Behavior of the Bagnore Steam/CO2 Geothermal Reservoir, Italy

Abstract

Abstract A commonly encountered geothermal steam reservoir consists of a gas cap overlying an aquifer. In addition to steam, the gas cap may contain substantial amounts of carbon dioxide and other noncondensable gases. A mathematical model for describing the pressure and composition histories of such reservoirs is presented. The model evaluates the change in states of a homogeneous vapor/liquid region as a mixture of steam and carbon dioxide is removed from the gas cap and as recharge water enters the liquid region. The production history of the Bagnore steam field in Italy was evaluated using this model. It is concluded that early reservoir behavior was controlled by a blowdown of carbon dioxide in the gas cap, while long-time behavior is being controlled by water influx. Introduction This paper presents a lumped-parameter model of a vapor-dominated geothermal reservoir containing a significant quantity of noncondensable gas. The formulation and methodology presented may be applied to noncondensable gases in general. However, this work is concerned only with the effects of carbon dioxide and dissolved carbonates on the producing characteristics of a geothermal steam field. Model calculations are compared with the observed history of the Bagnore steam field in Italy. It is concluded that the behavior of this field can be explained in terms of multicomponent vapor/liquid equilibrium considerations in conjunction with energy influx from local reservoir rocks and liquid influx from an external source. Lumped-parameter models traditionally have been used in studies of oil and gas reservoirs. Here they are known more commonly as material balance methods1 since material conservation is the only balance law used. The extension of such techniques to pure water/steam geothermal systems requires in principle only the addition of appropriate energy balances. This has been done by Whiting and Ramey2 for a liquid-dominated field and by Brigham and Morrow3 for hypothetical vapor-dominated systems. The application of such models to multicomponent geothermal systems requires again only the addition of chemical species conservation equations for each additional component. For example, Grant4 has applied such a model to a liquid-dominated reservoir in New Zealand that contains substantial amounts of dissolved carbon dioxide. The model described in this paper is an extension and coupling of the Brigham and Morrow and Grant models. It is an example of a class of models that may be useful for analyzing the behavior of vapor-dominated geothermal systems. This paper used the model to evaluate the production history of a gas-cap geothermal reservoir significantly affected by the presence of carbon dioxide. Lumped-Parameter Producing-State Model This section discusses the producing-state mathematical model. First the thermodynamic system is described, next the constituent equations of state are presented, and then the development and numerical solution of the governing equations are discussed.