Hydraulic fracturing is a long-established method of stimulating a well to improve the inflow or outflow potential. Hydraulic fracturing is the most successful stimulation method used by the oil and gas industry, and is also used for water injection and production wells around the world, even for drinking-water wells. Hydraulic fracturing creates a crack in the earth that is then filled with a highly conductive material (proppant). This fracture has a large inflow area compared to an unstimulated wellbore and provides a high-permeability path for the fluid to flow in or out of the reservoir.
Hydraulic fracturing has a long history of being used in hot dry rock (HDR) geothermal applications since the 1980s (Murphy & Fehler, 1986). In those often very tight reservoirs, the aim is to create fracture networks that generate the reservoir flow capacity. In high-permeability formations, fracturing can potentially double the productivity of a well. In low-permeability formations, well performance can be increased by a factor of 5–10 in most cases.
In this paper, we focus on two different scenarios of geothermal stimulation. The first is for permeable, porous formations where the heat exchange happens through the perfect contact between the fluid and the porous reservoir. Stimulation may then be necessary to create a small fracture if the pressure drop near the well is too large due to insufficient reservoir permeability. The other scenario is a formation at great depth, where the formation permeability is so extremely small that very long propped fractures would be needed to obtain sufficient flow or even where the porous system does not provide sufficient heat exchange but the heat exchange has to be facilitated by an artificial or stimulated fracture network: a so-called Enhanced Geothermal System.
For porous, permeable formations we will present examples of fracture treatments that can increase the flow rate so that the economics of the project is improved. In some formations, stimulation is then a contingency in case of poorer than expected reservoir quality. A worst-case well with a large skin value of 20 can perform with stimulation like a base-case unstimulated well. In other formations, stimulation will be integral to well design in order to optimise the project performance. For those cases the Coefficient of Performance can be improved from 7 to 25 with the aid of stimulation.
In Ultra-Deep Geothermal (UDG) recovery, the targets are reservoirs below 4000 m, because industrial heat demand requires a minimum temperature of 120°C up to 250°C. For an economic business case, the rate over a period of 15 to 25 years should be from 150 to 450 m3 h−1, depending on the boundary conditions.
Shallower reservoirs in the Netherlands often show very high permeability, but at great depth the target layers could have very low permeability (Veldkamp et al., 2018). Several stimulation methods can be used, of which hydraulic fracture stimulation with water (proppantless) is the primary candidate. Other stimulation methods are propped fracturing in sandstone, acid fracturing in carbonates and thermal stimulation.
For a geological play that is attractive for UDG in the Netherlands, the most likely stimulation method is with water fracturing, because propped fracturing would require a huge amount of proppant that is very costly. Based on analogues and conceptual designs, the expected flow rate is estimated under selected boundary conditions.