Geomechanical controls on hydraulic fracturing of the Bakken Formation

Christopher D. Hawkes, Mostafa Gorjian, Donna Beneteau

Dans les comptes rendus d’articles de la conférence: GeoRegina 2014: 67th Canadian Geotechnical Conference

Session: Engineering Geology and Rock Mechanics

ABSTRACT: The objective of the work presented in this paper was to measure mechanical properties on Bakken Formation core samples, and to assess the relationship between mechanical properties and geological attributes of this formation. To the best of the authors™ knowledge, no core-based measurements of mechanical properties have previously been published for the Bakken Formation from the Canadian portion of the Williston Basin, which impedes our understanding of hydraulic fracture behaviour in this tight, oil-bearing formation. Samples were taken from the lower, middle and upper members of the Bakken Formation from two wells in southeast Saskatchewan, and measurements of tensile strength [41 tests], fracture toughness [12 tests], and static elastic properties (during unconfined compression) [18 tests] were conducted. Testing also included compressional and shear wave acoustic velocity measurements on all samples, and wireline geophysical logs were analyzed to provide context for the laboratory testing. Results suggest that the shales of the upper and lower members of the Bakken Formation tend to have mechanical properties that differ notably from the middle member. Most importantly, in terms of properties affecting fracture height growth, both the tensile strength and fracture toughness of the shales are consistently lower than the values measured on adjacent strata present in the middle member. These results suggest that out-of-zone height growth would not be mitigated by the shales. The implications of dynamic elastic properties on fracture height growth, however, are less clear. Published microseismic monitoring data available for a hydraulic fracture treatment of the Bakken Formation strongly suggest that the shales of the upper and lower members are not effective barriers against vertical height growth, and that fracture geometries do not conform to the traditional conceptual model of a hydraulic fracture (i.e., a discrete plane oriented normal to the smallest principal in-situ stress).

RÉSUMÉ: L'objectif du travail présenté dans cet article était de mesurer les propriétés mécaniques du formation Bakken, et d'évaluer la relation entre les propriétés mécaniques et les caractéristiques géologiques de cette formation. Les échantillons ont été obtenus à partir des membres inférieurs, moyens et supérieurs du Bakken, et les mesures de résistance à la traction [41], les tests du ténacité à la rupture [12], et les des propriétés élastiques [18 essais] ont été menées. Les résultats obtenus suggèrent que la croissance verticale de fractures ne sera pas atténué par les membres supérieur et inférieur du Bakken.

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