 2Study Methodology through 3 phases, Phase 1 is site screening & Ranking, Phase 2 is subsurface site evaluation for safe containment & Phase 3 is complete site feasibility including wells and surface facility.
The applied approach is an optimization under uncertainty approach. In which we optimized four main variables while considering subsurface geological uncertainties, the four optimized variables are well location, perforation bottom-up sequence, perforation timing, and CO2 Injection scheme. While the geological uncertain parameters considered are, geological sand and shale baffles distributions, porosity and permeability transforms, relative permeability end points and hysteresis between draining and imbibition and aquifer salinity. This ensemble of optimized reservoir-geomechanics simulations were analyzed under uncertainty to determine the optimum strategy for CO2 plume containment.
It was concluded through this ensemble of geological scenarios that safe CO2 trapping mechanism can be increased by 30% using a time and rate managed sequential injection scheme starting from the bottom of the formation and upwards. the dynamic simulation results show that this technique helps increasing the contact area with the saline water by invading most of the reservoir interval through leveraging local shale baffles to enhance both structural trapping and solubility mechanisms, Also leveraging high pressure to overcome low solubility in highly saline water and creating convective water density currents, hence increasing CO2 solubility in water as well as residual trapping of CO2 even in structurally marginal sites.
This scientific approach unlocked the potential for CO2 sequestration in ultra deep saline aquifers within the western desert to be deployed for a long-term and safe sequestration of CO2 while considering complex subsurface structure to pave the way for future successful CCS projects in similar environments.