Abstract

Use of carbon dioxide (CO2) as a fluid for hydraulic fracturing has been considered to stimulate oil production, to enhance shale gas recovery and to extract hot dry rock geothermal energy. In these projects, CO2 is usually injected into rocks at a depth more than 1,000 m. In the temperature and pressure at that depth CO2 usually becomes a supercritical state (SC-CO2), and viscosity of the SC-CO2 is very low around 5% of that of water. So, it is important to understand the behavior of SC-CO2 in rock. Thus, we made hydraulic fracturing experiments using SC-CO2 and water in two granite specimens for each fracturing fluid under a true tri-axial loading condition. The results of the experiments suggested that low viscosity fluid like SC-CO2 is expected to induce more three dimensionally and widely spreading cracks under considerably lower breakdown pressure than water.

1 Introduction

Carbon dioxide (CO2) is injected into the underground rock for a variety of purposes. It is often used for miscible flooding to enhance oil recovery in depleted petroleum reservoirs, and the use of CO2 as a frac- turing fluid for well stimulation has been considered because it eliminates formation damage and resid- ual fracturing fluid (Sinal & Lancaster 1987, Liao et al. 2009). Using CO2 as fracturing and circulat- ing fluid has also been proposed in hot dry rock geothermal energy extraction, because it eliminates scaling in the surface piping due to the inability of CO2 to dissolve mineral species (Brown, 2000). Recently, since shale has a greater affinity for CO2 than methane (Nuttall et al. 2006), the CO2 injection to enhance shale gas recovery has been considered as well (Kalantari-Dahaghi 2010). For all of these pur- poses, it is necessary to understand the behavior of CO2 in rock. It is also important to know how injected CO2 infiltrate into the surrounding rock mass in CO2 capture and storage projects (Xue et al. 2006, Nooner et al. 2007).

In these projects, CO2 is usually injected into rocks at a depth of more than 1,000 m. The temperature and pressure at that depth usually makes CO2 a super- critical state, while the lower temperatures in special geological conditions create a liquid state. The viscos- ity of liquid CO2 (L-CO2) is one order lower than that of normal liquid water, while that of the supercritical state (SC-CO2) is much lower still. To clarify fracture behavior induced with injection of the low viscosity fluid, we have already conducted hydraulic fracturing experiments using SC- and L-CO2 in granite speci- mens under hydrostatic loading, and have discussed differences in the results between the two fluids (Ishida et al. 2012). In this paper, we report the hydraulic fracturing experiments using SC-CO2 and water in granite specimens under a true tri-axial loading condi- tion. From the results, we discuss differences between the two fluids focusing on the breakdown pressure and distribution of located acoustic emission (AE) sources, adding data of our experiments with injection of oil having a few hundred times larger viscosity than water.

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