Stimulation of Conventional Geothermal Wells with Propped Fractures

Abstract

Abstract Conventional hydrothermal ("low enthalpy") geothermal reservoirs are already being developed in the Netherlands as a source of heat for greenhouses, district heating. Many of these wells are left with a positive skin after being drilled, and could therefore benefit from stimulation. In order to evaluate the potential benefit of propped fracture stimulation for this type of reservoir, a history match of a real geothermal injection and production doublet was made using an industry standard 3D reservoir simulation model. This model was then used to perform production and injection forecasts for different completion scenarios, including propped fracturing. All available data (logs, PLT, welltests) was used in creating and history matching the reservoir simulation model. The model was history matched to the individual welltests for both the production and injection well and to a 3 year data set of production and injection data. In order to match the long-term pressure trend, an increase in the size of the thermal fracture in the injection well was needed. The history matching results were compared to the predicted increase in the size of the thermal fracture using a new 3D FEM fracture model. The same model parameters were also used to design propped hydraulic fracturing treatments. The propped fractures were also placed in the reservoir simulation model for both the production and injection wells, to predict the performance of the wells with fracture stimulation. Although the actual wells used in this study had negative skins to begin with (after drilling), many wells drilled in the same reservoir (Rotliegend in the Netherlands) have positive skins to begin with. Our simulations showed significant improvement in well injectivity and productivity using propped fractures, compared to a well with a typical initial skin of +2 seen in new open-hole wells in this reservoir. Injection pressure was also reduced significantly, to below the regulatory limits on "safe" injection. The new 3D FEM model showed that injection well thermal fracture propagation was strongly affected by small differences in the Young's modulus between the higher and lower porosity layers in the reservoir. This paper shows, based on actual well data from a real geothermal doublet, the expected benefit from hydraulic fracture stimulation is significant, especially when the wells have a positive skin after drilling. The effect of small differences in rock stiffness on thermal fracture propagation in a geothermal injection well is also a new and surprising result.