title: Comparative geothermal well performance – supercritical and sub-critical
Creators: name: Rivera, Julius
name: Carey, Brian S.
Publisher: name: GNS Science
Year: 2023

description: The report documents output forecasts for 4500 to 6000 m deep geothermal well configurations penetrating hydrostatic pressure conditions in the Taup&#333; Volcanic Zone (TVZ), New Zealand (NZ), across a range of reservoir temperatures from 450 to 600°C. A range of wellhead parameters are computed using the supercritical capable GFlow wellbore simulator. In the simulation models the fluid is assumed to be pure water. The work identifies a range of output that might be possible from supercritical wells and for comparative purposes the outputs are reported as thermal and exergetic power, with values up to 220 MWth and 80 MWex across a range of wellhead conditions up to 220 bar with wellhead temperatures ranging from 250 to 420°C. Production wellhead data from 45 producing wells in six geothermal fields in the TVZ has been assembled. From the data, thermal and exergetic power has been computed enabling comparisons to be made between the simulated supercritical well forecasts and actual subcritical production well output data. From this analysis it is observed that a TVZ supercritical geothermal well might produce the work equivalent of a larger subcritical geothermal production well and up to four times the work (power) output of the median subcritical TVZ production well from the 45 well dataset that has been assembled. Wellhead temperatures are forecast to be greater for the supercritical wells (250 to 420°C) than recorded in the subcritical production well dataset (less than 264°C). Supercritical well output predictions documented by Albertsson et al. (2003) for a 5000 m Icelandic well encountering 550°C conditions have been reviewed with their predictions compared to the New Zealand TVZ subcritical production well data. There are a number of NZ subcritical production wells that are about as powerful in exergetic terms as the simulated Icelandic supercritical well (~80 MWex) with the Icelandic supercritical well being about four times as powerful exergetically as the “median” of the 45 NZ production wells in the comparative dataset. GNS was unable to match the output from the Icelandic supercritical simulation using typical model input parameters in GFlow, forecasting an output of 48 MWex at the 195 bar Albertsson et al. (2003) wellhead pressure condition, which is only ~2.5 times the median TVZ production well value of 20 MWex. The Icelandic output testing of the IDDP-1 well at Krafla undertaken between 2010 to 2012 was reviewed, with GNS Science simulating the well output using the GFlow wellbore simulator with the reservoir and well configuration data from Ingason et al. (2014). Thermal power of up to ~140 MWth and optimum exergetic power of ~55 MWex at wellhead pressures below 70 bar were forecast. Ingason et al. (2014) provides insight into potential wellhead fluid velocity constraints; they were recommending operating at fluid flow rates less than 40 kg/s in the carbon steel 9 5/8” production casing and wellhead componentry. Casing and wellhead materials selection along with site specific reservoir aspects will influence and constrain recommended wellhead fluid velocities for wells producing from supercritical conditions. A range of further studies are discussed and recommended including: • Understanding the depositional potential through the phase transition from supercritical to superheated steam that is expected under flowing wellbore conditions; • Material studies, considering corrosion, erosion and fatigue of casing materials, well and wellhead componentry; • Well studies, quantifying the thermal, pressure and transient stresses from starting, stopping and changing flow rates in wells up to 6000 m deep; • Reservoir engineering techniques applicable to low density compressible fluids in ultra-hot / supercritical geothermal resources; • Downhole logging tools capable of withstanding the high temperatures and pressures expected.