Hydraulic fracturing of large igneous rock samples under triaxial compression

ABSTRACT

Ph. Siebert1, K. Willbrand2, N. Weber3, M. Feinendegen1, T.-P. Fries4, C. Clauser2, M. Ziegler1

Geotechnical Engineering, RWTH Aachen Universitiy1

Applied Geophysics and Geothermal Energy, E.ON Energy Research Center, RWTH Aachen University2

Computational Analysis of Technical Systems, RWTH Aachen University3

Structural Analysis, Graz University of Technology4

Hydraulic fracture experiments on igneous rock specimens of size 450 x 300 x 300 mm³ have been carried out, in order to verify a 3D fluid-induced fracture propagation code based on the Extended Finite Element Method. The code will become part of a numerical layout tool for deep geothermal fracture systems. Therefore experiments were designed with the purpose of code testing. We started with the penny shaped crack scenario which allows a comparison with analytical models. Secondly we looked at the reorientation of a crack whose initial orientation is not aligned with the preferred fracture plane regarding to the stress state. Tests were conducted on three different types of igneous rock: Granite, Basalt and Gneiss. The testing device (Fig. 1) enables the initiation of hydraulic fractures in rock samples under triaxial compression. Fracture propagation is monitored by a set of ultrasonic transducers.

Figure 1: Testing device for hydraulic fracturing experiments on rocks under triaxial compression

After setting up a primary stress state by flat-jacks, fluid is injected into a borehole in the rock specimen until a fracture initiates. Due to continued injection the hydraulic fracture propagates. Specific injection procedures with different fluids and flow rates are conducted to create slowly propagating fractures in the limited volume of the rock specimen. This is of importance for the verification of the numerical code, since the process of fluid-induced crack propagation is modelled in a quasi-stationary way. Acoustic emissions during the injection are recorded to monitor and analyze the fracture development. Up to 32 ultrasonic transducers are mounted on the specimen surfaces. Furthermore the state of stress and the deformations of the specimen and the abutment are monitored and interpreted to get a comprehensive insight in the fracturing process. Material parameters are determined in standard rock mechanics tests (e.g. uniaxial compressions test, Brazilian test, fracture toughness by chevron notch specimen). The key aspect of the contribution will be the experimental part of the project. Our testing device will be introduced, experimental results will be discussed and the methods of verification presented.

KEYWORDS

hydraulic fracturing, fracture propagation, laboratory experiments, acoustic emissions, numerical simulation, triaxial compression