A Study of the Geomechanic and Microseismic Responses of Different Rocks to the Injection of CO₂: Implication for Carbon Sequestration

Injectivity and formation integrity are factors that affect the feasibility of CO₂ sequestration. High injection pressure increases the displacement ability and solubility of CO₂ in in-situ fluids and hence its effective storage which has commensurate economic impact. However, a possible hazard is the creation of fractures that could lead to compromising formation integrity. To optimize injection pressure, operators perform leak-off test using a water-based fluid which has considerably different properties from CO₂. Thus, the rock geomechanical response (breakdown pressure (P_b), damage extent and microseismic activity) to the injection of these fluids can vary. In this paper, we investigate how CO₂ injection influence rock fracturing as compared to water and the implication for CO₂ sequestration.

We conducted multiple triaxial fracturing tests on 2.5% KCl brine saturated sandstone samples using CO₂ and water. The tests were done on cylindrical samples with dimensions of 4″ in diameter and 5.5″ in length. Samples with different petrophysical and elastic properties were used. We recorded the injection pressure and monitored acoustic emissions (AEs) concurrently using an array of sixteen 1 MHz piezoelectric transducers. The AEs were used to estimate the events' hypocenter location and their attributes. After fracturing, vertical plugs were taken along the main fracture and used for permeability measurements. We imaged the fractures using scanning electron microscope (SEM) to study fracture morphology and performed a statistical analysis of the primary fracture width.

CO₂ reduced P_b considerably (by as much as ∼30%) as compared to water for all the sandstones. The difference in CO₂- and water-induced P_b varied among sandstones and this variation had a negative correlation with moduli. CO₂-induced permeability was at least an order of magnitude greater than that of water over the entire range of confining pressures. Physical examination revealed that CO₂ fracturing created fully developed bi-wing fractures that spanned the entire sample length contrary to that of water fracturing which traversed only half the sample length. The number of AEs in CO₂ fracturing was several times greater and their location showed broader distribution perpendicular to the fracture plane. Our statistical analysis of the primary fracture from the SEM images showed that CO₂-induced fracture width is approximately 6 times that observed in the water-induced fracture.

We observe that CO₂ fracturing occurs at a lower P_b and thus the P_b value estimated from leak-off test would be an over-estimation of the actual P_b of the formation. CO₂ fracturing results in greater fracture length and permeability due to the decompression energy released by sudden gas expansion in cracks, consequently, such fractures can propagate over longer distances. These fractures will be highly transmissive and could act as CO₂ migration pathways. From the foregoing analysis, it is imperative to know the exact P_b of the formation with CO₂ to safely operate the sequestration zone.