Geothermal technology : Circular C-108

Abstract

In Hawaii, geothermal reservoirs are expected to occur 4,000-8,000 feet below sea level. Rotary drilling rigs likely to be used ip Hawaii are rated for drilling to a maximum depth of about 16,000 feet. Holes may be drilled perpendicular to the ground surface or directional to almost any desired angle from ground surface. The local subsurface geology and the avrulability of well control techniques and blow-out prevention equipment minimizes the risk of well blow-outs. Approximately ten acres of cleared land may be needed to site the wells necessary for a 25 megawatt power plant. Drilling mud and cuttings may be disposed of at the drill site sump or can be removed if required. While in the production zone, return air is likely to contain hydrogen sulfide which can be abated by a caustic soda abatement system. After well completion, up to eight hours of unabated venting may be necessary to clear rock debris. Well casing intergrity is essential if usable water aquifers are to be protected. Steam piping from well-head to power plant may be placed about five feet above ground on saddles or may be buried for safety and aesthetics. Before a power plant becomes operational the State Department of Health must issue permits regarding the quality of the air and fluids discharged from the plant. The proposed DOH regulations require a 98% H2S removal and a maximum concentration of about 25 parts per billion H2S at the property line. Abatement systems are available which can meet these standards, eg. the Stretford abatement system. Contingency abatement systems are likely to be designed into the power plant. The plume exiting the cooling tower should consist entirely of water vapor. The use drift eliminators in the cooling tower should prevent water droplets from exiting with the vapor. Liquid effluent should be piped into deep injection wells. If the silica content of the effluent is high, a silica dropout system may be utilized to prevent injection well plugging. The surface area for a 25 megawatt power plant may be about seven acres. Roads and electric transmission lines may be constructed or upgraded to accomodate geothermal exploration, development, and production activities. The County of Hawaii geothermal noise guidelines limit noise to 55 decibels by day and 45 decibils at night at nearby residences. Abatement technology exists to abate noise to acceptable levels.