Characterization of natural fractures of the Upper Devonian Duvernay Formation in the Kaybob Area, Alberta
Creators: Garcia Rodriguez, Guido
Publisher: name: University of Alberta Library
Year: 2024

<p>The organic-rich mudstones of the Upper Devonian Duvernay Formation constitute an unconventional reservoir in the Western Canada Sedimentary Basin, developed for over a decade using hydraulic fracturing technology. Integrating natural fracture characterization with lithofacies analysis, mechanical properties examination, and regional structural study can define the mechanical stratigraphy of mudrocks, reconstruct stress fields, and elucidate basin-scale fluid flow during fracturing. Determining the geometry, orientation, and fracture intensity of natural fractures is crucial for fracture modeling, horizontal well placement, and optimizing hydraulic fracturing and production operations.<br>
This study focuses on characterizing natural fractures in the Duvernay Formation in the Kaybob area. It seeks to determine fracture intensity, controlling factors of fracture propagation, and the relative timing of fracturing in the Upper Devonian organic-rich mudrocks beneath the Alberta Basin. Objectives include analyzing lithofacies, mechanical properties, type, geometry (height, length, aperture), orientation, intensity/spacing of natural fractures, and fracture structural settings. Results on fracture morphology and distribution in the Duvernay Formation are compared to descriptions and structural analyses of fractures in the Upper Devonian Perdrix Formation in the Front Ranges and Foothills of the Nordegg area, serving as an outcrop analog of the Duvernay Formation. The aim is to correlate phases and timing of fracturing in these areas.<br>
The research is based on the description and analysis of approximately 418 meters of core, core tests, and well logs in seven vertical wells, as well as around 4.4 kilometers of<br>
Formation Micro-Imager (FMI) logs in four horizontal wells in the Kaybob area. 3D structural modeling (90 km x 90 km) and facies modeling (2 km x 2 km to 10 km x 12 km) were conducted to interpret the lateral distribution of lithofacies and fractures in the Duvernay, Majeau Lake, and Leduc Formations.<br>
Seven lithofacies are identified in the Duvernay Formation core, with three constituting nearly 95% of the total thickness. Facies F2 exhibits ductile behavior with low calcite content, high illite, and total organic carbon (TOC), and low elastic and strength parameters (UCS, YM, TSTR). Facies F3 displays moderate stiffness and strength with intermediate calcite, illite, TOC, and elastic and strength parameters. Facies F5 shows stiff behavior with high calcite content, low illite, TOC, and high elastic and strength parameters. The lower fracture intensity in facies F2 and F5 can be related to more ductile mechanical properties of facies F2, which are not favorable for fracture propagation, and higher thickness of beds in facies F5.<br>
Analysis of drilling-induced fractures and breakouts in FMI logs helped determine the orientation of the present-day maximum horizontal stress (SHmax) as N53° ± 2°, with SHmax&gt;Sv. Tensile calcite-filled vertical fractures (#2003) in the Duvernay Formation form three main sets oriented parallel, orthogonal, and at approximately 20° to SHmax. Open and subhorizontal fractures and polished slip faces are subordinate. Fracture intensity in mudrocks varies from approximately 1 to 1.6 fract/m in vertical wells and from around 3 to 4.5 fract/m and m²/m³ in horizontal wells. The highest fracture intensity occurs in thinly bedded organic- and carbonate-rich mudrocks of facies F3 compared to more ductile organic-rich biosiliceous mudrocks of facies F2 and stronger and stiffer carbonate-rich nodular mudrocks of facies F5.<br>
Hydrocarbon generation and fluid expansion are significant mechanisms for fracture development in shales. In the study area, thermal maturity organic-rich mudrocks, fluid pressure, and burial depth of the Duvernay Formation increase from the northeast to the southwest. Estimated fracture intensity from vertical wells similarly increases from the northeast to the southwest. Proposed phases of fracturing in the Duvernay Formation during burial and uplift are based on fracture characterization and regional data on fracturing in Upper Devonian carbonate rocks of the West Shale Basin.<br>
Fracture intensity in organic-rich mudrocks of the Perdrix Formation is higher than in the mudrocks of the Duvernay due to increased deformation in the Front Ranges. Two deformation phases are identified, involving subhorizontal calcite-filled tensile fractures and bedding-orthogonal tensile and transtensional fractures during the Laramide orogeny, and NNW-SSE open subvertical fractures likely during Eocene extension.<br>
The detailed investigation of natural fractures in the Duvernay and Perdrix Formations in core, image logs, and outcrops provides geoscientists and engineers with crucial information for fracture modeling. This information can enhance exploration, drilling, and production practices of the source rock reservoir in the Western Canada Sedimentary Basin.</p>