Vertical Two-Phase Steam-Water Flow in Geothermal Wells
Authors: Gould Thomas L. 
Published: December 2007

The efficient design and operation of a geothermal wellbore requires an understanding of the complex interactions of heat transfer, fluid flow, phase change, flow regime change, and steam-water slip. The equations flow and concepts presented here account for those interactions and are used in simulating two-phase steam-water in a wellbore.

Introduction

The current examination of our country's energy resources has resulted in new and greater interest in geothermal power. There are geothermal installations in New Zealand, Italy, California, Mexico, Japan, U.S.S.R., and elsewhere. Many of these operations have long been producing power from reservoirs that are principally vapor-dominated systems, where essentially dry steam is produced at the wellhead and used directly for power generation. Unfortunately, vapor-dominated systems are relatively rare and current geothermal projects in the U.S. propose to use aquifers that produce hot water or a steam-water mixture. New Zealand is the leader in developing the technology required to operate these two-phase steam-water systems.

A typical aquifer, when first discovered, will produce undersaturated water at the wellbore sand-face. produce undersaturated water at the wellbore sand-face. As the fluid flows up the wellbore and pressure is lost, the fluid eventually flashes, producing a steam-water mixture in the well. The two-phase mixture is usually separated at the wellhead, with the steam being used directly to generate power." More advanced designs call for the fluid heat to be transferred to a secondary fluid, such as freon or isobutane, thereby using the potential of both steam and hot-water fractions. potential of both steam and hot-water fractions. The secondary fluid is used to generate power and the condensed water is reinjected into the formation. This approach reduces environmental problems and makes more efficient use of the available thermal energy. The reinjected water maintains aquifer pressure and prolongs the producing life of the field. As production continues over a period of months, the aquifer pressure will usually drop to the saturation point, and fully developed two-phase steam-water flow will be produced by the formation at the sandface. The two-phase mixture then flows towards the surface, experiencing pressure drop and heat transfer. In many applications, minerals are deposited on the wellbore casing where the mixture is flashing.

The design, analysis, and operation of geothermal systems requires sophisticated applications of the principles of fluid mechanics, heat transfer, and flow principles of fluid mechanics, heat transfer, and flow through porous media. The mathematical equations that describe a geothermal wellbore will be presented in this paper. The simulation of the combined effects of heat paper. The simulation of the combined effects of heat transfer and two-phase flow in wells has not previously been fully presented. Various portions of the technology have been published, but in a piece-wise fashion. The aim of this work is to develop an integrated approach to the problem, an adequate treatment of which must account for problem, an adequate treatment of which must account for the following effects:Two-phase pressure drop,Flow regime change,Phase change (fluid miscibility),Relative steam-water velocity (slip),Heat transfer from the fluid.

All these effects are interrelated.

JPT

P. 833