Geothermal Well Completions: A Critical Review Of Downhole Problems And Specialized Technology Needs
Authors: Snyder Robert E. 
Published: December 2007

Typical geothermal wells produce low pressure steam or hot brine from relatively shallow depths. However, effects of 350–700 degrees F temperatures and 100% aqueous environments create completion and operating problems that are foreign to even experienced oil d gas well operators. Unusual problems with casing that is otherwise properly designed for basic tension, burst and collapse properly designed for basic tension, burst and collapse are presented. Serious results of primary cementing limitations caused by severe hole conditions are emphasized. And state of the art completion methods and special problems in three major geothermal development areas are reviewed.

Introduction

In several areas in the Western United States, geothermal resources have been favorably assessed and wells are now being drilled and completed with the potential to supply power to electric generators, when potential to supply power to electric generators, when such plants are installed. Such wells typically will flow large volumes of superheated brines from which commercial quality steam will be separated. A viable operation is already supplying electricity in a nearby development in Cerro Prieto, Mexico. And dry steam wells are being produced commercially from non-typical, geothermal reservoirs in The Geysers area of northern California. These developments represent a variety of downhole conditions and reservoir characteristics ranging from those in highly underpressured, air drilled, fractured volcanics to conventional-appearing sediments with unique problems caused by high volume, hot water flows.

This paper is an overview of state of the art technology, recognized problems and limitations relevant to completion and production of geothermal wells. Technical discussion will include:Casing failure modes - -design considerations, and effects of unique hole conditions and geothermal fluid environments.Completion methods - - operations and problems in Western U.S. and Cerro Prieto (Mexico) fields, and productivity/injectivity considerations.

Studies were conducted under contracts with Sandia Laboratories of Albuquerque as part of a federally funded program designed to promote industry development by significantly reducing the cost of drilling and completing geothermal wells. Two important objectives are early definition of practical problems and rapid technology transfer. Some aspects of the technology will apply to geopressured geothermal wells planned for Texas and Louisiana. And Los Alamos Scientific Laboratory's Hot Dry Rock Development Program in New Mexico, sponsored by DOE, is an experimental geothermal operation with related technology needs.

CASING DESIGN CONSIDERATIONS

Geothermal wells are relatively shallow, typically 5,000–9,000 feet. Reservoirs are normally underpressured relative to a full column of fresh water. And wells are produced at maximum attainable rates through open casing strings to minimize friction loss. Factors that limit casing diameter are cost, drilling and cementing problems in large diameter holes, and collapse rating limitations. The entire well/gathering system should be optimized as friction loss in flowlines must be considered. In dry steam wells, too-large casing may reduce flow velocity and cause downhole condensation.

Proper injection well design will be equally important. Reinjection of cool brines to the producing zone will be required in certain large-scale geothermal projects in sedimentary basins. Injection wells will projects in sedimentary basins. Injection wells will have large diameter tubing, and completion programs will consider reservoir injectivity, per well pressure/ rate requirements a spacing. Presently, plant condensate is injected as a disposal requirement at The Geysers and reinjectivity is being evaluated in the Imperial Valley. However, in Cerro Prieto, separated water and plant condensate flow to a large surface lake in uncultivated desert.

An example geothermal well completion is illustrated in Fig. 1.