Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Compositional and Textural Variability of Shales as Hindrance to Understanding Shale Fracturing
(1) Geoscience Technology, Noble Energy Inc., Houston, TX.
(2) Wattenburg Business Unit, Noble Energy Inc, Denver, CO.
Deciding whether to develop shale reservoirs is difficult, because they need fractures to support hydrocarbon flow at commercial rates. Unfortunately, shale fracturing is poorly understood, largely because the term “shale” includes polymineralic rocks of different compositions and textures and whose major components (clays, quartz, carbonates, feldspars, and organic matter) have different mechanical properties. If one accepts the premise that each shale reservoir has unique mechanical facies, then it follows that use of the unqualified term “shale” hinders progress in understanding shale fracturing.
Shales that are derived from a common provenance can exhibit heterogeneity of composition dependent on environment of deposition (EOD). Shaw and Weaver (1965) found that clay content increases (feldspar and quartz content decrease) with increasing distance of transport in Mississippi fluvial and deltaic deposits. Other published studies show that EOD correlates with composition of connate waters, in turn correlative with shale texture and hence influencing mechanical properties. For example, increasing salinity causes kaolinite to flocculate more readily than does smectite, thus giving greater salinity-dependence of texture in kaolinitic shales than in smectitic shales. Oxic waters favor clay deposition in biogenic pellets. Anoxic waters favor shales that are mechanically anisotropic rather than isotropic, by favoring laminae (paucity of benthic burrowers) and organic films on clays that inhibit flocculation.
Perhaps placing shales in a sequence stratigraphic framework (e.g., Sneider, 2003) that also includes consideration of their provenance and hydrologic evolution can organize their classification in a geologically practical way that aids the prediction of their mechanical properties.
Nonetheless, even if the word “shale” meant only one specific, monomineralic, anisotropic, very fine-grained rock that always has the same texture, its mechanical properties would still be complicated and highly sensitive to rates (especially strain rates and fluid-pressure equilibration rates). The rate-dependence follows from effective-stress dependence (Terzaghi, 1936; Hubbert, 1951) of rock deformation. Small variations in volumetric strain within a shale reservoir (low matrix-permeability) can potentially cause significant variations of local fluid pressure and thus of local effective stress and therefore in rock response to both natural and anthropogenic deformation.