Mechanical properties and modeling of seal-forming lithologies. Final report Page: 4 of 9
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FINAL TECHNICAL PROGRESS REPORT
Research Objectives and Summary
The focus of this research supported by DOE Basic Energy Sciences for a three
year period and extended one additional year at no extra cost, has been on the mechanical
and transport properties of two sedimentary lithologies, rocksalt and phyllosilicate clay-
bearing shale, that commonly serve as structural traps to hydrocarbons Experimental
determinations of physical properties of these rock types have been combined with
numerical modeling to examine the development of structural traps and the roles of fluids
in natural deformations, applying depositional and tectonic loading conditions and
entertaining different histories for the presence of fluids. Laboratory efforts have been
directed towards determining realistic constitutive relationships that are accurate over a
wide range of strain rates and improving our understanding of the physics and
micromechanics of deformation. Modeling efforts have applied these laboratory-based
relationships to follow the initiation and evolution of salt structures and overlying
stratigraphic units for structures with well-constrained geologic histories, stratigraphic
controls on loading and timing, and microstructural information relevant to flow stresses.
Many of the research objectives defined by our original proposal have been
completed and we have addressed new questions that emerged during the course of our
investigations. The nonlinear relationships between strength and strain rate for rocksalt
and illite-bearing shale have been evaluated over a wide range of experimental conditions
(210 < T < 2000C, 3 < Pe <400 MPa, 10-9 < < 10-3 s-1) and numerical modeling of salt
diapirs with geologic constraints on loading and ascent rates has extended predictions of
mechanical response of rocksalt to strain rates of 10-15 s-1. Fluids are extremely
important to the mechanical behavior of both rock types. Experimental results for shale
include its behavior in the absence of pore fluids, with only structural and adsorbed H20
layers on clays, as well as its drained and undrained response when saturated with brines
of varying composition. Models of salt diapir development have compared incubation
times for diapir initiation and ascent velocities that are constrained by geologic
observations and those predicted by power law, dislocation creep laws determined in our
laboratory for relatively dry rocksalt and the linear, solution-transfer creep law of Spiers
et al. (1988, 1990) for wet rocksalt.
Shale Deformation
Failure strength and permeability measurements for Wilcox shale have been made
with 1 molar solutions of NaCl, KCI, and CaCl2 as well as with distilled H20. Both the
mechanical and transport properties of shale are strongly dependent on effective mean
stress, exhibiting significant non-recoverable changes with increasing effective pressure
Pe associated with inelastic changes in pore geometry and structure, and smaller
reversible poro-elastic changes for decreasing pressure. In addition, failure strengths are
weakly dependent on strain rate and can be described by an exponential law of the form
= A(T, Pe) exp { (01 - (3) }, both when samples are essentially dry and when they
are saturated with fluids. Inelastic yield strengths depend on temperature for dry samples
at elevated Pe through an Arrhenius expression.1
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Kronenberg, A. K.; Russell, J. E.; Carter, N. L.; Shea, W.; Ibanez, W.; Mazariegos, R. et al. Mechanical properties and modeling of seal-forming lithologies. Final report, report, April 1, 1998; College Station, Texas. (https://digital.library.unt.edu/ark:/67531/metadc709163/m1/4/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.