Completion Of Hot Dry Rock Geothermal Well Systems
Authors: Pettitt Roland A. 
Published: December 2007

In the hot dry rock (HDR) concept of extracting geothermal energy, as developed by the Los Alamos Scientific Laboratory (LASL), a manmade geothermal reservoir is created by drilling a deep hole into relatively impermeable hot rocks, creating a large surface area for heat transfer by hydraulic fracturing, then drilling a second hole to intersect the fracture to complete the closed circulation loop.

The completion of HDR geothermal wells presents cementing problems that are unique. Current presents cementing problems that are unique. Current well depths are from 10,000 to 14,500 ft (3 to 4.4 km) with bottom hole temperatures up to 525 degrees F (275 degrees C). During investigation and development of the reservoir, water injection flows may lower the temperature of the entire wellbore to 104 degrees F (40 degrees C). As a result, the casing string may be subjected to temperature stress cycles representing differential temperatures of up to 455 degrees F (235 degrees C).

Experience has shown that the conventional high-temperature completion cement formulation of Class A or B portland cement, stabilized with 40% silica sand, does not withstand these cyclic stresses, and that rapid deterioration of casing-to-cement and cement-to-formation bonds occurs, which allows significant flow in the resulting microannulus. Investigations and tests (both laboratory and downhole) are being conducted by LASL, and other national and private laboratories, to determine other portland cement formulations, thermal setting cements, and special packer completion techniques which will better withstand the temperature, pressure, and flow requirements of HDR geothermal systems.

This paper will describe (a) the performance history of casing cement for the existing EE-1 injection well, (b) the proposed cementing plan for the EE-2 injection well presently being drilled, and (c) the completion system being tested in EE-1 for expanded use in EE-2.

Introduction

The Los Alamos Scientific Laboratory (LASL) for the past six years has been actively investigating the potential for and problems associated with extracting geothermal energy from hot, but essentially dry, rock at moderate depths.

In the Los Alamos concept, a manmade geothermal reservoir is formed by drilling into a region of suitably hot rock and then creating a large fracture, using conventional hydraulic fracturing techniques developed by the oil industry. After forming a circulation loop by drilling a second hole into the top of the fractured region, the heat contained in this reservoir is transported to the surface by circulation of water. The water in the earth loop is maintained as a liquid by pressurization at the surface, increasing both the rate of heat removal from the fractured reservoir and the amount of heat transported up the withdrawal hole.

To test this concept, a deep exploratory hole (GT-2) was completed in 1974 to a depth of 9610 ft (2929 m) in the granitic basement rocks of the Jemez Mountains of northern New Mexico at Fenton Hill (Fig. 1). The bottomhole temperature was 388 degrees F (197 degrees C). A near-vertical hydraulic fracture with a radius of about 400 ft (120 m) was created near the bottom of GT-2. Drilling of the second deep hole, Energy Extraction Well No. 1 (or EE-1), to intersect the fracture began in May 1975, about 250 ft (75 m) north of GT-2. It was completed in October at a depth of 10,053 ft (3064 m) and a bottomhole temperature of 400 degrees F (205 degrees C). Below 6240 ft (1957 m), EE-1 was directionally drilled to turn the hole through a 205 degrees spiral to create the underground connection. EE-1 was cased to a depth of 9600 ft (2926 m) with 7-5/8-in. casing and forms the injection well of the underground loop.