Fracturing Operations in a Dry Geothermal Reservoir

Abstract

ABSTRACT Fracturing and completing deep wells in hot, non-porous crystalline basement rock challenges conventional equipment use, procedures, and techniques common in oil and gas and normal geothermal completions. Fracturing operations at the Fenton Hill, New Mexico, Hot Dry Rock (HDR) Geothermal Test Site initiated unique developments necessary to solve problems caused by an extremely harsh downhole environment. Two deep wells were drilled to approximately 15,000 ft (4.6 km); formation temperatures are in excess of 600°F (315°C). The wells were drilled during 1979-1981, inclined at 35 degrees, one above the other, and directionally drilled in an azimuthal direction orthogonal to the least principal in-situ crustal stress field. The pair of wells form the injection and production wells of an energy extraction system which will be unique in reservoir development. The test site is located near the flank of a young silicic composite volcano, the Valles Caldera, in the Jemez Mountains of northern New Mexico. The deeper well was planned as the cold water injection hole; as such, it may be cooled from the static, conduction geothermal gradient to as low as 80°F (25°C). The upper, production well will be heated to over 500°F (260°C) along its entire length. The well pair has been designed to be a closed loop, heat-extraction system formed with hydraulic fracture connections between the 1200 ft (370 m) vertically-spaced wells. These production conditions, and the in-situ formation temperatures, strongly constrain all completion hardware, cementing formulations and procedures, and fracturing operations. Hydraulic fracturing experiments to connect the two wells have used openhole packers, hydraulic jet notching of the borehole wall, cemented-in isolation liners and casing packers. Problems were encountered with hole drag, high fracture gradients, H2S in vent back fluids, stress corrosion cracking of tubulars, and the complex nature of three-dimensional fracture growth that requires very large volumes of injected water. Two fractured zones have been formed by hydraulic fracturing and defined by close-in, borehole deployed, microseismic detectors. Initial operations were focused in the injection wellbore near total depth, where water injection treatments totalling 51,000 bbls (8,100 m3) were accomplished by pumping through a cemented-in 4-1/2-in. liner/PBR assembly. Retrievable casing packers were used to inject 26,000 bbls (4,100 m3) in the upper section of the open hole. Surface injection pressures (ISIP) varied from 4,000 to 5,900 psi (27 to 41 MPa) and the fracture gradient ranged from 0.7 to 0.96 psi/ft.