Drained and undrained triaxial stress path testing has been conducted on three clean unconsolidated quartz sands in order to investigate the influence of textural parameters on elastoplastic behavior. For all sands tested, pore collapse and porosity reduction were observed to be strong functions of the applied stress path, due to shear enhanced compaction. At elevated pressures pertinent to petroleum engineering practices, grain-crushing can be a significant mechanism of plastic strain accommodation, depending on both sand texture and applied stress path. Yield caps with an additional shape parameter allowing flattening of the ellipse accurately capture the observed onset of plastic yielding. For unconsolidated sands it is important to ascertain whether preconsolidation pressure or grain-crushing pressure controls the onset of plasticity under isotropic loading conditions.
1. INTRODUCTION
Approximately 90% of the world's oil and gas wells are drilled in siliciclastic reservoirs [1] with most new discoveries being made in either unconsolidated sands or weakly cemented sandstones [2]. Pressure depletion associated with hydrocarbon production can rapidly induce largestrain plastic deformational behavior in these weak "problem" geomaterials, resulting in well failures, solids production and general reservoir impairment.
Plastic strains are accommodated either through brittle shear failure or compactive pore collapse mechanisms depending on the exact stress path followed. For more deviatoric stress paths (relatively low mean effective stress, P', and high stress difference, Q) dilational microcracking occurs prior to shear localization into failure planes orientated at a given angle to the principal stress state [3]. However for more isotropic stress paths (relatively high P' and low Q) continuous plastic compaction occurs with no localization, leading to pore collapse and subsequent strain hardening on further loading.
Weak geomaterials with generally high porosity and low cohesive strength can deform plastically to large strain even under hydrostatic stress conditions. High porosity enables the material to deform irreversibly due to a purely contractant volumetric collapse mechanism. The locus of such compactive yield points associated with different stress paths defines a yield function or "plastic cap" which delineates the onset of unrecoverable porosity loss due to material implosion [4].
Classical elastoplastic constitutive laws that integrate both compactive pore collapse and dilational shear failure were originally derived by the soil mechanics community. Elastoplastic material properties are generated from experimental testing, however traditionally soil mechanics laboratory applied pressures have been in the kPa range whereas effective pressures relevant to the etroleum industry can be in the 10'sMPa range. Accordingly, we have tested a suite of unconsolidated sands at elevated pressures up to 130MPa in order to assess: (i) the performance of various cap models at elevated pressure; (ii) sand textural controls on elastoplastic material properties.
Rock strength prediction is fundamental to deformation-related problems in which no core is available for direct measurement, or for real-time numerical analyses such as wellbore stability assessment. Thus much effort has been expended in developing correlations between rock fracture strength parameters (cohesion, friction angle) and other physical properties such as rock composition/texture [5] and wireline-derived data [6]. With the upsurge in recent years of geomechanics modeling to answer reservoir engineering problems, the industry is increasingly applying strength correlations to populate dataspars
