Effect of Water Infiltration, Injection Rate and Anisotropy on Hydraulic Fracturing Behavior of Granite

Abstract

Hydraulic fracturing tests on Pocheon granite cylinders at seven different injection rates varying from 1 to 100 mm 3 /s were conducted. They were compared with sleeve fracturing tests in which borehole was sleeved, and therefore, water infiltration influence was excluded. Hydraulic fracturing behavior of granite is significantly influenced by water infiltration, which is closely related to the preexisting microcracks in granite as well as the cleavage anisotropy. There was a threshold injection rate to fracture the granite specimen under given stress conditions. When the injection rate is below the threshold, water infiltrated granite matrix with slow increment of injection pressure, and the specimen finally reached a full saturation without fracturing. Injection pressure developed nonlinearly with time during water infiltration, while approximately linearly when infiltration was excluded. For both hydraulic and sleeve fracturing tests, breakdown pressure increases with increasing injection rate. The breakdown pressures by sleeve fracturing were more than two times higher than those in hydraulic fracturing. X-ray computed tomography (CT) observations show that induced fractures are along the weaker cleavage parallel to the direction of the vertical stress. The higher breakdown pressure results in a larger aperture of fractures in hydraulic fracturing tests.

Application of Water Fracturing in Geothermal Energy Mining: Insights from Experimental Investigations

“…The pressurization rate is the change in the injection pressure per unit time. It is mainly affected by the injection rate and sample hydraulic parameters [20]. The drill hole pressurization and fracturing stages can be clearly identified on the injection pressure-time curve in the hydraulic fracturing tests.…”

“…The in-situ residual pressures of the induced cracks in samples G-1, G-2, MG-3, MG-7, and MG-8 were measured at different injection rates (5,10,15,20,25,30 mL/min), as shown in Figure 17a. We applied the next higher water injection rate whenever the pressure increase was stable.…”

“…For instance, Solberg et al [19] demonstrated that there was no significant difference in the breakdown pressures when the granite was tested for fracturing at a low injection rate of 0.0198-0.198 mL/min (0.33-3.3 mm 3 /s). However, some researchers found that rocks could not be hydrofractured with an injection rate lower than 1.5 mL/min (25 mm 3 /s) for Pocheon granite [20] and 5 mL/min for Strathbogie granite [13]; this is due to the fact that the lowest injection rate highly depends upon the hydraulic parameters of the rocks. In addition, laboratory studies showed that thermal cracking could change rock permeability [21,22], strength [23], elastic moduli [24], and other mechanical properties [25,26].…”

“…In experimental research on hydraulic fracturing of granite, only a limited number of studies have used real rocks and most studies have been conducted with artificial materials including concrete, cement mortar, and Perspex [27,28]. Even with real rock materials, most experiments used small cylindrical specimens (50 mm in diameter and 100 mm in height) for fracturing testing under a pseudo-3D confining pressure state [13,20]. In fracturing tests of such cylindrical samples, it is generally difficult to determine the morphological changes in the crack due to the small size of the sample.…”

Experimental Study of Fracture Propagation: The Application in Energy Mining

“…Previous studies on large scale hydraulic fracturing were mostly conducted on artificial cement samples with artificial materials such as cement mortar, concrete and Perspex [34,35]. Even though some studies used real natural rocks, most of the experiments used small cylindrical rock samples with 50-60 mm in diameter and 100-120 mm in height for hydraulic fracturing under a pseudo triaxial confining stress state [36][37][38]. The tensile and shear strengths of cement samples were much lower than that of granites.…”