Improved Fluid Circulation and Heat Transfer in Geothermal Reservoirs due to Superior Fracture Network in Hot Dry Rocks (HDR)

Abstract

As the reduction of carbon emissions becomes an increasingly pressing issue, a larger emphasis is being placed on the need for the development of renewable energy. One such option is geothermal energy which utilizes the heat from the earth’s crust; it presents a vast potential for the production of commercial scale base-load power generation. However, the conventional techniques used in the stimulation of hot dry rocks (HDR) geothermal wells are not very effective in producing a permeable reservoir for heat exchange between the rock mass and the working fluid. To increase the permeability of geothermal reservoirs, a new stimulation technique (developed by CSIRO - Commonwealth Scientific and Industrial Research Organisation) which involves isolating sections of the well for controlled planar fracture growth can be used. However, if these notches/fractures are placed too closely together they will interact with one another, resulting in a deviated fracture path. A two dimensional numerical model has thus been developed to study conditions under which adjacent fractures will interact with one another. This study aims to verify the numerical model through stimulating a number of granite blocks, and drawing comparisons between the observed fracture pattern and that predicted by the model. To achieve this goal, the stimulated and fractured granite blocks were sectioned and their fracture patterns were extracted using a MATLAB code, before being reconstructed in their respective positions. Stimulation was carried out firstly using conventional techniques, and then by trialling the method proposed by CSIRO. Observation of the reconstructed images showed good agreement between the model predictions and the observed fracturing patterns in two-dimensions. However, the three-dimensional pattern in the notched, perpendicular well-bore was observed as a ‘half cylinder’. This was counter intuitive as it was expected that radial symmetry of the fractures would be observed resulting in a ‘bowl’ shape. It was therefore concluded that while the model was unable to accurately predict the three-dimensional geometry of an array of fractures, stimulation through a notched perpendicular wellbore was very effective in the production of a controlled system of fractures with an improved fluid flow and heat exchanging surface area of the reservoir in comparison to the conventional techniques.