Imaging Fractures Using Temperature Transients From Perturbation Analysis—A Novel Surveillance Technique Applied to the Belridge Diatomite

Abstract

Results of a temperature transient analysis of data from Shell's 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 16 logging-observation wells.A perturbation analysis of the nonlinear pressure diffusion and heat-transport equations indicates that at a permeability of approximately 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 loggingobservation 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 AE 2 N-NE. The azimuth of the Phase II upper hydraulic fracture is determined to be 16 AE 2 N-NE in the northern half of the pilot, and estimated to increase to 21 AE 2 in the southern half of the pilot. The azimuth of the lower hydraulic fracture averages 19 AE 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 and 160 ft, respectively, 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.