Modeling of Geothermal Systems

Abstract

Summary During the last decade, the use of numerical modeling for geothermal resource evaluation has grown significantly, and new modeling approaches have been developed. In this paper, we present a summary of the current status in numerical modeling of geothermal systems, emphasizing recent developments. Different modeling approaches are described and their applicability discussed. The various modeling tasks-including natural-state, exploitation, injection, multicomponent, and subsidence modeling-are illustrated with geothermal field examples. Introduction A number of different methods for modeling the behavior of geothermal reservoirs are currently available to reservoir engineers. These methods vary widely in complexity and cost of application. In the selection of the proper method for a particular study, one must consider the amount and quality of field data available and the objectives of the study. Geothermal systems are generally very complex, exhibiting such features as fracture-dominated flow, phase change, chemical reactions, and thermal effects. Modeling studies must be carried out to analyze data from geothermal wells accurately and to estimate the generating potential of a system. When a model of a geothermal potential of a system. When a model of a geothermal system is developed, the existing field data must be carefully evaluated, and the important physical processes that occur in the system identified. After a plausible conceptual model of the field is developed, one must choose a mathematical (numerical) model that can realistically evaluate the performance of the geothermal reservoir and reliably predict its future behavior. We have found that modeling the natural state of a field before modeling the field under exploitation can give very valuable reservoir information. It not only tests the conceptual model qualitatively, but also gives estimates of mass and heat flow in the system. Furthermore, it provides consistent initial conditions for the exploitation provides consistent initial conditions for the exploitation models. The primary objective for geothermal reservoir modelinc, is to provide answers to important reservoir management questions relating to well decline, well spacing, generating capacity (power potential) of the reservoir, injection effects, and potential subsidence and scaling problems. These questions must be addressed by use of a problems. These questions must be addressed by use of a proper exploitation model that has evolved from the proper exploitation model that has evolved from the conceptual model and the natural-state modeling studies. In this paper, we present a brief review of geothermal reservoir modeling, emphasizing recent developments. The different modeling, approaches are described, and their advantages and limitations are discussed. We briefly describe the governing equations for mass and heat flow and discuss phase transitions and solution techniques. Examples illustrate the different methodologies for modeling of natural state, exploitation, injection, multicomponent flow, and subsidence. Finally, we identify problems of current interest in geothermal reservoir modeling. Earlier summaries of geothermal reservoir modeling are given by Witherspoon et al. and O'Sullivan. Physical Processes and Physical Processes and Conceptual Models In contrast to oil and gas reservoirs, geothermal systems are very dynamic in their natural state. There is continuous transport of fluid, heat, and chemical species. Important physical processes in geothermal systems include mass transport, convective and conductive heat transfer, phase change (boiling and condensation), dissolution and phase change (boiling and condensation), dissolution and precipitation of minerals, and stress change caused by precipitation of minerals, and stress change caused by pore-pressure changes. Most of these processes are pore-pressure changes. Most of these processes are strongly coupled; for example, phase change disturbs chemical equilibria, often resulting in precipitation/ dissolution of minerals that in time can alter porosities and permeabilities of the subsurface rocks. This in turn can permeabilities of the subsurface rocks. This in turn can affect the mass transport in the system. In modeling geothermal reservoirs, one must carefully evaluate which physical processes need to be considered in a specific modeling study. This will depend on the objectives of the study and the complexity of the geothermal system. Most currently available geothermal simulators consider only single-component mass and heat transport. In recent years, several simulators capable of modeling the transport of a second component, either a noncondensible gas or a dissolved solid, have been developed. Conceptual models of geothermal systems vary greatly in complexity. Perhaps the "simplest" geothermal systems are those created by hot water upflow through a single fault or at the intersection of two or more faults (e.g., Susanville and East Mesa, CA. ) A rather complex porous-medium-type geothermal reservoir is the Cerro Prieto field, Mexico (Fig. 1). The lithology consists of interlayered shale and sandstone beds. The detailed lithology shown in Fig. 1 has been determined mainly on the basis of wireline well logs. JPT P. 1007