To safely sequester carbon dioxide (CO2) in the subsurface, it is vital to maintain the injection pressure below the formation breakdown pressure, which is dominantly governed by lithology, principal stresses, and presence of natural fractures. Injecting CO2 at pressures below formation breakdown prevents the creation of injection-induced fractures and associated microseismicity which are undesirable occurrences for CO2 sequestration projects. However, higher injection pressure allows more formation fluids to be displaced, thus enhancing the effective storage capacity of the sequestration zone which benefits CO2 project economics. Leak-off tests are typically conducted with a water-based fluid to determine the breakdown pressure. But, considering the significant dissimilarity in the fluid properties of water and CO2 the resulting breakdown pressure (Pb), failure mechanism and extent of damage can vary. In this study, we investigate how different injectates (CO2 and water) impact rock breakdown pressure and fracturing and the implications for CO2 sequestration.

Multiple true triaxial fracturing tests were performed on 2.5% KCl brine saturated samples using CO2 and water. The tests were done on CO2 non-/exposed samples which were cylindrical with dimensions of 4” in diameter and 5.5” in length. Samples with different petrophysical and elastic properties were used. The injection pressure and acoustic emissions were simultaneously recorded in real time. We mounted an array of sixteen (16) 1 MHz piezoelectric transducers around the samples to capture acoustic emissions (AEs) which were used to calculate the events’ location, and attributes. After the fracturing tests, we took vertical plugs along the main fracture and measured permeability under confining pressure. We also imaged the fractures using the scanning electron microscope (SEM).

For all samples, CO2 reduced Pb noticeably as compared to water. The percentage by which Pb was reduced varied among the different sandstones. Similar Pb was observed for non-exposed and exposed samples fractured with CO2. The permeability of fractures induced by CO2 was consistently one order of magnitude greater than water induced fracture permeability, over the entire range of confining pressure (1000 psi to 4000 psi). Physical examination of the fractured samples revealed that CO2 fracturing created bi-wing fractures that spanned the entire length of samples, whereas fractures generated by water fracturing traversed only half of the sample length. The number of AEs in CO2 fracturing was considerably greater, and the AEs had broader distribution perpendicular to the fracture plane, compared to that of water fracturing. CO2 and water induced AEs had similar moment magnitudes, failure mechanism and frequency. SEM imaging of fractures revealed wider fracture aperture ( 1.4-6 times), several lose grains, rough fracture edges, secondary branching, and regions of intense microcracking in fractures created by CO2 injection than by water injection.

Based on the experimental results, we have observed that fracturing with CO2 occurs at a lower breakdown pressure; therefore, the Pb estimated from leak-off test (using a water based fluid) would be an overestimation of the actual Pb of the formation. Similarity in breakdown pressure of exposed and non-exposed quartz rich rocks means that the geomechanical response of a predominantly quartz rich formation during and before CO2 injection will likely remain similar. The lower breakdown pressure could be attributed to the lower viscosity and greater percolation ability of CO2, enabling it to reach pores and crack tips more easily to promote crack propagation. CO2 fracturing results in larger damage in both fracture propagation extent and permeability due to the sudden expansion of CO2, which releases energy to further the crack extension. Consequently, generated fractures can propagate over longer distances vertically which can potentially compromise the integrity of the seal above and below the storage zone. They also have greater transmissivity and thus could facilitate CO2 leakage by providing a pathway for migration. Therefore, precise knowledge of the formation's Pb during CO2 injection is essential for optimizing injectivity which consequently will promote accurate project economic evaluation and environmental protection. But the comparability between magnitudes, focal mechanism and frequency of acoustic emissions induced by water and CO2 injection means that lessons can be learnt from the abundant experience of conventional water injection. Laboratory measurements provide a controlled means to ascertain the true Pb and other geomechanical responses to CO2 injection. In terms of reservoir stimulation, CO2 as a fracturing fluid has the potential to lower operations cost, increase production, and minimize environmental impacts.