Abstract

Woodford shale has recently emerged as a potential gas play in the U.S. Mid-continent, following in the footsteps of the Barnett shale. Profitable production from these formations relies on the success of horizontal well drilling and hydraulic fracturing design, which in turn depends greatly on knowledge about rock formation anisotropic elastic properties and pore pressure coefficients (poroelastic properties). In this paper, a suite of recently developed wire-line logs such as the Element Capture Spectroscopy (ECS) log, Sonic Scanner log, and porosity logs, were run on the Woodford and Barnett formations. The results were utilized to estimate the anisotropic elastic and poroelastic properties of these gas shale formations using the GeoGenome™ model, a theoretical upscaling model backed up with unique laboratory testing on field retrieved preserved shale samples at a scale that has never before been investigated; that is, observing mechanics from displacements as small as 100 nm to 100 mm, and maximum applied forces not even reaching 1000 mN. Comparison between laboratory Ultrasonic Pulse Velocity measured data, Sonic Scanner log data, and simulated data has shown excellent agreement, and thus confirmed the reliability of the GeoGenome upscaling model in obtaining shale anisotropic mechanical properties of which some parameters cannot be obtained even with the most advanced sonic log. In addition, a new technique was developed using nano-indentation to extract the Woodford shale mechanical and strength properties from drill-cuttings. The measured stiffnesses and compressibility on Woodford shale drill-cuttings-size chips have shown great potential for full mechanical characterization of rocks from just the drill-cuttings, which until recently were regarded as having little value for rock mechanical characterization besides some existing simple empirical correlations. This new shale testing technique will undoubtedly help the industry to significantly reduce the overhead associated with full core retrievals at great depths.

Introduction

The Woodford and Barnett shale formations, deposited during the Devonian and Mississippian period, are currently the two most prolific natural gas resources of the U.S. Mid-continent with estimated reserves of billions of cubic feet per well or multi-trillion cubic feet per field (web source:Panhandle Oil, 2006; Janwadkar et al., 2008). However, the intrinsically low permeability of Woodford and Barnett shale often hinders the production from these gas formations unless extended reach drilling, horizontal well drilling, and multi-stage hydraulic fracturing are employed (Matthews, 2007; Janwadkar et al., 2008). Moreover, more often than not, these stimulations only have short-lived profitable gas production. The geological complexities often associated with these gas fields require a thorough knowledge about the formations anisotropic mechanical properties, a necessity for successful drilling, completion, and stimulation plan (Ekbote & Abousleiman, 2006; Kanj & Abousleiman, 2007). For example, estimation of principal horizontal stresses, a crucial piece of information for hydraulic fracturing design and wellbore stability analysis, from overburden stress in shale formations requires the Young's moduli, Poisson's ratios, and the coefficients of pore pressure (Biot's coefficients) in directions perpendicular and parallel to the formation bedding planes(Amadei et al., 1987; Thiercelin & Plumb, 1994; Ekbote & Abousleiman, 2005; Al-Tahini & Abousleiman, 2008).

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