A Hydraulic Fracturing Technique For Dry Hot Rock Experiments In A Single Borehole
Authors: Ernst Peter L. 
Published: December 2007

Ernst, Peter L., Preussag Erdoel and Erdgas

To minimize cost of hot dry rock experiments it is essential to conduct these experiments in single boreholes. A hydraulic fracturing technique is proposed that permits changes in flow paths, flow rates and permits changes in flow paths, flow rates and heat exchange surface without drilling a second hole. The feasibility of this technique will be field tested in West Germany.

Introduction

The hot dry rock concept developed at the Los Alamos Scientific Laboratory requires drilling two holes into hot crystalline rock. These boreholes are connected at depth through one or more hydraulic fracs. This concept is field tested extensively at the Fenton Hill test site by LASL scientists. Preliminary results of these tests show that Preliminary results of these tests show that the concept is technically practicable and indicate a good chance of commercial feasibility.

Hydraulic fracturing of sediments is done routinely since a number of years, but experience in fracturing crystalline rock is scarce. Before a commercial hot dry rock energy system can be realized, additional data of an in situ properties of crystalline rocks and stress fields must be gathered. Theoretical concepts of hydraulic fracturing, flow through fractures, heat extraction rate, reservoir lifetime and many others have to be checked and improved by field testing.

A fracture system designed for scientific experiments must allow control of significant parameters like flow rate, flow path, heat exchange surface and others. To path, heat exchange surface and others. To finance as many experiments as possible from limited funds, cost of installation of such a test facility has to be minimized. The most expensive part of a hot dry rock test system is the drilling of boreholes. If a system can be realized that uses only one borehole, significant cost reduction will result. Even in case, boreholes reduction will sediments for other purposes can be deepened into hot crystalline rock and used for hot dry rock experiments, it is essential to be able to perform these tests in only one borehole. We have attempted to design such a system theoretically and hope to realize and test it in the near future.

SYSTEM DESCRIPTION

Figure 1 shows the schematic design of the fracture circulation loop. At least three fractures will be created in a cased and perforated borehole and connected to each perforated borehole and connected to each other. Injection and production will be done through tubing and annulus. Casing size will permit running of two parallel strings of permit running of two parallel strings of tubing, so that perforated intervals are accessible separately. A cased hole was chosen to ensure fracture initiation at defined points and to avoid the use of open hole packers. The fractures will be kept open by fluid pressure only, as is done in the possibility to inflate and deflate fractures partly or as a whole, and ensures minimum restriction of flow path.

To make sure that fractures will grow together, it is essential that both borehole and fractures be vertical. A borehole can be drilled vertical, but the inclination of hydraulic fractures cannot yet be controlled. It is indicated by theory and experience that, in the absence of significant and non- horizontal tetonic stresses, hydraulically induced fractures will be vertical to within 1 to 3 degrees at those depths, where the necessary elevated temperatures will be encountered. The theory for connection of two parallel adjacent fractures is described by Hsu and Sarda.