Some reservoir engineering calculations for the vapor-dominated system at Larderello, Italy

Various reservoir properties are calculated for the Larderello vapor-dominated system using available published data. Bottom-hole flowing properties are calculated from measured wellhead data. Whereas wellhead temperatures measured at a particular time tend to change systematically with changes in flow and pressure, calculated bottom-hole temperatures tend to be constant for two sample wells; while for a third, bottom-hole temperatures decrease with increasing flow. Bottom-hole temperatures calculated from wellhead data taken over several years can be constant, increase, or decrease for particular wells. A steady-state model for steam flow to a well is used with calculated bottom-hole data to show that the effect of non-Darcy flow is important. The initial mass of fluid in place for the northeast zone of Larderello (56 km2) is estimated, using data on shut-in pressures and total mass production. Reservoir thickness needed to store this mass of fluid is calculated as a function of porosity and initial fraction of water in pores. Representative values are 19 km of thickness, assuming 5% porosity with steam alone, and 832 m, assuming 20% porosity and 10% of pore volume as liquid water.