Studying the Effect of Preconsolidation Stress on Stress-Strain Behavior of Sands

Date
2018-05
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Abstract

The pore volume compressibility of unconsolidated sand reservoirs is a principal mechanism controlling reservoir economics. Core handling and testing protocols for unconsolidated sands are significantly different than for consolidated materials. A single stress step equilibration is compared to stress cycling equilibration techniques for measuring the uniaxial compressibility.

The resulting uniaxial compressibility curves are compared, as are pre- and post- test CT scans of the samples. In high compressibility samples, stress cycling at 1250 psi resulted in substantial brittle grain failure compared to the single step protocol. The measured compressibility from stress cycling was as much as 23% lower than that obtained from single stress step protocol. For the more quartzose sands, compressibility curves from the two methods overlaid (within experimental error) over a range of depletion of up to 8000 psi. Far less grain fracturing was also observed for this lithology. For unconsolidated sand reservoirs that are rich in feldspar or ductile lithic fragments, the two laboratory test protocols result in dramatically different estimates of compressibility at low depletion stresses. A samples’ mineralogy, texture and stress history must be considered while designing an equilibration protocol.

The second portion of the thesis explores the effect of preconsolidation on the stress-strain behavior of sand packs. Comparison to Modified Cam-Clay (MCC) models are made and the necessity for a double yield surface model is discussed.

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Keywords
Modified Cam-Clay, Double Yield Surface, Unconsolidated Sands, Triaxial Test, Multistage triaxial test, Preconsolidation, Uniaxial Test, Compressibility, Sample Preparation, Sand pack
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