Long-period, long-duration seismic events during hydraulic stimulation of shale and tight-gas reservoirs — Part 1: Waveform characteristics

Abstract

Long-period long-duration (LPLD) seismic events are relatively low-amplitude signals that have been observed during hydraulic fracturing in several shale-gas and tight-gas reservoirs. These events are similar in appearance to tectonic tremor sequences observed in subduction zones and transform fault boundaries. LPLD events are predominantly composed of S-waves, but weaker P-waves have also been identified. In some cases, microearthquakes are observed during the events. Based on the similarity with tectonic tremors and our observations of several impulsive S-wave arrivals within the LPLD events, we interpret the LPLD events as resulting from the superposition of slow shear-slip events on relatively large faults. Most large LPLD waveforms appear to start as a relatively slower, low-amplitude precursor, lacking clear impulsive arrivals. We estimate the energy carried by the larger LPLD events to be ∼1000 times greater than a ∼M W − 2 microseismic event that is typical of the events that occur during hydraulic stimulation. Over the course of the entire stimulation activity of five wells in the Barnett formation (each hydraulically fractured ten times), the LPLD events were found to cumulatively release over an order of magnitude higher energy than microearthquakes. The large size of these LPLD events, compared to microearthquakes, suggests that they represent slip on relatively large faults during stimulation of these extremely low-permeability reservoirs. Moreover, they imply that the accompanying slow slip on faults, probably mostly undetected, is a significant deformation process during multistage hydraulic fracturing.

Seismo‐Mechanical Response of Anisotropic Rocks Under Hydraulic Fracture Conditions: New Experimental Insights

“…Due to these concerns, the monitoring of hydraulic fracturing is becoming increasingly important with the detection of microseismicity during multistage horizontal fracturing proving particularly effective as a monitoring tool (Eaton et al, ; Hurd & Zoback, ; Pearson, ; Warpinski et al, ). Several studies have investigated the seismic characteristics of these microseismic signals and have reported the occurrence of long‐period seismic events and “tremor‐like” events in various reservoirs during hydrofracture (Bame & Fehler, ; Das & Zoback, ; Ferrazzini et al, ; Kumar et al, ; Mitchell et al, ). Non‐Darcian flow and fracture resonance in wellbore‐scale features have also been noted as likely sources of this diagnostic seismicity (e.g., Tary et al, ).…”

Weighted-elastic-wave interferometric imaging of microseismic source location

“…Although in most cases the S-wave energy is stronger than that of P-wave energy (Das and Zoback, 2013), there are specifi c situations in which the P-wave energy is equivalent to or stronger than the S-wave energy. To ensure the high precision and resolution of the source location, different imaging resolutions or weighting coefficients are chosen according to the characteristics of the microseismic data.…”

Changes of Slip Rate and Slip‐Plane Orientation by Fault Geometrical Complexities During Fluid Injection

“…The slip responses along a permeable fault during fluid injection do not follow the trend seen in earthquakes where seismicity follows a decaying main shock‐aftershock pattern. Instead, the injection‐induced slow slip is generated that is associated with swarm‐like seismicity that expands outward from the injection point and occurs over a long period of time (Das & Zoback, ; McClure & Horne, ; Rutledge et al, ; Shapiro et al, ; Zoback et al, ). Quantification of the coupled fluid migration and shearing along permeable faults is thus recognized as critical for understanding injection‐induced seismicity and the potential for dynamic, unstable slip nucleation.…”