Unconformity Geological term that refers to an ancient surface that has been buried and now represents a discontinuity or break between coherent masses of rock or strata.

Vadose Zone A hydrogeological term that represents the volume of rock and unconsolidated materials between the ground surface and the underlying water table, in which pores and cracks are...

Vapor-Dominated Reservoir The highest grade of geothermal resource in which self-flowing wells produce dry steam from a reservoir containing a mixture of hot water and steam. Larderello,...

Wairakite A zeolite mineral of hydrothermal origin that was first identified in the Wairakei geothermal field in New Zealand by Alfred Steiner. It forms clear to white prismatic crystals,...

Well Casing Large diameter pipe that is assembled and inserted into newly drilled intervals of a well. The upper portions of a cased well are commonly cemented in place to provide a smooth...

Wellhead Surface equipment at the top of a well (oil, gas, geothermal) that provides a structural and pressure-containing interface for drilling and production equipment. It is used to...

Well Logging Methods used to survey and characterize the physical and geological characteristics along and near the wall of a well, including the temperature, pressure, fracture density,...

Zircon Zirconium silicate (ZrSiO4) is a minor or accessory mineral in igneous and metamorphic rocks. It contains measurable albeit minor amounts of uranium and lead, which make it amenable...

Zonal Isolation The process of operationally isolating specific intervals or zones along a wellbore to perform well intervention activities, such as stimulation.