Passive seismic data recorded in observation wellbores for hydraulic fracture monitoring are usually used to detect P- and S-wave arrivals from induced microseismic events. The time arrivals for these events are used to compute source locations to build a fracture location map. Along with these arrivals are a number of other coherent signal and noise events. The primary objective of this thesis is to examine various other types of coherent signal and noise on four microseismic datasets. This thesis is made up of four separate studies.
The first study examines numerous coherent low-frequency (<100 Hz) arrivals characterized by conspicuous dispersion and quasi-linear moveout. Their apparent velocities and dispersion characteristics are consistent with Lamb waves, an elastic guided wave that propagates within finite media such as plates or cylindrical casing. Lamb waves may be used for monitoring degradation of casing cement over time.
The second study examines two datasets to characterize frequency shifts in discrete narrow passbands caused by wellbore acoustic transmissivity due to geophone clamping. The passbands changed throughout two fracture stimulations in proportion to the hydraulic fracture pressure. Analytical and finite-element models suggest that these temporal variations may be caused by a relative change in geophone clamping force as the pressure external to the wellbore varies. This sensitivity suggests that passive seismic monitoring can be used for downhole detection of relative stress changes.
The third study focuses on frequency content variations of 20 high S/N P- and S-wave events. The 20 events were grouped into four clusters based upon a geolocation algorithm and event cross-correlations. Spectral analysis shows variation in the signal within event sets. A 2-D elastic finite-difference simulation using a layered model shows that this variation could be ascribed to path effects associated with propagation of the signal from the source to the receivers.
The fourth study examines a potential long-period long-duration (LPLD) seismic event and analyzes the phenomena in detail. LPLD events observed on microseismic data have been likened to deep low-frequency tremor attributed to slow-slip processes along pre-existing fractures or strike-slip regimes at subduction zones. The event may be the result of hydraulic fracture fluid leak-off out of the intended zone into a pre-existing fracture.