Item

Abstract

Radial water jet drilling uses the power of a focused uid jet, which is capable of drilling multiple laterals of about 100 m length out of an existing well and thereby stimulating the well with full control on the operational parameters like initial direction of the lateral, length, uid pressure etc. In contrast to hydraulic stimulation treatments, this technology can potentially provide a network of enhanced uid pathways around a geothermal well to intersect with existing high permeable structures like fracture or karst systems within the reservoir, independent of the ambient stress eld. Applying RJD, laterals typically have a diameter ranging from about 25 mm to 50 mm, depending on jetting parameters like pressure and ow rate as well as rock properties. Drilling a single lateral in a cased well requires approximately 12 hours, as the casing has to be penetrated using a coiled tubing operated milling bit before jetting into the formation. In case the target zone is open-hole, jetting a lateral is considerably faster. Compared to conventional hydraulic stimulation treatments with required uid volumes of more than 1000 m3, only a fraction of this is needed for RJD (< 1 m3). In addition, no pressure will be applied to the reservoir, thereby reducing environmental risk as well as the risk of induced seismicity considerably. Although RJD is investigated and applied in the hydrocarbon industry, applications in geothermal wells are very rare. If the technology can be shown to increase the eciency of a geothermal well, it will provide an interesting alternative to conventional hydraulic stimulation treatments. RJD shows highest eciency in terms of performance increase in reservoirs with low permeability (< 10 mD). The most important criteria for the well are the minimum diameter (4 1/2" OD casings) and maximum along hole depth (about 5 km). So far, RJD operations have been performed in wells with a an inclination of up to 46 . Technologies, however, have been developed to perfom RJD operations even in horizontal well sections. Depending on the initial production; for tight gas reservoirs the gas production can be improved with a factor 4-7, simulation for geothermal wells suggest a potential performance increase by a factor of up to 3 when 8 laterals of 100 meter are successfully drilled and geological conditions are favourable. Since the potential increase depends on the type of the geothermal reservoir as well as its properties, the improvement factor has to be conrmed by eld experiments. Currently no major hazards to the well have been identied. The main risk associated with a RJD treatment appears to be sand production from the open-hole completion. However since the amount of experience and well-documented cases is limited, not all risks may have been identied at this moment in time. Major uncertainties in the production estimates are the long-term (>1 year) stability of the jetted laterals and the eect of sub-surface heterogeneity. The jet-ability of typical geothermal reservoir rocks is also not well documented. As the jet-ability strongly depends on physical rock properties and in-situ reservoir conditions, which are signicantly dierent to typical hydrocarbon reservoirs, the feasibility of RJD in dierent geological settings has to be evaluated. Although, RJD presents a low cost stimulation method with currently no major identied risk to the well nor to the environment, experience with RJD in the geothermal industry is rare. Field applications are therefore key to evaluate the potential of the RJD stimulation technology for geothermal applications.