Preliminary Study of Discharge Characteristics of Slim Holes Compared to Production Wells in Liquid-dominated Geothermal Reservoirs

This paper presents a theoretical study of the discharge characteristics of slim holes compared to production wells. Assuming that (1) the boreholes feed from an all-liquid zone, and (2) the feedzone pressure and temperature are independent of borehole diameter, calculations have been carried out for a variety of borehole diameters. The wellhead pressurelflowrate relationships for the various borehole diameters do not collapse to a single curve, even when flow rates are adjusted to account for differences in cross-sectional area. The area-scaled discharge rate declines with a decrease in borehole diameter. Both frictional pressure gradient and heat loss effects are more significant for the smaller-diameter slim holes than for the larger-diameter wells. The difference in heat loss effects is probably responsible, at least in some cases, for the difficulty encountered in inducing deep slim holes (depths >> 300 m) to discharge. Scaling up the discharge capacity of slim holes to those of production wells by the cross-section area ratio provides a conservative estimate of production-size hole discharge.