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Characterization of Limestone Interlayers in Marcellus Shale and its Effect on Production in Southwestern PA


Marcellus Shale is an organic-rich mudrock that contains several limestone interlayers. These limestone interlayers could cover a relatively small or large area, and their thickness varies across the Appalachian Basin. The Purcell Limestone is one of the most important limestone layers in the Marcellus Shale, which has divided the Marcellus Shale into Union Spring Member and Oatka Creek Member. The Purcell Limestone covers a large area of Appalachian Basin with thickness of more than 5 feet in many areas. The other limestone layers are generally thin and locally distributed. The existing of these limestone layers influences the Marcellus Shale reservoir on both reservoir evaluation and shale gas production. It is necessary to identify all the limestone interlayers to accurately estimate the thickness of organic-rich mudrock, the shale gas reserve, and the reservoir heterogeneity. More importantly, these limestone interlayers have altered the mechanical properties of Marcellus Shale. Once the thickness of a limestone interlayers becomes greater, it would affect the vertical growth of hydraulic fractures, which will be a critical uncertainty in the design of well trajectory and hydraulic fracturing job.

The southwestern region of Pennsylvania is popular among operators because of the thick and organic-rich section of Marcellus Shale. The thickness of Purcell Limestone ranges from 0 to over 20 feet in this region. The variations in thickness of Purcell Limestone and presence of other thin limestone interlayers place a distinct influence on shale gas production from Marcellus Shale. Through analyzing the features of the production data and the reservoir properties of Marcellus Shale in this region, this research attempts to answer how the limestone interlayers impact the reservoir quality, hydraulic fracturing, and consequently the shale gas production. Well logs plus well information and drilling data were utilized to analyze the spatial distribution of Purcell Limestone and other thin limestone interlayers, and a 3-D model of Marcellus Shale with these limestone layers was constructed. The production data of Marcellus wells was collected to analyze the production profiles in different wells, while the trajectory data of Marcellus wells were used to understand the shale gas production zone(s). Finally, all these information were integrated to determine the effects of limestone interlayers on shale gas production from Marcellus Shale.