Geothermal Production Using Radial Jet Drilling In A Naturally Fractured Carbonate Reservoir

Abstract

Many potential geothermal reservoirs in the world are in formations in which flow is dominated by fracture permeability rather than matrix permeability. In many cases, fracture density and the resulting permeability is highest near faults and decreases with increasing distance to faults. This poses a challenge for development of geothermal resources which should be near the fault from the production perspective but away from the fault to reduce seismic hazard. From a safety perspective, the distance between faults and the geothermal wells, especially the injector well, should be sufficient. Sufficient in this case depends on a large number of factors including a geomechanical analysis of the fault and its stress state and the geological and petrophysical properties of the formation. Particularly (re)injecting cold fluids in the injector well increases seismic risk because of the unfavorable effect of cold water and high pressure on the stress condition of faults. In this paper, we discuss a geothermal system in which the near-fault permeability is large enough for economic production, but permeability further away from the fault is too low. A possible doublet in such an area could be designed as follows: a production well near the fault and an injector well further away from the fault. This system is stimulated to achieve economically viable rates. Here Radial Jet Drilling (RJD) is investigated as a possible technique for stimulation. RJD has originally been developed in the petroleum industry, but might also be of interest for geothermal production as an alternative for hydraulic or shear fracturing. For RJD, small diameter (1 inch), short laterals (up to 100 m) are jetted into the formation using hydraulic jets. The jets are not steerable and the resulting laterals remain open hole. To simulate the potential benefit resulting from the RJD, the uncertainty in the radial path is taken into account. It is assumed however that the jet drilling can cross the (small) fractures. The range in the increase of the Injectivity Index for 8, 12 or 16 laterals of up to 100 m length is calculated. On average, the Injectivity Index doubles for 12 laterals, but the uncertainty range is large (35%). The results also depend on the imposed fracture distribution which determines the horizontal anisotropy of the permeability.

AAPG Datapages/Search and Discovery Article #90345 © 2018 AAPG European Region, Geothermal Cross Over Technology Workshop, Part II, Utrecht, The Netherlands, April 17-18, 2018