Geomechanical assessment of continuum-discontinuum characterization of clay shale caprocks

• Author / Creator

  Heikal, Abeer

• The Clearwater Formation is a stratigraphic unit of Early Cretaceous (Albian) age that is a laterally continuous unit consisting largely of shale with a minor component of interbedded siltstone or sandstone. Throughout its geological history, the depositional environment of this formation was exposed to complex weathering and loading/unloading processes that transformed and altered the typical sedimentary (argillaceous) formation to an over consolidated material with inherent epigenetic discontinuities such as fissures and fractures. From a stiffness or strength perspective, this clay shale formation can be considered a hard-soil or a soft-rock. For engineering assessment and design involving these clay shale deposits, careful geologic in situ characterization, detailed laboratory testing, and representative numerical modelling are needed.
  In Alberta, Clearwater Formation clay shale (CCSh) serves as the caprock for almost all steam assisted gravity drainage (SAGD) projects. Prior to commencing a thermal recovery project, an assessment for caprock integrity is typically carried out to ensure the caprock will serve as an effective containment seal of reservoir fluids over the life of the elevated pressure and temperature SAGD operation. Shallow caprocks (e.g. less than 200 m depth) are generally more vulnerable to deformation due to their low confinement, hence there is a need for a more rigorous evaluation of its behaviour and integrity. This rigorous assessment includes accurate geological characterization including an estimation of normal and shear stiffnesses (Kn and Ks) of the potential discontinuities within a CCSh caprock. To date, limited research has been conducted on these properties with most approaches adopting assessment methods developed for fractured hard rocks.
  This research initially examined the discontinuous nature of the CCSh through the processing of a unique light detection and ranging (LiDaR) dataset captured for an exposed mining bench of the Clearwater Formation. The LiDaR point cloud was used to extract the fracture characteristics in the CCSh using MATLAB scripting and successfully constructed a 3D triangulated irregular network (TIN). A geologically constrained discrete fracture network (DFN) was subsequently built based on the LiDaR dataset.
  Finite element analyses were conducted to explore the potential differences in fracture deformation mechanisms between a hard rock and hard soil/soft rock. The finite element models were validated against corresponding laboratory mechanical responses where subsequent analyses confirmed the expected response that fractured soft rock masses do not follow typical deformability and compliance conventional relationships developed for hard rock masses. A new boundary-identification scheme to identify the transition from soft rock to hard rock conditions was proposed based on a rock mass compliance ratio.
  Three dimensional geomechanical simulations were conducted of a CCSh caprock (CCShC) overlying a SAGD reservoir. The simulations were conducted in order to determine the extent to which overburden confinement, caprock discontinuity configuration and the SAGD MOP affect caprock deformability. It was found that, mechanically modelling the fractured CCShC above SAGD as a continuum results in an inaccurate estimation of the surface uplift. The latter may lead to forming undesirable pathways within the caprock for the pressurized fluids underneath it, compromising the caprock basic hydraulic seal.
  The three-dimensional distinct element code (3DEC) was used to study the equivalent mechanical parameters and representative elementary volume (REV) of a soft rock mass considering strength anisotropy and scale effect. Following various attempts, a mechanical REV could not be estimated for the soft CCSh. In addition, the equivalent mechanical parameters could not fit in a tensor form. Accordingly, the soft CCSh cannot be represented in an equivalent continuum model form. This provided additional evidence that soft rocks do not behavior in a similar manner to hard rocks, and the mechanical models developed based on a hard rock matrix assumption must not be applied to soft rock masses such CCSh.
  To constrain the values of normal (Kn) and shear stiffness (Ks) of fractures in a soft CCSh, results of consolidated drained (CD) direct shear laboratory tests on pre-existing discontinuities within CCSh specimens were analyzed. Based on numerical simulations of the tests, Kn and Ks of pre-existing discontinuities were estimated. A novel approach was introduced to generally estimate the Kn of discontinuities in soft rock masses at zero change of pore pressure using 3D coupled (hydromechanical) numerical simulations. While in general for hard rock systems, Ks will increase or remain constant for increasing discontinuity displacement, for the soft rock CCSh specimens analyzed, Ks was found to decrease with increasing discontinuity displacement.

• Subjects / Keywords

  • Clay shale caprocks
  • Geomechanical assessment
  • Continuum-discontinuum characterization

• Graduation date

  Fall 2020

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-nyx8-hx78

• License

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