In-Situ Hydraulic Parameters for the Canirnenellis Granite Hot Dry Rock Geothermal Energy Research Reservoir
Authors: Pine Robert J. , Ledingham Peter 
Published: December 2007

A research hot dry rock (HDR) geothermal energy reservoir is currently under development in the Carnmenellis granite in Cornwall, U.K., at a depth of about 6,600 ft [2000 m]. This paper details the performance, analysis, and interpretation of hydraulic tests using low to medium pressures and flow rates. These tests allowed an evaluation pressures and flow rates. These tests allowed an evaluation of the in-situ hydraulic parameters relevant to initial water storage and long-term leakage. Tests were analyzed using both diffusion theory and an explicit, Coupled hydraulic/mechanical computer code.

At low fluid overpressures, the permeabilities, in terms of uniform diffusion, were in the range of 1 to 10d, rising to about 60 d at overpressures of about 725 psi (5 MPa). These figures imply that long-term leakage psi (5 MPa). These figures imply that long-term leakage from an HDR reservoir is minor. The computer model was able to simulate several cycles of injections and shut-ins at progressively higher pressures and flow rates with a single set of variables.

Introduction

An HDR geothermal energy research project is being conducted by the Camborne School of Mines (CSM) at their test facility at Rosemanowes quarry in Cornwall (Fig. 1).

The facility includes two deviated wells, RH11 and RH12, each drilled to a vertical depth of about 6,600 ft [2000 m] (Fig. 2). The holes are located within the Carnmenellis granite throughout their lengths, and are inclined at 30 from vertical in their lower sections to intersect the predominantly vertical natural jointing.

The primary objective of the project is to link the two wells through the natural jointing by using a combination of explosive pretreatment and hydraulic stimulation to create a heat transfer zone (or reservoir) for the circulating water. However, it was first necessary to characterize the hydraulic behavior of the rock mass with modest flow rates and fluid overpressures before the natural joint structure was modified by major hydraulic injections. The resulting information then could give reference conditions for comparing the results of the high energy tests. The information is relevant for determining the leakage losses at the limits of the stimulated reservoir. It is also of interest to compare the results with those obtained at depths of 1,000 ft [300 m] in previous experiments at the same site to obtain a depth perspective for the hydraulic behavior.

Since the test depth is unique for crystalline rocks in the U.K. and is unusual elsewhere, the results are of interest for other projects involving facilities at comparable depths (e.g., liquid waste disposal and deep-mined hydroelectric pumped storage).

Geological Conditions

The Carnmenellis granite pluton was emplaced about 285 million years ago, and subsequently has undergone a complex tectonic history. The pluton is exposed at the ground surface over an approximately circular area of about 50 sq miles [130 km]. The Rosemanowes quarry test site is near the center of the exposure (Fig. 1). Topography is gently undulating with a maximum relief of about 330 ft [100 m].

There is a network of dominant, near orthogonal, vertical joints throughout the granite, with typical spacings of 3 to 10 ft [1 to 3 m] near the ground surface, 10 to 30 ft [3 to 10 m] at 2,500-ft [750-m] depth, and similar to wider spacing at 6,600 ft [2000 m]. Nearly horizontal joints also occur but less frequently. All joints are typically tight, away from zones of stress relief.

In-situ stresses have been measured in some detail. At a depth of 6,600 ft [2000 m], the vertical, maximum, and minimum horizontal stresses are approximately 7,500 psi [52 MPa], 8,700 to 10,200 psi (60 to 70 MPa) and 4,400 psi [30 MPa], respectively. The horizontal stress directions and major near-vertical joint set strike orientations are shown in Fig. 3.

The fabric of the Carnmenellis granite consists of megacrysts of alkali feldspar set in a coarse ground mass of plagioclase, alkali feldspar, micas, and trace minerals. Grain size is variable, with megacrysts up to 0.75 in. [20 mm] long and crystals in the ground mass typically 0.08 to 0.2 in. [2 to 5 mm] in diameter. The permeability of the fabric is extremely low, with values permeability of the fabric is extremely low, with values of 10(-3) d obtained by laboratory water diffusion tests. The mechanical properties may be characterized by a uniaxial compressive strength of about 19,600 psi [135 MPa], with dimensionless triaxial strength parameters m and s of 21 and 1.0 per Hoek and Brown, parameters m and s of 21 and 1.0 per Hoek and Brown, Young's modulus of 9.43 × 10(6) psi [65 GPa], and Poisson's ratio of 0.2. Poisson's ratio of 0.2. JPT

P. 1982