Carey, B.S.; Rae, A.J.; Bixley, P. ; Carson, L.B.; Alcaraz, S.A.; Chambefort, I. 2021 Prognoses for two supercritical well designs. Lower Hutt, N.Z.: GNS Science. GNS Science report 2021/36. 14 p.; doi: 10.21420/8KAM-7312
Abstract:
This report has been assembled to provide parameters to assist with the design of geothermal wells that are expected to encounter supercritical geothermal conditions at depth in the Taupo Volcanic Zone (TVZ). To advance well design, two synthetic well prognoses have been developed, structured to define the conditions expected to be encountered and to be contained by a supercritical geothermal well in the TVZ drilled to a depth of ~6 km. Two sites were selected for hypothetical wells: (i) in a location midway between Ohaaki and Ngatamariki (SC-1), some distance from a currently identified geothermal system, and (ii) on the margin of the Rotokawa Geothermal System (SC-2). The main difference in the prognoses is the expected depth from the surface to the metasedimentary basement rocks; in SC-1 the contact is predicted at 2800 m, whilst in SC-2 it is predicted to be ~2000 m. Above the basement metasedimentary rocks is a volcanic stratigraphic pile. The wells are anticipated to reach 400 and 500°C at depths of about 4000 and 6000 metres respectively. Pressure-depth conditions expected are hydrostatic from the surface down to the metasedimentary contact, and probably sub-hydrostatic in the metasedimentary rocks. Though it is challenging to predict the composition of supercritical fluids, we propose an analogy here based on the physicochemical conditions of supercritical fluids that have been variously encountered in other parts of the world compared with analyses from subcritical conditions encountered in the Rotokawa Geothermal System in the TVZ. Based on the geoscience information, we outline what might be expected from the well once constructed. Equipment, materials, and drilling and well testing procedures are proposed that could reasonably be expected to ensure a satisfactory well completion and an acceptable well life. There is significant preparatory work required to prepare for the drilling and testing of an exploratory supercritical well, the development of methods, procedures and tools capable of withstanding the anticipated conditions are discussed (auth)
