In-situ stress measurement in the Carnmenellis granite—II. Hydrofracture tests at Rosemanowes quarry to depths of 2000 m

Abstract

Hydrofracture stress measurments were conducted in the Carnmenellis granite at the Camborne School of Mines Hot Dry Rock Geothermal Energy Research site at Rosemanowes Quarry near Penryn, Cornwall, U.K. Tests were conducted at depths of up to 2000 m in borehole RH 12, which was drilled with nominal diameters of 311 mm and 216 mm and at an inclination of up to 32° from vertical. The tests were successful in establishing reliable values for the minimum horizontal stress, and estimates of the maximum horizontal stress. Stress anisotropy was very significant throughout the tested depth, with maximum and minimum horizontal stresses of about 70 and 30 MPa respectively at a vertical depth of 2000 m. The overburden stress at this depth was estimated as 52 MPa. The bearing of the maximum horizontal stress was established in a separate programme of overcoring stress measurement at approximately 130° relative to true north. The stress magnitudes determined by the overcoring tests were close to those measured by the hydrofracture tests at the same depth. The effects of hole inclination, test procedure and premature cracking of the borehole wall due to high compressive stresses are discussed in relation to the in-situ stress interpretation.