Imaging Hydraulic Fractures Using Temperature Transients in the Belridge Diatomite

Abstract

Abstract* Results of a temperature transient analysis of Shell's Phase I and Phase II Diatomite Steamdrive Pilots are used to image hydraulic injection fracture lengths, angles, and heat injectivities into the low-permeability formation. The Phase I Pilot is a limited-interval injection test. In Phase II, steam is injected into two 350 ft upper and lower zones through separate hydraulic fractures. Temperature response of both pilots is monitored with sixteen logging observation wells. A perturbation analysis of the non-linear pressure diffusion and heat transport equations indicates that at a permeability of about 0.1 md or less, heat transport in the Diatomite tends to be dominated by thermal diffusivity, and pressure diffusion is dominated by the ratio of thermal expansion to fluid compressibility. Under these conditions, the temperature observed at a logging observation well is governed by a dimensionless quantity that depends on the perpendicular distance between the observation well and the hydraulic fracture, divided by the square root of time. Using this dependence, a novel method is developed for imaging hydraulic fracture geometry and relative heat injectivity from the temperature history of the pilot. The azimuth of the Phase I hydraulic fracture is determined to be 14 ± 2° N-NE. The azimuth of the Phase II upper hydraulic fracture is determined to be 16 ± 2 ° N - NE in the northern half of the pilot, increasing to 21 ± 2° in the southern half of the pilot. The azimuth of the lower hydraulic fracture averages 19 ± 4°. The hydraulic fractures are found to be symmetric around both injectors with an estimated length of 200 ft. Increased steam injection after the first year of pilot operations caused what is interpreted to be horizontal fractures toward the west in the G cycle and the east in the M cycle. These features are imaged to be at least 100 ft and 160 ft along the hydraulic fracture azimuth. These conclusions are compared to tiltmeter data, microseismic data, and a simulation history match of pilot performance. Microseismic events recorded in the pilot are apparently not diagnostic of heat delivery to the formation.