Stress Analysis Using Source Mechanisms from Failure along Joints and Faults in the Marcellus Formation during Hydraulic Fracture Stimulation Treatment
Williams-Stroud, Sherilyn C.1; Hulsey, B. J.1
(1) MicroSeismic, Inc., Houston, TX.
Microseismic events detected during a stimulation treatment of a horizontal well in the Marcellus Formation were mapped with a monitoring array of geophones located on the surface above the well. The events formed well-defined trends that could be interpreted to be parallel to both the northeasterly J1 and northwesterly J2 joint directions that occur in the Appalachian Basin. The signal to noise quality was very good, and source mechanisms were determined for a number of events in the result. The J1 joints are known to be nearly parallel with the direction of maximum horizontal stress in most of the Appalachian Basin, and 3 of the 7 stages of the hydraulic fracture stimulation treatment showed trends parallel to the J1/SHmax direction. The source mechanisms inverted from events along these trends confirm that the failure occurs along fracture planes parallel to SHmax, with nearly pure strike slip displacement. Relatively strong microseismic activity was detected repeatedly along a trend nearly perpendicular to SHmax throughout stages 4 to 6. A large amount of the materials used to treat these stages appears to have been lost into the formation; well log information and subsurface mapping support the interpretation that the activity resulted from reactivation of a NW-striking fault during the stimulation treatment. The source mechanism inversion of events from this microseismic activity show the same strike-slip source mechanisms identified for the J1-parallel trends. Because the source mechanism solution is non-unique, analysis of the geologic and in-situ stress context is required to determine which failure plane of the solution is most likely. We present three possible scenarios for interpreting this result, 1) microseismicity in the fault zone originates not from the fault, but from fractures within the fault damage zone that are optimally oriented to slip assuming SHmax azimuth is sub-parallel to J1, 2) reactivation occurs on both the northeasterly- and northwesterly-striking fractures as a result of reservoir deformation related to opening mode failure along the fault plane, and 3) reactivation occurs on only the fracture planes parallel to the northwesterly-striking fault because SHmax is not parallel to the J1 joints but in an azimuth where both J1 and J2 joints are favorably oriented for strike-slip failure induced by the stimulation treatment.
AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.