Investigation on Reservoir Stimulation Characteristics in Hot Dry Rock Geothermal Formations of China During Hydraulic Fracturing

Abstract

Geothermal energy has been widely proposed as a potential renewable energy to replace traditional fossil fuel energy. Hot dry rock (HDR) reservoir which contains abundant geothermal energy widely distributes in China. The Gonghe Basin in Northwest China is chosen to develop the Chinese first HDR field operation project. HDR is a low-permeability, high temperature and hard granite without fluid. Developing HDR requires water cyclically flowing between injection and production wells to extract heat energy. Hydraulic fracturing, as a key reservoir stimulation technology, can create the path of fluid cyclically flowing. However, few studies have investigated hydraulic induced artificial fractures in HDR geothermal formations. This paper investigated HDR geothermal reservoir stimulation characteristics and fracture patterns during hydraulic fracturing. Reservoir stimulation was conducted with a true triaxial hydraulic fracturing apparatus which could establish a real HDR formation environment in the laboratory. The factors affecting breakdown pressure and fracture creation were investigated via experiments and numerical simulations. This study could be used to evaluate and design reservoir stimulation in field HDR geothermal operation.

Appendix

The Figure Appendix Figs.

Fig. 12

a A complete outcrop rock sample; b The outcrop rock sample were cut into 300 × 300 × 300 mm cubes

12,

Fig. 13

a The water outlet hole in the flat jack; b the water inlet on the rock sample surface, the black color ring is the rubber ring and the white color wrapped the black color is the PTFE sealing tape

13,

Fig. 14

a The photo of the fractured rock sample, the black lines indicate fractures by a marker, red water flows out of the rock from these fractures; b The cut sample

14,

Fig. 15

Brazilian tensile strength for different temperatures

15 and

Fig. 16

The relationship of granite stress and aperture for Gonghe site

16 could help readers understand this study better. Figure 12 shows the field sampling and processed granite samples. Figure 13 shows the sealing measures at the contact between the rock sample and water inlet. Figure 14 a shows a fractured rock sample, the red fluid flows out along the induced fractures, Fig. 13 b shows a cut rock sample. Figure 15 shows the Brazilian tensile strength results of the granite at different temperatures. Figure 16 shows the relationship of stress and aperture of the granite in Gonghe Basin.

Figure 12 a shows the field sampling in Gonghe Basin, the granite outcrop (Fig. 1a) is processed into 300 × 300 × 300 mm cubes (Fig. 1b) for laboratorial hydraulic fracturing experiment.

Figure 13 a shows the water outlet hole in the flat jack. Figure 2b shows the rubber ring and the Poly Tetra Fluoroethylene (PTFE) on the injection tube head.

Figure 14 a shows a rock sample after hydraulic fracturing. Figure 14 b shows that a rock sample was cut into two parts.

Figure 15 shows the Brazilian tensile strength results of the granite at different temperatures. With the increase of temperature, the Brazilian tensile strength almost linearly decreases.

The validation of Eq. (2) for Gonghe site was shown in Fig. 16, which is the relationship of granite stress and aperture, the constant of  – 0.15 is obtained by fitting the experimental results, then ${e}_{0}$ and ${e}_{\mathrm{inf}}$ is got, the value of ${e}_{0}$ is about 30 μm, the value of ${e}_{\mathrm{inf}}$ is about 5 μm, more detail about the relationship of stress-vs-aperture functions could be seen in our previous study (Ma et al. 2019). More validations and applications about this equation in hydraulic fracturing numerical simulation can be referred in previous researches (AB 2010; Höekmark et al. 2006; Hökmark et al. 2010; Liu et al. 2004; Rutqvist and Tsang 2008).