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Abstract

The deliverability of liquid-dominated geothermal reservoirs is presented in terms of reservoir performance, inflow performance, and wellbore performance. Water influx modeling is used to match the performance of Wairakei in New Zealand, and Ahuachapan in El Salvador. The inflow performance is given in terms of a linear productivity index for liquid-only flow, and a solution-gas drive productivity index for liquid-only flow, and a solution-gas drive relationship for two-phase flow. A 9-5/8 in. production well is assumed, flowing 250 deg. C water from 900 m depth, and with a well-head pressure of 100 psia. A geothermal development model that couples reservoir deliverability and power plant performance is used to illustrate how the development cost of geothermal electric power projects can be estimated. power projects can be estimated

Introduction

The factors affecting the economic feasibility of geothermal projects are many and complex, primarily because of the site projects are many and complex, primarily because of the site specific nature of geothermal resources and their utilization. Reservoir performance is perhaps the major cause of uncertainty in geothermal field development decisions, at least in comparison to the performance of surface facilities and power plants. This uncertainty means that it is difficult to estimate the total development cost of liquid-dominated resources for electric power production. This may be the reason why issues of geothermal resource exploitation and power plant operations tend to be dealt with separately in the literature.

Generalized costs for geothermal electric power projects worldwide are reported by Cataldi and Sommaruga. For a typical 30-50 MW power plant they estimated the following total costs: vapor-dominated reservoirs 1600-1700 ﹩/kW, of which 40 % relatto fluid supply, 50 % to power plant, and 10% other costs; liquid-dominated reservoirs 2400-2500 ﹩/kW, of which 50 % relate to fluid supply, 30 % to power plant, and 20 % injection and other activities. The corresponding cost of electricity was estimated in the range 20-80 mills/kwh. We observe that in liquid-dominated developments the reservoir related costs amount to 70 % of the total development cost. Tester states that these costs are usually 50-60 % of total cost, and can even reach 75 %. Clearly, the cost of geothermal electric power depends greatly on the nature of the resource and its performance.

In this paper we couple the main reservoir and economical issues of geothermal electric power in a geothermal development model. We consider the effect of deliverability on the cost of geothermal electric power from liquid-dominated resources. The overall performance of a reservoir/wellbore system with time is what we call deliverability. It has three components: reservoir per-formance, inflow performance, and wellbore performance. The elements per-formance, inflow performance, and wellbore performance. The elements of the geothermal development model selected here, are specific to three liquid-dominated reservoirs (Ahuachapan, Roosevelt Hot Springs, Wairakei), forming two composite systems, that we use to investigate wellhead and central power plant options. assuming fixed steamfield development and power plant costs.

GEOTHERMAL DEVELOPMENT MODEL

Decision making in geothermal developments deals with an objective, choices, and constraints. The objective is the primary feature of development. Typically, it would be an economic objecfive such as profit maximization or cost minimization. The objective provides the criterion for selecting the best strategy (choices) of development. In addition, when an objective is picked, the impact of the choices on the objective must be defined. The optimum strategy strongly depends upon the objective.

Development choices incorporate the features of development which can be controlled and selected by the developers. In essence, they represent the strategy of the developers in exploiting the resource. Examples of choices for a geothermal development would be well design and spacing, and power plant design and capacity.

Constraints impose limitations upon the choices - developers must work within them in order to effectively develops the resource. The most important constraints are the ones imposed by the resource and market. The resource constraints would include such items as temperature, chemical impurities, and system deliverability. The market for electricity imposes price and demand constraints; economic constraints such as demand levels or budget limits also exist. Other important constraints are restrictions on fluid disposal.

Once the objective, choices, and constraints of development have been described, the optimum development strategy can be approached in an analytical fashion. The development choices represent variable. For each selection of variable there is a corresponding value of the objective.

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