Abstract:
Microseismic monitoring by downhole geophones, surface seismic, fiber-optic distributed acoustic sensing (DAS), and distributed temperature sensing (DTS) observations were made during the hydraulic fracture stimulation of the MIP-3H well in the Marcellus Shale in northern West Virginia. DAS and DTS data measure the fiber strain and temperature, respectively, along a fiber-optic cable cemented behind the casing of the well. The presence of long-period long-duration (LPLD) events is evaluated in the borehole geophones, DAS data, and surface seismic data of one of the MIP-3H stimulated stages. LPLD events are generally overlooked during the conventional processing of microseismic data, but they represent significant nonbrittle deformation produced during hydraulic fracture stimulation. In a single stage that was examined, 160 preexisting fractures and two faults of suboptimal orientation are noted in the image logs. We identified two low-frequency (<10 Hz) events of large temporal duration (tens of seconds) by comparing the surface seismic data, borehole geophone data, and DAS amplitude spectra of one of the MIP-3H stages. Spectrograms of DAS traces in time and depth reveal that the first low-frequency event might be an injection noise that has footprints on all DAS channels above the stimulated stage. However, the surface seismic array indicates an LPLD event concurrent with the first low-frequency event on DAS. The second LPLD event on DAS data and surface seismic data is related to a local deformation and does not have footprints on all DAS channels. The interpreted events have duration less than 100 s with frequencies concentrated below 10 Hz, and are accompanied by microseismic events. Introduction Hydraulic fracturing of unconventional shale reservoirs is necessary to enhance the reservoir permeability. Hydraulic fracturing has been undertaken by various operators since the 1940s (Montgomery and Smith, 2010). Companies carry out a multistage perforation followed by a high-pressure fluid/proppant slurry injection to create long hydraulic fractures within low-permeability reservoirs. These hydraulic fractures combined with significant stimulation of the bounding natural fracture network increase the stimulated reservoir volume and subsequent reservoir production. The present-day stress orientation within the reservoir exerts the greatest influence on the direction of hydraulic fracture growth. However, the density, orientation, and openness of natural fractures/faults can also affect the direction and complexity of hydraulic fracture propagation. Brittle failure along preexisting fractures generates small-magnitude microseismic events (MSEs) as highfrequency seismic waveforms with clear Pand S-arrivals. These MSEs are interpreted to result from shear slip on preexisting fractures and faults near induced hydraulic fractures (Rutledge and Phillips, 2003; Warpinski et al., 2004; Das and Zoback, 2013a). Although microseismicity is used as a direct measure to calculate the stimulated reservoir volume, the correlation is debatable (Sicking et al., 2012; Wilson et al., 2016). Energy balance between MSEs and injection energy has been compared in several studies (Boroumand and Eaton, 2012; Warpinski et al., 2012; Kavousi et al., 2017; Kumar et al., 2017b). Results show that the energy released in the form of MSEs is only a small portion of the energy supplied to the reservoir during hydraulic fracture stimulation as estimated from treatment pressure and injection volume. This deficit suggests that energy release by other deformation mechanisms are important components of the input-output energy balance associated with hydraulic fracturing. Recently, long-period long-duration (LPLD) events have been considered to be a potential source for energy consumption by slow shear slip on relatively large faults (Das and Zoback, 2011, 2013a; Mitchell et al., 2013; Kwietniak, 2015). Das and Zoback (2013a) show that LPLD events release 1–2 orders of magnitude more enWest Virginia University, Morgantown, West Virginia, USA. E-mail: pkavousi@mail.wvu.edu; tom.wilson@mail.wvu.edu; tim.carr@mail.wvu.edu. NETL DOE, Pittsburgh, Pennsylvania, USA. E-mail: abhash.kumar@netl.doe.gov; richard.hammack@netl.doe.gov. Georgia Tech University, Atlanta, Georgia, USA. E-mail: haibin.di@ece.gatech.edu. t Special section: Distributed acoustic sensing and its oilfield potential Interpretation / February 2019 SA1 D ow nl oa de d 11 /1 9/ 19 to 1 57 .1 82 .1 54 .1 89 . R ed is tr ib ut io n su bj ec t t o SE G li ce ns e or c op yr ig ht ; s ee T er m s of U se a t h ttp :// lib ra ry .s eg .o rg / ergy than observed MSEs and may contribute much more to reservoir stimulation than that associated with observed microseismicity. Zoback et al. (2012) show that fracture and fault orientations relative to the present-day SHmax could determine the slip behavior. They propose through modeling that misaligned faults undergo slow slip, whereas well-oriented