Geothermal well design identifies the equipment, materials and drilling procedures required to ensure a satisfactory well completion to achieve an acceptable well life. The subsurface conditions likely to be encountered during drilling are the fundamental inputs needed for the well design process.
Advancing our knowledge of the deep subsurface is a core goal of the GNG programme – but we don’t want to wait until the end of the programme to think about well design. So we are working from what we know already from geothermal systems in the Taupō Volcanic Zone, and international experience, to advance well designs for 6 km deep exploratory geothermal wells.
What underground conditions will deep drilling likely encounter?
Work during 2021 has seen the development of two synthetic well prognoses (download report), which define conditions expected to be encountered down to a depth of 6 km, with temperatures in the 400 to 500°C range expected over the 4 to 6 km interval, and pressures of up to about 45 MPa. If these conditions are encountered in the exploratory well, then Aotearoa New Zealand’s first supercritical geothermal well will have been drilled.
The two sites selected were (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), a producing geothermal field. Both sites were purposefully located above zones of deep low-resistivity, identified in publicly available magnetotellurics models. The locations are possibly close to magmatic intrusions, which are a heat source, feeding heat and hot fluids to shallower levels underground. The inference from this is that these deep low resistivity conductors are hosted in volumes of fractured, possibly permeable rock.
The main difference in the geology of the two prognoses is the expected depth from the surface to the contact with the metasedimentary basement rocks; in SC-1 the contact is predicted at 2800 m, whilst in SC-2 it is predicted at ~2000 m. Above the basement metasedimentary rocks is a volcanic stratigraphic pile. Pressure - depth conditions expected are hydrostatic from the surface down to the metasedimentary contact, and probably sub-hydrostatic in the metasedimentary rocks.
Though challenging to assess, the prognosis report proposed the composition of the geothermal fluids based on analogy with the physicochemical conditions of supercritical fluids that have been encountered in other parts of the globe, and compared with those known to exist in the subcritical conditions already drilled into at the Rotokawa Geothermal System.
What is expected from the supercritical well once constructed?
The well designs are anticipated to be challenging regarding materials of construction, to withstand the thermal stresses, temperatures, pressure and fluid chemistry. The prognosis report highlights a need to develop a range of tools and equipment capable of withstanding the conditions that are anticipated in order for a 6 km supercritical well, and the prospect from which it draws, to be characterised through post drilling well testing activities. There is significant work required (which is not a part of the GNG programme) to prepare for the drilling and testing of an exploratory supercritical well.