ABSTRACT

INTRODUCTION

Hydraulic fracturing is the most common method used to create an artificial fracture for extracting heat from a hot dry rock (HDR) geothermal reservoir[ 1]. To establish a heat extraction system in a HDR reservoir, it is essential to confirm the actual geometry of the fracture or of the associated fluid invasion zone. Hypocentral locations and fault plane solutions of microseismicity are often used to identify the orientation and area of the fracture [2]. However, it is difficult in the field to evaluate the results of these methods, and hydraulic fracturing experiments have been carried out under controlled conditions at laboratory and small field scales[3-5]. Majer and Doe [3] measured acoustic emissions (AE) during hydraulic fracturing of a salt block, and in a small scale field experiment? Only tensile events were observed in the salt block experiment, but both tensile and shear events were commonly recognized in the field. Falls et al.[5] carried out measurements of AE during hydraulic fracturing in large granitic cores under an unconfined stress state and reported that double-couple mechanisms predominated.

We conducted laboratory hydraulic fracturing experiments in cubic blocks, 20 cm on a side to investigate the fracturing mechanism using acoustic emissions. We evaluated the effect of fluid permeation on the fracture mechanism by using different fracturing fluids and different rock types.

SPECIMENS AND EXPERIMENTAL PROCEDURE

Blocks of acrylic resin and three types of rock were used for the experiments. The acrylic block was selected as an impermeable and homogeneous material. A coarse-grained, Inada granite and a fine-grained, Akiyoshi marble were chosen as permeable, crystalline media. Komatsu andesite was chosen as a low permeability non-crystalline rock.

Test specimens were bi-axially loaded using flat jacks to 6.1 MPa and 12.2 MPa, as shown in Figure 1. The coordinate system used in the experiments is also shown in Fig. 1. For the acrylic block, only 2.6 MPa and 4.6 MPa were applied, since its breakdown pressure is two times higher than in the rocks. To study the effect of fluid viscosity on the fracturing mechanism, both water and oil(viscosity of 70 cP) were used as fracturing fluids. The fracturing fluid was injected through a mini-packer into a 1 cm dia. hole drilled in the center of the block. To 'minimize the effect of anisotropy, the injection hole was drilled perpendicular to the rift plane. A constant flow rate of 4 cm3/min was used for all experiments.

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