Development of a Design Tool for Hot-Dry-Rock Fracture Systems

 

Development of a Design Tool for Hot-Dry-Rock Fracture Systems

In a joint project together with two other departments at RWTH Aachen University (the Institute of Geotechnical Engineering and the Institute for Computational Analysis of Technical Systems) and the Institute of Structural Analysis at Graz University of Technology efforts are being undertaken to improve the technology of deep geothermal energy exploration.

The Hot-Dry-Rock Concept

The Hot-Dry-Rock (HDR) technology has great potential for large-scale conversion of geothermal energy into electric energy. Pressurized water is used to fracture hot rock at depth of about 3 km - 5 km in order to create an engineered underground heat exchanger. For producing geothermal heat, cold water is injected down to the fractured hot rock and heated up while passing through. Brought back to the surface it is used to drive a steam power plant.

The Project in a Nutshell

The controlled siting and creation of a fracture system in deep and dense formations needs to be developed to the point where the engineered fracture system is designed as to serve the purpose of an efficient heat exchanger between the hot dry rock and the a steam power plant driving water cycle. At the current stage of the project we develop a fracture propagation code based on the Extended Finite Element Method (XFEM) which will be verified against large-scale hydraulic fracturing experiments in the laboratory. A future stage envisages verification of the code in the field and its coupling with the thoroughly verified and tested heat and mass transport code SHEMAT-Suite on the reservoir scale for providing a layout design tool for fracture systems to be used as an efficient heat exchangers at depth.

Laboratory Experiments

The built-up of a large-scale true-triaxial testing facility for little permeable rock samples has been accomplished at the Institute of Geotechnical Engineering (GiB) in 2013. Compressive stresses can now be set independently along all three axes of a 30 x 30 x 45 cm3 rock sample, horizontally up to 15 MPa and vertically up to 30 MPa. In the center of the 30 x 30 x 45 cm3 rock sample fluid can be injected into a small open borehole section and pressurized for breaking the rock. As a result, a fracture propagates along the direction of maximum stress in the 3D stress field set by the flat jacks. During the injection process, acoustic emissions are recorded for monitoring and analyzing the development of the fracture with time. Up to 32 ultrasonic transducers are mounted on the rock sample’s surfaces. Furthermore, the states of stress, deformation, and strain are monitored.

Development of the Numerical Design Tool

In the current stage of the project we focus on the development of the XFEM fracture propagation code and its verification at laboratory scale. The final layout tool will be applied at reservoir scale and coupled to a heat and mass transfer code capable of predicting long-term heat production from the engineered heat exchanger and its economic feasibility. Fracture layout, borehole positions, and flow rate as controllable parameters will be optimized under the constraints of geology, maximum energy yield, and economic viability.

A number of physical processes are involved in a fluid driven fracture: the rock deformation due to the induced fluid pressure on the crack faces, the fluid flow within the fracture and the fracture propagation. These physical processes are modeled separately in the XFEM fracture propagation code and solved within an iterative coupling. The strongly non-linear and non-local behavior makes fluid induced fracture modeling a challenging task. The extended finite element method (XFEM) is particularly suited for dealing with discontinuities. Unlike necessary mesh refinement around discontinuities in FEM, XFEM employs additional problem-specific functions (enrichments) added to the approximation space to account for these discontinuities.

Numerical Code Verification

Currently, the XFEM fracture propagation code is verified at laboratory scale. For this purpose, specific injection procedures with different flow rates and fluids have been used in the fracture experiments for ensuring a slow fracture propagation. This is important for verifying the code since the volume of the rock sample is limited and the process of fluid-induced crack propagation is modeled numerically under quasi-steady-state assumption. Material parameters are determined in standard rock mechanics tests (e. g. uniaxial compression test, Brazilian test, fracture toughness by chevron notch specimen).

Outlook

As a follow-up phase of this project, we plan to (1) verify the XFEM fracture propagation code with field data from shallow boreholes in basement rock, (2) analyze the important aspect of shear displacement in the engineering of deep geothermal heat exchangers by using and adapting the above mentioned testing facility and (3) prepare an interface between the fracture propagation code and SHEMAT‐Suite.

This project is financed by the German Federal Ministry for Economic Affairs and Energy (BMWi).