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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.
In September, a technical workshop was held on the topic of injection-induced seismicity in Banff, Canada. It brought together industry and technical experts to discuss the increasingly important topic of induced seismicity associated with various injections during oil and gas activities. The event was cosponsored by the Society of Exploration Geophysicists (SEG), the Society of Petroleum Engineers (SPE), and the American Rock Mechanics Association (ARMA), serving as a follow-up to a previous meeting on the topic held in Broomfield, Colorado, in 2012. More than 120 professionals participated, with the majority traveling from the United States (60%) and Canada (30%), and some from Europe (10%) and one each from Japan and Colombia. The attendees represented oil and gas companies (34%), academia (25%), service companies (25%), and government organizations including national laboratories, geological surveys, and regulatory bodies.
A coupled hydro-mechanical model is used to evaluate fault activation associated with hydraulic fracturing in the Horn River Basin. The model is used to simulate hydraulic fracture growth through a discrete fracture network, examining the pore pressure diffusion and associated fracture dilation and shearing. Based on the geomechanics, the seismic activity can be predicted and used to compare with the actual seismicity monitored during the fracture treatment. The synthetic microseismic prediction includes location, timing and magnitude of the activity and can be used to validate the geomechanical attributes and calibrate the model to match the field data. Applying such a microseismic geomechanics approach not only improves the interpretation of the microseismic image but also improves the understanding of the geomechanical response of the reservoir.
In this study, the impact of the hydraulic fracturing on a preexisiting fault was examined to quantify seismic hazard. A geomechancial model was created to investigate a Horn River Basin hydraulic fracture and the associated seismic magnitudes. The model was designed to investigate the mechanism of fault activation and the impact of fracturing at different locations around the fault. The study indicated that the stimulated fracture network had to grow directly into the fault in order for the injection pressure front to trigger fault slip. Geomechanical assessment of absolute seismic hazard can be used to modify the engineering design prior to operations to minimize the seismic hazard including the placement of the well, and modifiy staging along the well to avoid fracturing in the regions likely to lead to fault activation. In scenarios where induced seismicity occurs during the treatment, the method can also be used to examine operational changes to lessen the relative seismic hazard.
With hightened public concerns of environmental issues with hydraulic fracturing, attention is raising around the few isolated cases of injection-induced seismicity. An increasing number of reports have recently been made of felt seismicity associated directly with hydraulic fracture treatments or disposal of waste water from extraction of unconventional resources. In order to safely and efficiently develop unconventional reservoirs in areas of concern, industry protocols have been developed to deal with induced seismicity issues. Typically these protocols rely on local seismic monitoring to define traffic light systems, where operations are modified depending on the seismicity levels. As part of these protocols, methodologies are required to assess the seismic hazard both prior to the initiation of operations in addition to modifications to planned operations when required by traffic light levels.
ABSTRACT: This paper details back analysis conducted at two mines with the purpose of informing exposure management procedures to manage seismic risk. The analysis was intended to help each mine determine which blasts should have an exclusion, what the area of exclusion should be and when re-entry should occur. Short-term seismic responses to blasting were assessed to evaluate the dependence of blast type, location and size on the likelihood of triggering a response and the magnitude of the triggered response. Exclusion procedures should be targeted towards blasts most likely to trigger a seismic response. The area of exclusion is best defined on a case-by-case basis but the spatial distribution of seismic responses to blasting are useful to discover general trends. This was done by evaluating the distance in each axis between the event and the blast. The number of events on each grid point on the XY, YZ and XZ planes is evaluated to allow for a ‘heat-map’ of event location probability. In general, our recommendations for each mine were for a minimum exclusion distance that should be extended if there are known high hazard zones such as active structures, pillars and abutments. Given there are multiple re-entry assessment methods used throughout the industry, the back-analysis investigated six different methods of determining re-entry time. Three methods use a raw or absolute threshold value for re-entry and three others use thresholds relative to the background seismicity. Each re-entry assessment method was tested using pre-set (pre-determined) re-entry, real-time re-entry and a combination of the two (a pre-set minimum re-entry before a real-time review). The results of the back analysis allowed the mines to determine their optimal re-entry procedure. Once the mine has chosen a tolerable level of hazard outside of exclusion, they choose the re-entry technique which will result in the shortest average re-entry time for this level of hazard. The most effective method varies slightly depending on the measure of success used and the typical re-entry time. The management of seismic risk requires a holistic approach and exposure management is simply one component that works together with other control measures.
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