faults and fractures undergo brittle failure in response to increased fluid pressure. The reason might be that fluid pressure propagates faster along well-oriented faults than misaligned faults and triggers a rapid slip. Das and Zoback (2013a) suggest that LPLD events result from slow shear slip on preexisting faults that are unfavorably oriented in the present-day stress field or have high clay content. Das and Zoback (2011) analyze borehole seismic data from hydraulic fracturing in the Barnett Shale. They interpret the LPLD events as a low-frequency energy release (between 10 and 80 Hz) that lasts from tens of seconds to minutes. The LPLD events are usually characterized by low-amplitude arrivals and incoherent phase, making the phase picking very difficult (Das and Zoback, 2011; Eaton et al., 2013). LPLD seismic events have similarities with observed tectonic tremors in subduction zones and transform faults (Caffagni et al., 2015). Tectonic tremors are assumed to be accompanied by slow shear slip of plates in transform faults or subduction zones (Obara, 2002; Shelly et al., 2006; Nadeau and Guilhem, 2009). Das and Zoback (2011) suggest that similar phenomena could happen during hydraulic fracturing when there is slow slip on preexisting faults of suboptimal orientation. They propose that this nonbrittle deformation process could contribute to reservoir production by significant permeability enhancement. Mitchell et al. (2013) analyze the seismic waveforms from surface and downhole geophone arrays during hydraulic fracturing of a horizontal well in the Cline Shale of West Texas to detect LPLDs. The spectrogram of the stacked waveforms from the downhole array revealed the presence of several LPLDs; however, no LPLDs were detected in the surface recordings, most likely because of low signal strength. Eaton et al. (2013) study seismic waveforms recorded during the hydraulic fracturing of a well in a Montney gas reservoir in British Colombia and identify several LPLD events. They observe LPLD events at frequencies less than 10 Hz and propose that complexity of preexisting natural fractures could affect the spectral frequency of LPLD events. Analysis of distributed acoustic sensing (DAS) data requires that fiber distortions and regional tectonic events unrelated to the local reservoir response be ruled out as possible LPLD events. Recently, Caffagni et al. (2015) and Zecevic et al. (2016) point out that the regional seismic events can be misidentified as LPLD events due to their similar waveform characteristics, overlapping frequency content, and apparent velocity. For regional earthquakes, codas of Pand S-waves that are multiply reflected and scattered may also produce the effect of long-duration signals with ambiguous arrival times similar to LPLD events (Aki, 1969; Aki and Chouet, 1975). In the recent past, Kumar et al. (2017a, 2018) identify several LPLDs in the surface seismic data recorded during hydraulic fracturing of the Marcellus Shale wells at the current study site in Monongalia County, West Virginia. To avoid any misinterpretation between an LPLD event and known or unknown regional earthquakes, Kumar et al. (2017a, 2018) analyze seismic waveforms from the nearest stations of the USArray and cross-checked regional earthquakes. Kumar et al. (2017a, 2018) find that no temporal correlation between the LPLD events detected from surface broadband stations and known catalog events, suggesting a local source of deformation as the cause of the LPLD events. The absence of regional earthquake activity during stimulation allows us to explore the DAS and distributed temperature sensing (DTS) data for possible LPLD events associated with hydraulic fracturing of a horizontal Marcellus Shale well in Monongalia County of West Virginia. Fiber-optic DAS and DTS data record the strain or strain rate and temperature around the wellbore, respectively. Fiber-optic sensing technology has been used by oil and gas companies since 1990s to monitor steam injection, injection profiling, acid injection profiling, and hydraulic fracture diagnostics (Karaman et al., 1996; Sierra et al., 2008; Glasbergen et al., 2010; Rahman et al., 2011; Holley and Kalia, 2015). DAS is sensitive to the vibrations in the local environment around the fiber, and it provides a measure of the relative axial strain or strain rate of the optical fiber (Tanimola and Hill, 2009). The frequency content of DAS data has been studied by several researchers. Ghahfarokhi et al. (2018) show that seismic attributes, such as instantaneous frequency, dominant frequency, and energy, could be applied to DAS data to better monitor hydraulic fracturing. Jin and Roy (2017) show that very low-frequency strain-rate DAS signals could reveal information about the stress shadow, fracture length, density, and width. However, low-frequency DAS data can be significantly affected by temperature variations around the fiber during crossstage flow communications. Elimination of thi
