LAUSR

In recent years, an increasing number of engineering operations related to underground fluid injection have appeared. These involve fields like not only traditional oil and gas recovery, but also some unconventional engineering such as CO2 geological storage (Metz et al. 2005; Holtz et al. 2001; Spycher and Pruess 2010; Rogala et al. 2013) and acid gas re-injection (Chakma 1997). In some of these processes, the injection pressure should be controlled to prevent the fracturing of the formations to ensure the mechanical stability of sites (Hawkes et al. 2005; Li et al. 2014; Rutqvist and Tsang 2002; Streit and Hillis 2004). In some others, artificial fracturing is usually required to improve the permeability of the reservoirs. In many occasions such as CO2 fracturing or injection, the formations are actually saturated with twophase pore fluids. Accordingly, one of the common issues in such processes is the hydro-fracturing failure behavior of the formation rocks under the action of gas–water mixtures, which might be significantly different from that under a single-phase fluid. The hydro-fracturing failure behavior of rocks under this condition would be influenced by effects of partial pressures, the change of pore flow, and the physical and chemical reactions between the fluids and rocks. Therefore, a hydro-fracturing device and the test method for rocks saturated with controlled two-phase pore fluids are valuable for the experimental investigations. The surrounding rocks at the bottom hole are generally under a true triaxial stress state with a negative minimum principal stress (Matthews and Kelly 1967). In the existing test techniques, the method based on the hollow cylinder could implement similar stress state on the specimen by imposing the axial, the confining, and the internal pressures. A series of this type of test devices have been developed since the first hollow cylinder testing device for splitting tests of sandstone by F. D. Adams (Adams 1912; Jingnong and Mianba 1986; Périépj 1990; Lee et al. 1999; Song et al. 2001; Kanj et al. 2003; Monfared et al. 2011). Moreover, many investigations of mechanical behaviors of rocks have been conducted by researchers by the hollow cylinder tester (Zhang et al. 2010; You et al. 2010). Therefore, the hollow cylinder test method has been actually widely used. However, none of the existing hollow cylinder-based testers is capable of testing the hydro-fracturing behavior of rocks saturated with controlled two-phase fluids. In this work, the traditional hollow cylinder tester was upgraded to make a novel hydro-fracturing tester for rocks saturated with controlled two-phase fluids to meet many new emerging needs as mentioned previously. One of the highlights of this device is the additional function of saturation control of the gas–water pore mixtures in the rock specimen. The controlled stress state is built the same way as the traditional hollow cylinder tester, while the two-phase pore fluids are controlled to form specific partial pressure or saturation. * Bing Bai [email protected]