Abstract

The primary purpose of stimulating fractured shale reservoirs is the extension of the drainage radius via creation of a long fracture sand pack that interconnects with natural fractures thereby establishing a flow channel network to the wellbore. However, there is limited understanding of a successful method capable of stimulating Utica Shale reservoirs. Indeed, most attempts to date have yielded undesirable results. This could be due to several factors, including formation composition, entry pressure, and premature pad fluid leak-off. Furthermore, stimulation of Utica shale reservoirs with acid alone has not been successful. This treatment method leads to a fracture length and drainage radius less than expected resulting in poor well productivity.

In this work, geological data is first examined for the reservoir. Laboratory data are then presented to address the unique mineralogy and mechanical properties of the Utica shale. The high percentage of acid soluble carbonate and dolomite suggests an acid treatment to lower entry pressures. This treatment can be the main stimulation of a vertical or horizontal well since natural fractures are present, or the acid breakdown can precede a gelled acid or proppant laden water frac or crosslinked fracturing fluid treatment. Experimental results reveal the impact of clays, potential generation of fines both siliceous and organic, acid solubility, low temperature biological activity, potential for scale generation and the prevalent problem of recovery of injected fluids. Acid solubility is presented vs. time and acid strength. Conductivity data is presented for gas fracs, matrix acidizing and proppant fracturing of the shale. The adsorption, as well as the regained relative permeability to gas is examined vs surfactant type to allow the selection of an additive package that will optimize fluid recovery and improve relative permeability to gas. Information obtained from this study can be used to optimize fracturing treatments of Utica shale reservoirs in the Appalachian Basin.

Introduction

As interest in drilling and producing shale reservoirs throughout North America increases due to the success of the Barnett, Woodford, and Fayetteville shales, numerous potential reservoirs that have previously been undeveloped are being examined for their potential. The organic-rich, low-permeability Upper Ordivician Utica Shale is one such reservoir that displays many attributes which may result in a commercially viable play of great areal extent. This interest is driven largely by increased natural gas prices and improved completion technologies. Indeed, there may be no better example of the role of technology in natural gas recovery than the Late Mississippian Barnett Shale of the Fort Worth Basin, which provides an analog for exploration of similar unconventional reservoirs throughout North America. Nevertheless, there is no universal production model method of stimulating each and every unconventional reservoir that exists. The Utica Shale compares favorably with such organic-rich units as the Middle Devonian Marcellus Shale of the Appalachian Basin and the Upper Cretaceous Lewis Shale of the Green River Basin (Table 1).

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