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The deformation that occurs during hydraulic fracturing is investigated in the context of a few recently reported examples of induced earthquakes. Energy balance considerations are used to compare radiated microseismic energy with the hydraulic fracture injection. The microseismic energy is typically a very small portion of the hydraulic energy unless fault activation releases stored tectonic stresses. Nevertheless, the microseismic energy is found to be smaller than the hydraulic energy for all cases examined. Similarly, the concept of conservation of injected volume is used to compare microseismic volumetric deformation strength with the total injected volume. Both the energy and volume balance suggest significant aseismic deformation typically occurs, but the injected volume and corresponding energy may suggest a potential upper bound on the largest magnitude earthquake that may occur.
Summary Hydraulic fracturing is an essential technology for hydrocarbon extraction from both conventional and unconventional reservoirs. Recently, concern has developed regarding induced seismicity generated in association with multistage fracturing of horizontal wells in shale reservoirs. Microseismic monitoring of hydraulic fractures, which has been a routine service for over a decade, can provide information about the levels of seismic activity commonly found during fracturing. A review of thousands of fracture treatments that have been microseismically monitored shows that the induced seismicity associated with hydraulic fracturing is very small and not a problem under any normal circumstances. Results are presented for six major shale basins in North America in which hundreds to thousands of fracture treatments have been conducted in predominatly gas reservoirs. This paper reviews the methodology, the data, and the interpretation of the microseismicity.
Summary Induced seismicity associated with hydraulic fracturing has become a significant regulatory issue in Western Canada, after the occurrence of felt events in three specific reservoirs. Seismicity results from elevated pressure of the stimulated fracture network reducing the effective clamping force and triggering slip of tectonically stressed faults. Several published examples indicate activation over progressively larger fault regions, as multiple stages interact with critically stressed faults in various relative orientations to the treatment well. Geomechanical modeling is used to examine progressive fault slip from multi-stage fracturing and the associated seismicity. The modeling explores different operational scenarios to examine progressive fault activation as hydraulic fracture stages sequentially pressurizes more of the fault. Testing these mitigation strategies on faults in different orientations provides potential operational guidelines to support seismic traffic light systems, typically used to mitigate injection induced seismicity. Simulations show that the amount of injected fluid interacting with the fault plane controls the intensity of observed seismicity for a specific fault. Stages farther away from the fault can have an impact on fault slippage but with a delayed effect. Sequence of propagation of the hydraulic fracture stages compared to fault orientation is important. If the first stage is closest to the fault, more of the injected fluid will interact with the fault, triggering a large slipping patch on the fault plane. Successive stages will have a lesser effect due to stress shadowing. However if the first stage is the most distant from the fault, slippage on the fault plane will be gradual, thus reducing the amount of seismic moment release. The sequence in which wells on a multi-well pad are stimulated could also impact the associated seismicity. Introduction There has been increasing occurrences of recent seismicity associated with fault activation during hydraulic fracturing, resulting from elevating pore pressure on optimally-oriented, pre-existing faults leading to triggered release of stored tectonic energy (Maxwell, 2013). Anomalous seismicity is similar to microseismicity, although larger magnitude fault seismicity corresponds to inelastic slip over a larger area. However, triggered fault slip has in certain conditions lead to felt ground shaking. For example, three Western Canadian reservoirs located close to the tectonically-active trust belt have experienced anomalous activity: specifically at localized regions of the Horn River Basin, Montney and Duvernay Shales (Atkinson et al., 2016). Operators and regulators in Canada have proactively engaged the issue, establishing operational practices including seismic monitoring for a traffic light system to guide seismic hazard mitigations strategies. Clearly the topic continues to be of concern and establishing mitigation best practices is increasingly important.
There is concern that hydraulic fracturing (HF) near an existing critically stressed fault could trigger slip and generate a seismic event of significance. To this end, a mathematical discontinuum model representing a naturally fractured rock (NFR) mass containing a fault is created, and fault slip response to fluid injection is studied. UDEC™ is used as a Discrete Element Method (DEM) approach to define the seismic moment, and also to assess the influence of injection and slip on further shear slip propagation. In most of the simulations, slip and dilation take place along a number of oriented fracture surfaces, so it is reasonable to assume that in a real case, microseimic emissions would accompany each slip "event". The moment magnitude scale (Mw) is used to measure the size of the microseismic event in terms of the energy released. Seismic moment is a measure of the total energy (work) released during a seismic event and is used to measure its size. From another aspect, the injection energy is calculated as the total energy required for achieving the hydraulic fracturing; this is viewed as energy to open fractures against the ambient stress field and much of it is potential stored energy that can be released and create discontinuities (work of fracture), deformations (W=F·d), heat and radiated seismic energy. A small amount of the total released energy is transformed to radiated seismic energy and another small amount results in tensile fractures. Fracture energy is used to calculate the work required to create fracture surfaces. Comparing the energy release by fracturing or seismic events indicates their contribution to the total amount of released energy or total work. The amount of energy associated with the stick-slip event is small compared with the overall work done to generate the distortions (i.e. increases in aperture by forcing them open) in the rock mass, and much of the energy in slip is lost as heat from the plastic deformation, rather than as radiated seismic energy.
Microseismic monitoring shows the spatial distribution and magnitude of seismicity associated with slip of bedding planes and natural and incipient fractures. Work calculations have been undertaken to look at the various sources of energy storage (elastic strain energy) or dissipation (slip), and likely the energy losses from viscous dissipation could be addressed as well, helping to clarify HF processes in NFRs.
Abstract A vast number of the reported cases of increased seismicity of moderate magnitude (Mw > 0) earthquakes seem to be tied to some form of fluid injection activitiy, being it wastewater disposal by injection into deep wells or high pressure fluid injection into oil and gas reservoirs to hydraulically fracture the rock and improve hydrocarbon recovery. Regulations have been proposed to implement traffic light systems to dictate the responses that the industry needs to take based on either the magnitudes or observed particle velocities or accelerations on the surface. In order to relate the seismic hazard potential in seismically active areas, empirical ground motion prediction equations (EGMPE) are used to relate event parameters like magnitude and location to site characteristics such as peak ground acceleration (PGA) or peak ground velocity (PGV) which tend to be how building codes are parametrized. Therefore, local hazard assessment near hydraulic fractures that generate relatively large magnitude events need to be estimated more precisely by developing and using local EGMPEs. Hybrid deployments combining 15Hz downhole and low frequency near-surface geophones can be used to accurately capture both the localized microseismic events and any large magnitude events associated with hydraulic fracture monitoring across North American basins – Horn River, Eagle Ford, Barnett, and Montney for example. In our studies events with M>0 are observed for completions in these formations. While in many cases the magnitude of these events is too small to be felt on the surface, there are reports of higher magnitude events which have been sensed by workers on site and the local population. The exact relationships between magnitudes and shaking are not necessarily one-to-one. Shaking also varies based on the stress release of the events. As summarized recently by Hough (2014) for other fluid-induced seismicity, the lower stress releases typical for these sequences results in on-average less shaking than is observed for equivalent magnitude tectonic events. In order to quantify shaking over a seismogenic volume, we show how to develop EGMPEs based on the North-American examples. The EGMPE methodology developed in this study can be extrapolated for similar earthquakes of larger magnitude and included into future probabilistic hazard and risk analysis for induced seismicity as related to hydraulic fracture stimulations.