Factors controlling borehole performance

Abstract

Geothermal boreholes have been studied to estimate maximum discharges when drawing on (a) a source of pressurized hot water, (b) a source of dry or superheated steam. The case is initially considered where infinite permeability exists at depth (which is common in areas similar to that at Wairakei, New Zealand) and in which only geometry of the hole controls the flow-rate. Also investigated was the effect on discharge of increased hole size throughout the depth or over fractions of the depth, with resultant flow increases quantitatively established. The relationships derived, enable fall in borefield discharge to be predicted for known decline in temperature and pressure within the reservoir; conversely the actual decline in field discharges can be used to confirm such changes. For boreholes such as (a) above, the enthalpy may be accurately determined by a method using the maximum running pressure, the wellhead pressure at which imminent collapse of the steam-water mixture occurs, and the bottom hole pressure. A critical discharge pressure tapping located at the pipe outlet face may then measure flow-rates direct over a range of wellhead pressures and hence supply the data required to plot the borehole characteristic curve. The conditions were also considered where rock permeability controls the flow into the bottom of boreholes either (c) from radial fissures or (d) from permeable-porous media. Because «draw-downå (drop in bottom hole pressure under discharge) is proportional to the square of flow for case (c) and directly as the flow for case (d), it appears that nearly all boreholes operate with discharge supplied from fissures even if these are often very narrow.