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Results
Summary Fracture networks stimulated by hydraulic fracturing follow pre-existing planes of weakness in the rock. We analyze a stimulation program in a tight gas formation in Texas. Microseismic Frequency-Magnitude analysis is used to infer fracture and stress characteristics. Data from a microseismic Shear-Wave Splitting analysis gives dominant fracture orientation. The static stress interaction of the microseismic events on the fracture network are then calculated. The resulting changes in Coulomb Failure Stresses (?CFS) thus constrain the zone of stress induced cracks and fissures, which are activated below microseismic detectability.
- Geology > Geological Subdiscipline > Geomechanics (0.51)
- Geology > Structural Geology > Tectonics > Plate Tectonics > Earthquake (0.50)
Summary With the increasing preference for long lateral wellbores for recovery of unconventional gas, and the lack of suitable vertical observation wells to monitor microseismic activity related to the treatment of these wells, more and more hydraulic fracture stimulations are being monitored with geophone arrays deployed in horizontal wellbores. However, since fracture height growth is often the most important concern in microseismic monitoring, single or multiple horizontal arrays do not provide adequate depth resolution to satisfy the needs of the petroleum industry. The study presented here explores the effectiveness of using horizontal arrays to provide the necessary depth constraint to provide realistic estimates of height growth in hydraulic fracture stimulations. Microseismic events from six stages of a hydraulic fracture treatment were recorded using two horizontal sensor arrays in adjacent wellbores. The events were located with a depth constraint, which confined the event locations to the depth of the treatment wellbore, and without a depth constraint. Comparison of the two sets of results reveals that the depths obtained without confinement are highly uncertain, with the scatter of the locations in depth increasing with greater distance from the sensor arrays. The event distributions in all stages tend to follow isochronal misfit surfaces upwards and away from the treatment well. Based on the results presented here, we conclude that horizontal array geometries are not able to effectively resolve depth for events associated with hydraulic fracture treatments excepting when events occur between arrays. Confining the event locations to a horizontal plane at reservoir depth is a reasonable assumption to obtain meaningful event distributions in plan view. The interpretation of the results in plan does not change significantly with and without the depth constraint, and the shapes of the misfit fields indicate that there is not a marked improvement in the fit of the waveforms when events drift vertically away from the treatment well.
Summary Long period, long duration (LPLD) seismic events are relatively low amplitude signals that appear to be generated by slowly slipping faults during stimulation of a gas shale reservoir. They are remarkably similar in appearance to tectonic tremor sequences observed in subduction zones and transform fault boundaries. The ratio of the amplitudes on the three components and apparent velocities indicate that these signals are predominantly shear waves. In most cases, a few micro-earthquakes occur during the LPLD events, most likely generated on small fault segments associated with the slowly slipping faults responsible for the LPLD events. Interestingly, the hydraulic fracturing stages associated with the most LPLD events in the data set investigated lie exactly where there is a significant low amplitude anomaly in the 3D seismic data. We believe this results from a large density of pre-existing fractures and faults in this part of the reservoir. An image log in a nearby horizontal well shows the highest density of fractures and faults in the same general area. This region also shows the highest perturbation in pore pressure during hydraulic fracturing, From the spectrum of LPLD events, it is apparent that a significant part of the low frequency energy of the LPLD signals is not being recorded due to the instrument response of the 15Hz geophones. Despite this, we estimate that the moment carried by the larger LPLD events is ~10-20 times that of Mw ~ -1 microearthquake. The relatively large size of these LPLD events suggests that slow slip on faults is an important process affecting the stimulation more than microearthquakes. Two processes appear to control whether a fault slips rapidly as a microearthquake or slowly and stably. Laboratory friction date indicate that shales with high clay and kerogen tend to slip stably (Kohli and Zoback, 2011). In addition, slip along poorly-oriented faults that occurs due to reduction of normal stress by high fluid pressure is expected to propagate slowly. Taken together, stimulating slip on pre-existing, faults in response to elevated fluid pressures can help optimize field operations and improve recovery.
- Geology > Geological Subdiscipline (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Mudrock > Shale (0.94)
- Geology > Structural Geology > Tectonics > Plate Tectonics > Earthquake (0.72)
Seismic Imaging of Hydraulically-Stimulated Fractures: A Numerical Study of the Effect of the Source Mechanism
Shabelansky, Andrey H. (Massachusetts Institute of Technology) | Malcolm, Alison (Massachusetts Institute of Technology) | Fehler, Mike (Massachusetts Institute of Technology) | Bakku, Sudhish K. (Massachusetts Institute of Technology)
SUMMARY We present a numerical study of seismic imaging of hydraulically stimulated fractures using a single source from an adjacent fracturing-process. The source is either a point force generated from the perforation of the casing of the well or a double-couple as is typically observed from the induced microseismicity. We assume that the fracture is sufficiently stimulated to be imaged by reflected seismic energy. We show for a specific monitoring geometry of hydrofracturing that not only different waves (P and S) but also different source mechanisms from the same region form an image of different parts of the target fracture and thus add complementary information. The strategy presented here might be used as an additional monitoring tool of the hydrofracturing process.
- Research Report > New Finding (0.46)
- Research Report > Experimental Study (0.46)
Microseismic Using a Near-Surface Array – A Case Study in the Haynesville Shale, East Texas
Gangopadhyay, Abhijit (BP America) | Johnston, Rodney (BP America) | Lucas, Jennifer (BP America) | Ramirez, Jaime (BP America) | Gillham, Travis (BP America) | Peña, Victor (BP America) | Wirnkar, Fabian (BP America)
Summary We report on the analyses of microseismic data collected using a near-surface array during fracture stimulation of two horizontal wells in the Haynesville shale, east Texas. Processing of the data reveal relatively high noise levels that present significant challenges in unmasking the underlying microseismic event signals. Orientations of fractures resulting from the well stimulations as observed from analyses of all located events range between 25° and 130°, and thereby suggest that the well azimuths are geomechanically consistent with the regional stress field. The number and size of microseismic events appear to be influenced by increase in bottom hole injection pressure caused by increase in pumping pressure and/or proppant concentration. Average fracture length and height obtained from the analyzed data considering either side of the well bore together are ~1200 feet and ~440 feet respectively. The results from the project confirm some aspects of well design and completions strategies, while providing important insights into improving future plans.
- Geology > Petroleum Play Type > Unconventional Play > Shale Play > Shale Gas Play (0.74)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Mudrock > Shale (0.64)
- North America > United States > Texas > Haynesville Shale Formation (0.99)
- North America > United States > Texas > Fort Worth Basin > Barnett Shale Formation (0.99)
- North America > United States > Louisiana > Haynesville Shale Formation (0.99)
- North America > United States > Arkansas > Haynesville Shale Formation (0.99)