Most Larderello wells produce superheated stem, with a noncondensible gas content of 3-10 percent by weight, of which typically 95 percent consists of C02 (D'Amore and Truesdell, 1984). depletion data are available the gas content of the discharge shows remarkably small temporal variations. changes by no more than 10 to 20 percent over several decades, and there are only few wells where gas content changes by as much as s factor of 3, rrhich is still a rather modest change. In many wells there is a tendency for gas content to increase for several years, then stabilize, and eventually decrease (Sestini, 1970; D' Amore and Truesdell, 1979; Calore et al., 1982). For most wells for which long-term In many cases the gas-steam ratio The present paper is concerned with the origin of C02 in the Larderello discharges from a reservoir engineering point of view. Specifically, we consider the question of fluid reserves (water and C02) at Larderello, and we carry out numerical simulations to obtain insight into temporal trends of C02 released from idealized models of vapor-dominated systems. Important constraints in the modeling arise from observed temperatures and enthalpies at Larderello. Discharge enthalpies are generally close to those of saturated steam near 25OoC (2.8 MJ/kg), with aome degree of superheat which tends to increase with time. Flow rates of most wells decrease rapidly during the first few years of production and subsequently decline very slowly (Sestini, 1970; Weres et al., 1977; D'Amore and Truesdell, 1979).

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