Engineered Geothermal Systems - IESE Modelling Research Group
The Institute of Earth Science and Engineering (IESE) is performing numerical simulation of Engineered or Enhanced Geothermal Systems (EGS). In order to accurately simulate EGS, it is important to accurately depict as much of the relevant physics as possible. To that end, we have constructed a medium description that is based on the true spatial fluctuation of material properties taken from well log and well core data, a ‘poroperm’ medium.
The poroperm medium is characterised by fracture density distribution that follows a pink noise spatial fluctuation and a related long-tailed permeability distribution that describes the fracture connectivity in the medium. Small changes in fracture density result in large changes in permeability. Specifically, we are simulating fluid percolation flow and heat flow through a deformable poroperm medium using the finite element method. Due to spatial fluctuations in porosity, and thus permeability, fluids tend to percolate through native permeability pathways in the medium.
The key to sustainable and productive EGS lies in the enhancement of those native percolation pathways to allow sufficient flow rates without allowing cold water to pass to the receiver wells.
We are seeking to understand the physics of injection induced strain damage that result in permeability enhancement of the existing native permeability pathways. Our goal is to design an EGS volume that can sustainably and economically produce electricity for the life of the geothermal field.
Figure 1. An example of three horizontal well bores through a ‘poroperm’ medium. The blue well is a cold injection well, and the two red wells are receiver wells that remove hot water and/or steam. The bright areas represent areas of high fracture density and opaque areas represent low fracture density. The spatial fluctuation in fracture density is well described by well log data. Fluids flow by percolation pathways present in the areas of high fracture connectivity that can be determined from well core data.
Figure 2. Vertical 2D section from Figure 1 that shows normalized fluid velocities. Fluids tend to percolate in native permeability pathways as a result of fracture connectivity in the rock. The goal in IESE’s native permeability enhancement is to stimulate these native pathways through shear deformation and damage to increase flow rates without compromising the heat of the reservoir.
Figure 3. Shear strain concentrations that result from injection over pressurization and the percolation of fluids through the native permeability pathways seen in Figure 2. The areas of high shear strain shown in this image will result in damage that increases the fracture connectivity. Increased fracture connectivity causes increased permeability, which in turn, causes increased fluid flow. Understanding and controlling the shear strain induced fracture connectivity enhancement is the key to economic and sustainable EGS.
To learn more about this research or to get involved, please contact Peter Leary or Justin Pogacnik