Characteristics of carbon dioxide fracturing in comparison to conventional water hydraulic fracturing: Evidence from acoustic emission monitoring of small-scale field experiments

Abstract

We conducted small-scale field hydraulic fracturing (HF) experiments using carbon dioxide (CO2) and water as fracturing fluids. A hole was drilled 10–14 m below the tunnel floor into hot granitic rock whose temperature can form CO2 supercritical state at HF pressure. Under the same injection rate of 10 mL/min, the volume of the acoustic emission (AE) hypocenter distribution induced by CO2 HF was approximately 27 times larger than that induced by water HF. In addition, when evaluating injectivity improvement through Injecting/Fall-Off Tests (IFOTs) using gaseous CO2 and water before and after CO2 HF and water HF, respectively, the injectivity of rock around the HF hole was improved by around 400 times larger by CO2 HF in comparison to a few times larger by water HF. The results suggest that CO2 HF can stimulate a much larger region and improve injectivity much more than water HF. In contrast, it has more possibility of leaking injected fluid through the stimulated region. On the other hand, when comparing the focal mechanisms of AE events in both CO2 HF and water HF with their occurrence times and locations, we found a tendency for tensile-dominant events to occur in intact rock, in contrast, shear-dominant events occur with a linkage between cracks, that is, the interaction of cracks. This finding suggests a solution for the paradox of the mechanism of HF in which shear events dominate in AE monitoring in actual production fields where there are various discontinuities and cracks, although the elastic theory expects tensile fractures.