| Title | Numerical Assessment of Stimulation of Geothermal Wells Via Hydraulic Fracturing |
|---|---|
| Authors | Francesco PIZZOCOLO, Jan Ter HEEGE, Peter FOKKER |
| Year | 2018 |
| Conference | Stanford Geothermal Workshop |
| Keywords | Stimulation Technique, Treatment, Hydraulic Fracturing, Coupled Numerical Model, Hydroshearing, Acidizing, Geothermal Well |
| Abstract | After years of exploiting steam, geothermal wells may need stimulation to remain economically profitable. A forward-looking production strategy necessitates the investigation of stimulation options in case production is diminishing. Hydraulic fracturing can be a suitable technique to improve steam production or to create new geothermal pay zones. The goal is to stimulate the area around the well by connecting pre-existing fractures or by creating new pathways for the steam through the initiation and propagation of new tensile fractures. Even when a dense fracture systems results in large leak off volumes,, it has been shown that it is possible to induce tensile fractures of tens of meters around the injection well. Performing such a treatment in a geothermal reservoir presents several important challenges. The technical and geological challenges that might affect the effectiveness and efficiency of the stimulation were investigated. A major issue is the way in which many geothermal wells are completed: a long perforated liner (up to 1500 meters) surrounded by an empty annulus (open hole section). Because of the high leak off that is typical for a densely fractured reservoir, a large volume of injected fluids will be lost to the geothermal field before the fracturing pressure is reached. This undermines the effectiveness of the stimulation. Moreover, the typical absence of a stress barrier, the induced fracture will not be vertically confined. The difference in effective stress between the top and the base of the perforation interval might cause the fracture to propagate above the top of the geothermal reservoir instead of into the field. Packers or temporary plugs will not be effective, because the fracture fluid will flow upwards through the annulus. An effective solution to isolate a wellbore section can be to expand the liner via hydraulic or inflatable expanders. This would create a sealing contact with the wellbore walls. If a hydraulic fracturing treatment is assessed to be technically feasible, it must be carefully tailored to the geothermal field. A close connection between dynamic flow simulators and geomechanical models is essential to numerically simulate the effectiveness of this stimulation technique, a close connection between dynamic flow simulators and geomechanical models is essential. To properly evaluate the stress changes induced by the pressure and temperature gradients in the geothermal field around the injection well, we linked in a 1-way coupled fashion numerical simulators and analytical solutions to connect the evolving fluid pressures and temperature with the changes in the stress field around the well. Different geological scenarios were analyzed and several sensitivities analysis on the mechanical properties of the rock matrix and of the fractures were performed. Moreover, a 2D geomechanical model was created to gain better insight on the behavior of the pre-existing fractures of the field during stimulation. The results of this model show that the induced overpressure generates considerable slip at the tip of the pre-existing fractures, diminishing towards the center of them. Cooling by the injection of cold fluid creates contraction of the rock surrounding the fracture and has an effect that is added to the effect of the overpressure. In our calculations the temperature gradient is mainly responsible for the opening the existing fracture. Hydraulic fracturing will not be the proper stimulation technique when isolation of a short section of the liner is not possible or is too expensive. The high injectivity that generates the high fracture leakoff can also be exploited to apply alternative stimulation approaches. The fast penetration of pressure and temperature fronts into the reservoir can make thermal and acid stimulation to be valuable alternatives to hydraulic fracture stimulation. |