Impact of supercritical CO2 injection on petrophysical and rock mechanics properties of chalk: an experimental study on chalk from South Arne field, North Sea.

Changes in chalk due to EOR by injecting supercritical CO2 (CO2-EOR) can ideally be predicted by applying geophysical methods designed from laboratory-determined petrophysical and rock mechanics properties. A series of petrophysical and rock mechanics tests were performed on Ekofisk Formation and Tor Formation chalk of the South Arne field to reveal the changes in petrophysical and rock mechanics properties of chalk due to the injection of CO2 at supercritical state. An increase in porosity and decrease in specific surface was observed due to injection of supercritical CO2. This indicates that a reaction between CO2 enriched water and particles takes place which smoothens the particle surface. Accordingly, partial increase in permeability was also noticed. An effect is also observed from the decrease in pore-space stiffness, calculated from sonic velocity. No significant effect on wettability as indicated by NMR T2 relaxation time was observed. Rock mechanics testing indicates that in 30% porosity chalk from the South Arne field, injection of supercritical CO2 has no significant effect on shear strength and compaction properties, while there is probably a slight decrease in stiffness properties. For both the Tor Formation and Ekofisk Formation, flooding with CO2 after waterflooding does not seem to affect the shear strength parameters. For the Tor Formation, the elastic deformation parameters seem to be reduced after CO2 injection. In contrast, the effect on the elastic moduli in Ekofisk Formation is insignificant. The time dependent properties for both Tor Formation and Ekofisk Formation do not seem to be affected by CO2 flooding in the relevant stress regime. Generally, the change in both petrophysical and rock-mechanics properties is insignificant in Ekofisk Formation, compared to the changes in Tor Formation, most probably due to the very little contact cement in Ekofisk Formation chalk. Time-lapse monitoring strategies may be required during a CO2-EOR process for the measurement of changes in reservoir properties that may cause deformation of and leakage from a reservoir. Results of this study will provide data for designing future monitoring strategies based on 4D seismic.