Downhole Tool Orientation and Hydraulic Fracture Monitoring of Induced Microseismicity

English Abstract:

Although tool orientation sources should be impulsive and band-limited and generate a time-varying wave, all sources have pros and cons. A perforation shot takes place in the treatment well, generally at the same depth as the anticipated microseismic activity; thus, it allows imaging of the rock volume of interest. Perforation shots have a tendency to exhibit P-, S_h-, and S_v-waves thanks to the asymmetry built into most perforation assemblies. A string shot is often located at a shallow depth (hence, inadequate rock volume illumination); it behaves like a slow-burning “explosion” over a short depth interval and typically generates good-quality P-waves appropriate for tool orientation, but often no S-waves sufficient for velocity model calibration. Surface/near-surface sources (e.g., dynamite, air gun, plasma sound source, thumper truck, nonexplosive electromagnetic pulse energy source, noise sources) can be employed. Ideally, one uses seismic vibrator trucks to generate P- and S-waves at far offsets and at different azimuths to minimize uncertainty. A downhole vibrating source generating both P- and S-waves can be used. Some sources can be tractored and thus are comparable to perforation shots in terms of pros and cons with the added benefits of repeatability (being nonexplosive) and time control (for velocity model calibration and potential crosswell profiling). In some stimulation assemblies, sliding sleeves may be used as they tend to generate decent P- and S-wave arrivals. Discerning sliding sleeve events among hydraulically induced events may be an issue, particularly if the pressure response curve is not available. A very early event may be used as a calibration shot, but one must assume that the detected early event occurs near the perforation. This assumption is often flawed, as pre-existing structures may allow stress transfer to occur or flow pathways may exist, resulting in the earliest events being some significant distance from the injection point.