Microseismic monitoring of hydraulic fractures is an important tool for imaging fracture networks and optimizing the reservoir engineering of the stimulation. The range of magnitudes of the recorded microseisms depends at the lower limit on the array sensitivity; while the upper limit varies significantly from site to site. In this paper the variation in the microseismic magnitude range is examined and compared with the injection and site characteristics. Although there are numerous potential factors effecting the seismic deformation, the energy of the pumping and state of stress appear to be the two dominant factors. However, interaction with pre-existing faults also results in increased deformation. Ultimately, this can potentially be used to design the stimulation to maximize the deformation. Characterization of the seismogenic potential is also important for seismic hazard assessment, as well as the design of passive monitoring. Introduction Over the last few years, microseismic imaging of hydraulic fracture stimulations 1 has become a widespread diagnostic technology. Microseismicity is used to image fracture geometry dynamics and optimize stimulations in a wide variety of settings. The resulting images are useful in both simple and complex fracture networks and able to detect fracture complexity resulting from injections in naturally fractured reservoirs. Particularly in North America, microseismic imaging has become a standard in development of both conventional and unconventional resource plays. Generally, the temporal locations of microseisms detected in an offset observation well are used to monitor the growth of the hydraulic fracture geometry. In most cases the hydraulic fracture is being created by tensile failure of the rock resulting from injection of fluids at pressures exceeding the minimum principal stress level, although the deformation mechanism associated with the recorded microseisms appears to be shear dominated deformations. Microseisms typically contain significant shear wave energy suggesting substantial shear deformation in the source of the microseismic energy, although fracture opening could occur simultaneous with the shear deformation and play a role in the permeability enhancement. One model to explain the shearing is stress changes or pore pressure increases associated with the primary hydraulic fracture 2, leading to induced shear failure. However, dog legs, offsets or other complexities along the hydraulic fracture could also result in localized shear deformations along a conventional tensile fracture. Intersections of a hydraulic fracture with oblique angle pre-existing fractures could also lead to localized shear deformation. Microseism signal analysis can be used to investigate aspects of the source characteristics of the shear deformation, although this may or may not provide insight into the stimulation objective of creating a permeable fracture possibly containing a fluid conductive proppant pack. An important microseism source attribute is the source strength or magnitude 3. Source strength is best quantified by seismic moment (product of shear modulus, shear displacement and area) which can be expressed with a moment magnitude scale, analogous to the well known Richter Magnitude scale. Investigating source strength has proven valuable in determining the effective detection range, by simply plotting magnitude versus distance between the microseisms and seismometers. However, the spatial extent of the seismic deformation has been postulated to image the extent and density of a stimulated fracture network in the Barnett Shale, and appears to provide a useful attribute that correlates with gas production in a case study examining several wells 4.
Geophysics Prowlram AK-50, University of Washin•lton, SeattleSeismicity and the orientation of fault planes from focal mechanisms indicate that Mount St. Helens is located at a dextral offset along the St. Helens seismic zone (SHZ): earthquake swarms occurring in this offset are related to volcanic eruptions. Because motion on the SHZ is in a right-lateral strike-slip sense, this dextral offset creates extension within a volume of the crust between the offset fault segments. This offset geometry is similar to that of geothermal areas along the San Andreas fault system. We apply a model derived from these geothermal areas to Mount St. Helens and find that the major differences between Mount St. Helens and the geothermal areas can be related to the ratio of the width of the offset between fault segments (1), to the seismogenic depth (h). At Mount St. Helens this ratio is < 1, whereas in the geothermal areas the ratio is m 1. We propose that when I/h < 1 as at Mount St. Helens, the regional minimum principal stress does not completely dominate the small volume under extension, and as a consequence, the opening geometry is poory established compared to the oblique crustal spreading that characterizes the geothermal areas where I/h m 1. Late Quaternary volcanic vents near Mount St. Helens strike northeast, similar to the strike of a set of pre-Quaternary faults and intrusive rocks that are mapped north of the volcano; in addition, the deepest earthquakes occurring within the extensional volume are aligned along a northeast striking fault. Since these northeast striking features are aligned approximately prependicular to the regional minimum principal stress, we infer that the spatial position of Mount St. Helens is controlled by the junction of the right-stepping offset of the SHZ with the older set of fractures and that these fractures are favorably aligned with respect to the contemporary regional tectonic stress directions for the transport of magma through the brittle crust. The sense of fault motions predicted by our model for local crustal extension is consistent with an apparent component of rightlateral shear measured from geodetic lines around Mount St. Helens during June and July 1980.
Microseismic mapping is a valuable tool for assessing hydraulic fracture stimulations. During a stimulation treatment, borehole, near-surface, or surface geophones record passive seismic data. The microseismic events are thought to be shear failures that occur around the opening of a tensile hydraulic fracture as it grows. Over the past several years, there has been a large growth in the number of hydraulic fracture monitoring surveys, along with the number of vendors who acquire and process microseismic data. Here we discuss two attributes of microseismic data which are associated with the size of the passive event: seismic moment and magnitude.
Economic development of tight gas reservoirs often rely on hydraulic fracturing to stimulate production. Passive microseismic mapping of these hydraulic fractures is a quickly growing technology to map fracture geometry and complexities created during these stimulations. Reliable microseismic locations depend on an accurate velocity model. In this study we examine velocities obtained from Vertical Seismic Profile (VSP) data, perforation shots from an adjacent well, and microseismic data to determine an anisotropic velocity model for improved microseismic hypocentral locations. We examine location uncertainties associated with uncertainties in the velocity model, in addition to uncertainties in arrival time data. The observed anisotropy is consistent with an effective media representation of the geological reservoir structure.
A combination of microseismic and surface-deformation monitoring with an array of tiltmeters was used to monitor the warm-up phase of a steam-assisted-gravity-drainage (SAGD) well pair. A sequence of microseismic events was recorded with signal characteristics that suggested deformation associated with thermal expansion of the wellbore, in addition to events apparently associated with induced fracturing in the reservoir. Integration of the microseismic data with volumetric strain, inverted from the measured surface deformation, indicates a discrete deforming region near the toe of the well. The volumetric strain also shows another region near the heel of the well, although the area is too far from the microseismic observation well for any associated microseismicity to be recorded. The central portion of the well pair did not have significant deformation, indicating poor steam conformance during this warm-up phase. A comparison of the temporal response of the microseismic deformation with the surface uplift suggests a lag between periods of accelerated seismic deformation followed by an associated period of accelerated uplift a few days later. This timing suggests the creation of a fracture network and related seismic deformation, which then fills with steam and starts to expand over a period of a few days. In a related paper (Du et al. 2007), stress changes associated with the volumetric strain are used to examine potential geomechanical failure zones that match the observed locations of microseisms. Together, the volumetric strain, computed stress changes, and failure zones could be used to calibrate a geomechanically linked reservoir simulator.
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