Prior to the drilling of the HGP-A well, it had generally been assumed that geothermal reservoirs on the Island of Hawaii were low-temperature liquid-dominated reservoirs which were constantly recharged from the ocean. Two numerical studies based on this conceptual model were carried out to investigate heat transfer and fluid flow characteristics in such a reservoir. It was found that (1) even under the most unfavorable conditions, i.e., in the absence of caprock for the prevention of heat loss and with constant recharge from cold seawater, it is possible to have a large amount of hot water at shallow depth, (2) the upwelling of the water table resulting from geothermal heating is small, and (3) the rate of contraction of isotherms resulting from the withdrawal of fluid from a production well depends not only on the withdrawal rate, but also on the size of the heat source and the relative location of the production well and the heat source. After the drilling of the HGP-A well had been completed in April 1976, it was found from the core samples that the permeability of the formation varies with depth, and that the low salinity of the water samples taken from the wellmore » indicates that there is a barrier between the well and the ocean, preventing the free flow of seawater. Thus, a third numerical model taking into consideration the layered structure of the formation, with recharge and discharge through an upper permeable boundary, was carried out to simulate the free convection processes at the Kapoho geothermal field. In addition, analytical studies on convective heat transfer from dikes and sills in an aquifer with high permeability have been performed, and analytical expressions for heat transfer rates and the size of the hot water zone from hot intrusives have been obtained.« less