Long period long duration seismic events during hydraulic stimulation of gas reservoirs [electronic resource]
Keywords: Geophysics.

This thesis attempts to identify and characterize the range of deformation processes that occurs during hydraulic stimulation of extremely low-permeability shale-gas reservoirs. The aim is to improve the mechanical understanding on how multi-stage hydraulic fracturing leads to reservoir stimulation. While monitoring the microseismicity that accompanies hydraulic fracturing provides information about the depth, extent and growth patterns of the induced fracture network, several lines of evidence indicate that microearthquakes are only part of the total deformation. The central topic of this dissertation is to describe a previously unknown class of Long Period Long Duration (LPLD) seismic events, much larger in magnitude than microearthquakes. Interpreted to represent sustained slow slip on relatively large faults, LPLD events can help explain the development of hydraulically conductive surface area and the overall increase in reservoir permeability associated with hydraulic fracturing. More importantly, the LPLD events are indicative of potentially widespread slow aseismic processes that enhanced the effectiveness of the hydraulic fracturing process. The first chapter introduces the problem of understanding hydraulic fracturing in shale gas reservoirs in the light of recorded seismic signals during the monitoring process. It also talks about the motivation for this research. The second chapter discusses the advantages of applying passive seismic tomography to a typical microseismic data set recorded in a single, nearly vertical well to monitor hydraulic stimulation of a shale-gas reservoir. In this method, the event locations and the velocity model are estimated simultaneously unlike the conventional industry practice of using a fixed velocity model derived from sonic logs and perforation-shots. Essentially the velocity model and the event locations are constantly updated till the desired fit is achieved. In this way we are making use of the information carried by the microearthquakes themselves for improving the model and consequently, the locations. This added flexibility not only makes it possible to accurately predict traveltimes of the recorded P- and S-waves, but also provides a convincing evidence for anisotropy of the examined shale formation. While we find that velocity heterogeneity does not need to be introduced to explain the data acquired for each stage of hydraulic fracturing, the obtained models are suggestive of possible time-lapse changes in the anisotropy parameters that characterize the stimulated reservoir volume. The third and fourth chapters discuss the principal topic of the thesis, which is identifying, characterizing and locating LPLD seismic events that have been observed during hydraulic fracturing in several shale-gas and tight-gas reservoirs. The LPLD events are low amplitude signals lasting for 10-100 seconds and are most conspicuous in the 10-80 Hz frequency band and are identified by their high amplitudes in stacked spectrograms in this particular frequency band. Band-pass filtering of the raw seismic data in this band is used to eventually isolate all the LPLD events. Third chapter of this thesis describes the waveform characteristic of the LPLD events and the fourth chapter discusses the location and mechanism of these events. LPLD events are similar in appearance to tectonic tremor sequences observed in subduction zones and transform fault boundaries. LPLD events, which are complex but coherent wave trains, have finite moveouts, the direction and amount of which confirms that they originate in the reservoir. They are predominantly composed of S-waves but weaker P-waves have also been identified. In some cases, microearthquakes are observed to occur during the LPLD events. Based on the similarity with tectonic tremor and our observations of several impulsive S-wave arrivals within the LPLD events, LPLD events have been interpreted as resulting from the superposition of shear slip events on relatively large faults. Using a method akin to empirical Green's function, the energy carried by the larger LPLD events is estimated to be about ~1000 times greater than a microseismic event of moment magnitude (MW) ~ -2, that is typical of the events that occur during hydraulic stimulation. In the course of the entire stimulation activity, LPLD events were found to release cumulatively, as much as two orders of magnitude higher energy than the 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. The fourth chapter describes how within the limitations of the recording geometry, it is possible to determine the general area in the reservoir from which the events originate for the two case studies in the Barnett shale. In the first study, LPLD events occur in the region where the density of natural fractures as well as the fluid pressure during pumping was highest. In the second case study, the LPLD events are observed to occur between two wells and seem to establish a hydraulic connection between these wells. In both data sets, the LPLD events occur in areas with very few microearthquakes. There are likely many faults in the reservoir not producing microseismicity that are slowly slipping in response to the stimulation; LPLD events are, however, expected to be generated where faults large enough to produce a sequence of slow slip events exist. A combination of factors such as high fluid pressure and/or high clay content is potentially responsible for slow slip on faults. This thesis identifies a number of topics for future studies (hopefully better data sets having improved recording geometries and sensors) to better study the relation of LPLD events to microearthquakes. LPLD events can provide useful insights into the primary deformation mechanism responsible for production post hydraulic stimulation. Knowing that these newly discovered events evidently caused by pervasive slow slip on relatively large faults are significantly impacting the stimulation of these extremely low-permeability reservoirs, it might be possible to better design reservoir stimulation by mapping the distribution of faults and fractures and areas with rock properties that favor slow slip.