Laboratory and numerical investigation of carbon dioxide injection-induced fracturing in geological carbon sequestration and enhanced oil recovery processes

Abstract

The carbon capture, utilization, and storage (CCUS), carbon dioxide (CO2) enhanced oil recovery (EOR) and fracturing, have posed the question about the mechanical integrity of the geological formation during CO2 injection. This study conducted experimental investigations on CO2 injection into synthetic concrete and Niobrara shale samples under various conditions to characterization the CO2 injection-induced rock failures and applied experimental findings in a numerical simulator to predict the breakdown pressure from CO2 injection. The fracturing experiments are conducted with a true-triaxial stress loading system on 8" cubic rock blocks. Compared to aqueous fracturing fluids, CO2 induces fracture at lower breakdown pressure, creates complex fracture with rougher surfaces, and activates natural fractures more easily. The results also yield a novel quantification relation between breakdown pressure and injectants' viscosities. Under the low-stress difference (313 psi), CO2 fractures typically deviate from the preferred fracture plane (PFP). The increment in stress difference forces the CO2-induced fractures to approach PFP. The pre-existing transverse fractures have a minor impact on the breakdown pressure of CO2 fracturing but result in significantly more complex fracture morphology under the low-stress difference. In water-saturated samples, the CO2 fracturing process exhibits some characteristics of hydraulic fractures due to the multiphase effects. The experiments on composite concrete samples show distinct behaviors between two materials with different strengths: the fractures in low-strength cores are always in the PFP, while the stress scheme controls those in the high-strength outer shell. With the modification with the relation of breakdown pressure and viscosities, TOUGH2-CSM successfully reproduced the experiment results for breakdown pressure, showing its capability to accurately estimate the breakdown pressure during CO2 injection to minimize leakage risks. This research provides more insights on the CO2 fracturing processes in aspects distinguishing from conventional hydraulic fracturing. Some of the findings can be easily incorporated into numerical simulators to guide field operations of CO2 injection into geological formations during CCUS and EOR processes.