Marcellus Shale Stimulation Barrier and Geohazard Assessment
Morath, Philip; Starr, Joel; Schanken, Luke
The term "shale reservoir" is somewhat of a misnomer, because while shales tend to have porosity, they do not have permeability. Thus, shale itself cannot be considered a reservoir; large amounts of proppant must be injected into the shale in order to "create" a reservoir. To achieve the most effective stimulation in the Marcellus Shale, it is critical to not only understand the properties of the Marcellus Shale, but also the bounding units.
The Marcellus Shale was deposited in the Appalachian Basin during the middle Devonian. It is bounded above by the Tully Limestone/Hamilton Shale and below by the Onondaga Limestone/Huntersville Chert. These rock units can serve as frac barriers based on modeling and have been confirmed as such using microseismic results. The bulk density of these bounding units is typically much higher than the bulk density measured across the Marcellus Shale. If the frac is initiated within the Marcellus Shale, the hypothesis is that the higher density bounding units will impede the propagation of frac energy, leading to a complex stimulation treatment contained within the Marcellus Shale.
In this study, the average bulk density across the Marcellus interval is compared to the average bulk density of the unit above and below the Marcellus Shale. If the unit above and below have similar bulk density measurements, it is assumed that the energy from stimulation treatment would remain focused within the Marcellus Shale. It is assumed that if the bulk density of the unit above the Marcellus is lower than the unit below, the stimulation energy would tend to focus up, whereas if it were higher, the stimulation energy would tend to focus down. Upward growth of the frac treatment has the greatest likelihood of occurring in the western portion of the Marcellus play, whereas downward growth is more likely in the eastern portion of the basin. It is worth noting that the underlying Huntersville Chert and Oriskany Sandstone were productive in the eastern portion of the basin.
Various reservoir properties were calculated using geophysical logs across the units underlying the Marcellus. Historic completion records were also examined and instances of production or shows of gas or water were recorded. When these data are combined with the fracture propagation hypothesis, there is a higher likelihood of a fracture treatment encountering bypassed, depleted, or wet zones beneath the Marcellus in the eastern portion of the play.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013