Characterizing Vertical Fracture Growth in Diatomite Using Seismic Source Parameters

Steam injections are frequently used to mobilize hydrocarbons in diatomite reservoirs, reducing the viscosity of the oil to facilitate drainage. These operations can induce deformation in the reservoir, and frequently the strain rates are rapid enough to propagate seismic energy. A properly calibrated microseismic system detecting these emissions can be used to outline growth of steam chambers and activation of stuctures around injectors by locating the sources of this energy. The worst-case scenario is that of steam connecting through a fracture network and releasing on the surface can not only be an inefficient use of energy, but also represents a potential hazard to operation in terms of damaged equipment and danger to personnel. Routine, rapid location of events outlining vertical growth of the process zone towards the surface during steam cycles is one obvious avenue of alerting operators to the possibility of steam escaping above ground. However, many sites feature the ubiquitous presence of above-zone events that may not lend themselves easily to detection of such trends. Another avenue to this increased understanding comes from a detailed analysis of the microseismic waveforms by inferring source characteristics such as moment, energy, and energy index (energy normalized by moment). Variations in this energy index for induced microseismic events can be used to observe variations in the failure process: high energy index events are characterized by more radiated seismic energy relative to other events of equal (moment) magnitude whereas low energy index events are partitioning less energy into the seismic waves and more into the deformation of the reservoir. Most events associated with the steaming in the diatomite follow a characteristic profile where there is an initial increase in energy index during injection followed by a decline at the end. However, examining events that progress upwards over several steam cycles, we see that these events radiate increasingly more energy through the progressive injection cycles, with the largest expression of energy occurring at the surface. This observation, that the fracturing process for these events is becoming more efficient at radiating seismic energy, suggests that less work is dedicated to creating fractures, and more work is done activating structures created from previous cycles facilitating growth to surface of a fracture network that creates a pathway for the possible escape of injected steam.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California