Application of Material and Energy Balances to Geothermal Steam Production

Abstract

The material-energy balance developed in this study has been used successfully to match performance and to forecast production for the Wairakei geothermal field of New Zealand. The equations should be applicable to other geothermal fluid reservoirs, provided the assumptions used are realistic. Introduction The basic study from which this paper was prepared was started as the result of the growing need throughout the world for increasing quantities of energy in all forms. Quite obviously, natural forms of energy that are readily available at low development cost are those in greatest demand. The underdeveloped countries - and particularly those having little or no petroleum resources - are the countries in which the most interest is being shown in the newer energy sources. One of the least expensive energy sources is natural geothermal steam. Although this form of energy has been recognized for centuries, it has been only during the past 20 years that serious efforts have been made to harness it. Natural geothermal steam energy in Italy, New Zealand, Mexico, Japan, and California is now being produced through wells to drive turbines and generate electricity. Further, active exploration for natural geothermal steam is being conducted in Hawaii, Fiji, Taiwan, Chile, Russia, Greece, and Katanga. It is surprising to find that most geothermal steam exploration is in the "steam seep" stage. That is, surface studies are made and exploratory wells are drilled in the general area of steam seeps. However, in the larger geothermal steam areas, there have been efforts to apply the most modern geological and reservoir engineering principles in order to define the reservoir parameters, particularly those relating to estimates of reserves and future productivity. This paper is concerned with the development of appropriate equations and techniques to facilitate these estimates. Production of natural steam or hot water presents problems different from those experienced in the production of oil and gas. For example, steam or hot water systems may be essentially single-component systems, while hydrocarbon systems are most frequently multicomponent fluid systems. Heat effects are much larger for water than for hydrocarbon systems; and the natural steam production may or may not be isothermal, while production of petroleum reservoirs is considered normally to be isothermal. Petroleum reservoir engineering principles may be applied to natural steam or hot water reservoirs if the inherent differences in the systems are considered. The basic considerations involved in geothermal steam reservoir engineering are: thermodynamics, physical and thermal properties of water, materials and energy balances, fluid influx, and performance matching and predicting. Thermodynamics Fig. 1 is a pressure-temperature diagram for the liquid-vapor region for pure water, showing the critical point and five other points representing possible initial conditions for a geothermal steam reservoir. JPT P. 893ˆ

A history of numerical modelling of the Wairakei geothermal field

“…Modelling studies of the effects of production from Wairakei began at The University of Auckland in the early 1980s; these were an outgrowth of theoretical studies of convection in a porous duced by Whiting and Ramey (1969); they represented the reservoir as a single box with a pressure-dependent storage coefficient and a pressure-dependent recharge of hot water. The derivation given by Whiting and Ramey (1969) is quite general, allowing for the reservoir to contain a two-phase mixture of water and steam, but the implementation of their model has been criticized by Grant et al (1982) because it assumed that liquid water conditions existed at Wairakei.…”

Lumped-parameter models for low-temperature geothermal fields and their application

“…However, for single-phase, low-temperature geothermal reservoirs, we can generally neglect energy balance and apply the mass balance only (Whiting and Ramey, 1969;Axelsson, 1989), because of the pressure-temperature behavior of geothermal reservoirs. Fig.…”

“…Several lumped-parameter models and modeling methods have been reported in the literature (Grant, 1983;Whiting and Ramey, 1969;Brigham and Neri, 1980;Castanier et al, 1980;Brigham and Ramey, 1981;Olsen, 1984;Gudmundsson and Olsen, 1987;Axelsson, 1989;Alkan and Satman, 1990;Axelsson and Dong, 1998;Satman and Ugur, 2002). Axelsson and Gunnlaugsson (2000) discuss the usefulness of lumped-parameter models in interpreting monitored production data for low-temperature geothermal fields.…”

Hawaii Geothermal Project: initial Phase II progress report

“…This information, in turn, may be used to predict the well performance. The methodology is similar to the Ramey/Whiting technique [18]. but extended to include the effects of additional parameters and a comprehensive sensitivity analysis.…”