Cylindrical specimens of Pocheon granite were used for laboratory hydraulic fracturing (HF) tests, and sleeve fracturing (SF) tests where the borehole was covered with membrane and fluid infiltration into sample was ignored. To investigate the effect of borehole diameter and specimen height on hydraulic fracturing behavior, all specimens have a fixed external diameter of 50 mm, while borehole diameter varied at 5, 8, 12 and 14 mm, and specimen height varied at 50, 70 and 100 mm. Most tests were conducted at a constant injection rate of 50 or 100 mm3/s. A few SF tests were compared at different injection rates varying at 5, 25, 50 and 100 mm3/s using both oil and water as the injection fluid. Experimental results show that breakdown pressure (BP) decreases with increasing borehole diameter, while it is insignificantly affected by specimen height. BP increases with increasing injection rate while the change is not significant at higher injection rates. The pressurization rate estimated from injection pressure curves ranges from nearly 0.003 to 1.2 MPa/s, and its impact on the hydraulic fracturing breakdown pressure was investigated. BP predicted by a fracturing mechanics model were similar to those measured values during experiments under a constant injection rate of 100 mm3/s, when assuming a 3 mm length of preexisting crack.
Hydraulic fracturing (HF) method has been widely used in reservoir engineering development and in-situ stress measurement. Hydraulic fracturing behavior varies depending on rock types. For development of Enhanced Geothermal System (EGS), where the target reservoir formations are usually hard crystalline rocks like granite and tight sandstone, understanding of hydraulic fracturing behavior of these rocks, particularly fracture initiation and propagation and the resulted breakdown pressure, is very necessary.
For an experimental study on hydraulic fracturing of rocks in laboratory, one has to decide sample and borehole size. The sample geometry will influence the experimental results. Table 1 lists sample sizes and borehole diameters applied in past experimental studies on hydraulic fracturing behavior of various granites. For cylindrical samples, the ratio of d/D has a wide range of 1:4 to nearly 1:50 and the ratio of D/L ranges from 1:1.2 to 1:3.5. Either through-going hole or half-going hole was used. For cubic samples in true tri-axial testing, the ratio of borehole diameter and the minimum side length d/s ranges from 1:6.4 to 1:20.
Haimson and Zhao (1991) reported that laboratory tests in boreholes that are at least 20 mm in dia. yield breakdown pressures that are essentially unaffected by borehole diameter size and are directly usable in interpretation of field data. Experimental results by Morita et al. (1996) showed a clear decrease of breakdown pressure of Berea sandstone when the borehole size increased from 38 mm to 100 mm. Detournay and Cheng (1992) proposed a mathematical model interpreting pressurization rate and size effects on the magnitude of hydraulic fracturing breakdown pressure. The rate effect is seen as a consequence of the interaction of two lengthscales: a diffusion length and a microstructural length.
In this study, we performed hydraulic fracturing tests on cylindrical granite samples, having three different ratios of D/L (1:1, 1:1.4 and 1:2) and four different ratios of d/D (1:3.6, 1:4.2; 1:6.3 and 1:10). Influence of D/L and d/D on the breakdown pressure and induced fractures was investigated. Additionally, sleeve fracturing tests where the borehole is covered with a latex membrane to prevent injection fluid infiltration into the rock specimen were performed at four different injection rates of 5, 25, 50 and 100 mm3/s. Oil and tap water, with viscosities of 52 cp and around 1cp at room temperature are used as injection fluids.