ABSTRACT
Nearly 30 estimates of the minimum earth stress were obtained from hydraulic stimulation tests in 5 boreholes at depths varying from 0.6 to 4.3 km in jointed granitic rock near the Valles Caldera in New Mexico. This site is located within the transition between the Colorado Plateau and Basin and Range provinces. Stresses were determined from open hole hydraulic fracturing experiments conducted with water at pressures up to 48.3 MPa (surface). The virgin bottomhole temperatures ranged up to 327°C. Pressure records show no evidence of rock breakdown, indicating injection into pre-existing joints. A typical pressure response consists of an early wellbore storage period followed by linear fracture flow, which then evolves into a steady pressure, fracture extension mode. Shut-in behavior is characterized by a sharp pressure drop followed by a more gradual decay. The minimum earth stress shows a general linear trend with depth, increasing with a gradient of 19 MPa/km. A low stress anomaly occurs from 2490 to 3100 m where the minimum stresses are as much as 28 MPa lower.
INTRODUCTION
Earth stresses were estimated from hydraulic stimulation experiments as a part of the Hot Dry Rock Project being conducted by the Los Alamos National Laboratory with funding from the US DOE, Japan's New Energy Development Organization and West Germany's Ministry for Science and Technology. The project site is located at Fenton Hill, Sandoval County, New Mexico, on the western flank of the Valles Caldera. Volcanism occurred at the Valles Caldera roughly 10 to 0.1 million years ago. The geology (Laughlin et al., 1983) consists of tuffs and other younger rocks underlain by precambrian metamorphic and igneous rocks that are the target of the Hot Dry Rock project. Five test wells and three side tracks have been drilled ranging in depth from 0.75 km to 4.6 km.
Earth stress information at Fenton Hill is derived from hydraulic stimulation experiments conducted in these wells. Although the primary objectives of these experiments were other than earth stress measurements, considerable data on formation pressure response and microseismic events were collected. The minimum compressive principal earth stress was deduced from hydraulic fracture extension pressures extrapolated to zero flow rate and from Instantaneous Shut-InPressures (ISIP). The directions of the principal stresses were determined from fault plane solutions obtained from first motions of fracturing-induced microearthquakes recorded on surface seismometers and from the envelope of the microseismic event locations determined from downhole geophone data (Murphy and Fehler, 1986). Corroborating data were obtained from laboratory differential strain analysis of core samples. Other techniques such as impression packers and borehole televiewers have not been used successfully because of the extremely hostile downhole conditions.
EXAMPLE OF A TEST
A typical pressure-time record from one test is shown in Fig. 1 and 2. This test, termed Experiment 2061, was carried out by injecting 1.38 million gallons of fresh water into well EE-3A at depths between 3827 m and 4017 m. In this well a "thief" zone at 3100 m accepts water at pressures significantly lower than those encountered at test depth, hence the test interval was isolated using an inflatable packer (Dreesen et al., 1986).
