In this paper we develop and demonstrate a method to estimate the reservoir pressure and a productivity index for vertical steam wells, from its production characteristic (also called output) curves. In addition, the method allows to estimate the radius of influence of the well, provided that a value of the reservoir transmisivity is available. The basic structure of the present method is: first, the measured well head mass flowrates and pressures are transformed to downhole conditions by means of a numerical simulator; then, the computed downhole va riables are fitted to a simple radial model that predicts the sandface flowrate in terms of the flowing pressure. For demonstration, the method was applied to several steam wells from the Los Azufres Leothermal field. We found excellent agreement of the model with this ample set of field data. As a bonus, the processed data allowed several inferences about the steam -producing zone of the reservoir: that thewells considered produce from relatively isolated pockets of steam, which are probably fed by nearby inmobile water; and that these feed zones are in poor hydraulic communication with the field surface waters. The main advantages of our method are that it provides a way to retrieve important reservoir information from usually available production characteristic curves, and that the method works from easily and accurately taken wellhead measurements.

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