--> Geomechanical Analysis of the Three Forks Formation and Implications for Oil Recovery

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Geomechanical Analysis of the Three Forks Formation and Implications for Oil Recovery

Abstract

The Williston Basin has become an important center for unconventional oil resources. The Bakken Formation accounts for most of the current production in the basin. However, as Bakken reserves become depleted, deeper reserves will become more important. Compared to the Bakken Formation, less is known about the underlying Three Forks Formation. Like the Bakken, the Three Forks is a tight reservoir that normally requires hydraulic fracturing to make production economical. Therefore, understanding the variability in geomechanical properties is critical for designing optimal artificial fracture stimulation schemes. Soft, ductile rock is not suitable for hydraulic fracturing because of its resistance to the propagation of artificial fractures; consequently, it is important to identify stiff, brittle rock types. A better understanding of rock mechanics in the formation should drastically increase the Three Forks recovery factor, which has been reported as 8.9% (plus or minus 5.32%). The objectives of this study are to (1) investigate controls on Three Forks geomechanics, (2) identify geomechanical facies within the formation, (3) relate geomechanical facies to stratigraphic facies, and (4) make recommendations that would improve hydrocarbon recovery. Analysis methods include mechanical hardness tests using an Equotip Piccolo device on cores distributed across the play area, core descriptions to compare with geomechanical facies, and triaxial testing using constant confining pressure to measure axial stress at failure on core plugs. Preliminary results identified geomechanical facies that correlate distinctly with stratigraphic facies identified in core description. Leeb hardness levels range from 540 to 820 HLD. In the Three Forks Formation, hardness generally decreases with depth but varies greatly within short intervals. The majority of structureless, sandy/silty facies yield higher HLD values. Facies with laminations, ripples, and large amounts of green clay yield much lower hardness values. Our correlative geomechanical model should help improve target selection and completion design, resulting in improved recovery factor for the Three Forks Formation.