The Middle Devonian Marcellus Shale Formation in the Appalachian Basin, which encompasses more than 3.3 trillion tonnes of organic matter, has low permeability (0.1 to 10 μd) and requires extensive fracture stimulation before the reservoir will yield gas in commercial volumes (de Witt, 1986). Since 2004, the application of horizontal drilling, combined with multi-staged hydraulic fracturing to create permeable flow paths from the shale units into wellbores, has resulted in a drilling boom for the Marcellus Formation (Engelder et al., 2009).
Effective hydraulic fracturing in unconventional shale reservoirs requires an understanding of the pre-existing discontinuities (impermeable/not effectively connected natural fractures) state and an assessment of whether at any point of the stimulation it is energetically favourable to move fluids within the reservoir. The dynamic nature of the local stress regime owing to hydraulic fracturing leads to stimulation of different fracture sets which could include bedding-parallel fractures and bedding planes. Describing the progression of the fracturing into the formation and ancillary issues of how far into the reservoir the rock has been stimulated and the stimulation containment provides the opportunity for operators to potentially control fracture behaviour and improve completion designs.
Evaluation of hydraulic fracturing generated microseismicity through seismic moment tensor inversion and further stress inversion techniques, where there is high signal-to-noise signals registered with a sufficient angular distribution of sensors around the events, can resolve the orientations of the fractures on which these ruptures occur. This process then identifies the predominance of fracture types, fracture orientations and dimensions within the stimulated reservoir.
The comparison of fracture network associated with a stimulation of different stages we can establish the importance of different fracture sets in effectively constraining the stimulation to the zone of interest. This information further allows operators to establish the stimulation effectiveness if production data is available.
Check out our full article published in First Break Journal Vol 35, No 4, April 2017 or contact us to get a copy.