Geomechanical controls on hydraulic fracturing of the Bakken Formation

Christopher D. Hawkes, Mostafa Gorjian, Donna Beneteau

In the proceedings of: GeoRegina 2014: 67th Canadian Geotechnical Conference

Session: Engineering Geology and Rock Mechanics

ABSTRACT: cal controls on hydraulic Fracturing of the Bakken Formation Christopher D. Hawkes, Mostafa Gorjian & Donna Beneteau Department of Civil and Geological Engineering Œ University of Saskatchewan Saskatoon, SK, Canada 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). RESUME 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. 1 INTRODUCTION The Bakken Formation in the Williston Basin contains vast oil reserves, but multi-stage hydraulic fracture treatments are generally required in order to achieve economic production rates from the siltstones and fine-grained sandstones that comprise the reservoir interval of the Bakken. The attributes (e.g., orientation, complexity, height, length, etc.) of hydraulic fractures are governed by rock mechanical properties and fabric (e.g., natural fractures) and in-situ stress regime. To date, considerable effort has been devoted to geological characterization of the Bakken Formation, and a modest amount of work on in-situ stress regime has been undertaken. However, at present there is a shortage of core-scale analysis of the mechanical properties that affect fracturing. In fact, 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. Herein lies the motivation underlying the geomechanical component of this project. The work presented in this paper represents the first phase of a project being undertaken to better understand the behaviour of hydraulic fractures in the Bakken Formation, with the long-term objective of developing predictive tools that can be used to optimize hydraulic fracture treatments. The specific objective of this first phase of the project was to measure mechanical properties on Bakken Formation core samples, and to assess the relationships between these mechanical properties and geological attributes. The scope of this phase spanned both reservoir and caprock samples, so as to advance our understanding of fracture propagation both within and out of zone. 2 GEOLOGICAL DESCRIPTION The Bakken Formation is present in the subsurface of northeastern Montana, northwestern North Dakota, southeastern Saskatchewan and southwestern Manitoba. In Saskatchewan, the Bakken Formation unconformably overlies the Big Valley and Torquay formations of the Three Forks Group and, in turn, is conformably overlain by the Souris Valley Beds or the Lodgepole Formation (Figure 1).

RÉSUMÉ: chanical controls on hydraulic Fracturing of the Bakken Formation Christopher D. Hawkes, Mostafa Gorjian & Donna Beneteau Department of Civil and Geological Engineering Œ University of Saskatchewan Saskatoon, SK, Canada 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). RESUME 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. 1 INTRODUCTION The Bakken Formation in the Williston Basin contains vast oil reserves, but multi-stage hydraulic fracture treatments are generally required in order to achieve economic production rates from the siltstones and fine-grained sandstones that comprise the reservoir interval of the Bakken. The attributes (e.g., orientation, complexity, height, length, etc.) of hydraulic fractures are governed by rock mechanical properties and fabric (e.g., natural fractures) and in-situ stress regime. To date, considerable effort has been devoted to geological characterization of the Bakken Formation, and a modest amount of work on in-situ stress regime has been undertaken. However, at present there is a shortage of core-scale analysis of the mechanical properties that affect fracturing. In fact, 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. Herein lies the motivation underlying the geomechanical component of this project. The work presented in this paper represents the first phase of a project being undertaken to better understand the behaviour of hydraulic fractures in the Bakken Formation, with the long-term objective of developing predictive tools that can be used to optimize hydraulic fracture treatments. The specific objective of this first phase of the project was to measure mechanical properties on Bakken Formation core samples, and to assess the relationships between these mechanical properties and geological attributes. The scope of this phase spanned both reservoir and caprock samples, so as to advance our understanding of fracture propagation both within and out of zone. 2 GEOLOGICAL DESCRIPTION

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Cite this article:
Christopher D. Hawkes; Mostafa Gorjian; Donna Beneteau (2014) Geomechanical controls on hydraulic fracturing of the Bakken Formation in GEO2014. Ottawa, Ontario: Canadian Geotechnical Society.

@article{GeoRegina14Paper440,author = Christopher D. Hawkes; Mostafa Gorjian; Donna Beneteau,title = Geomechanical controls on hydraulic fracturing of the Bakken Formation,year = 2014}