[CCoE Notice] Thesis Defense: Studying the Effect of Pre-Consolidation of Stress-Strain Behavior of Unconsolidated Sands

[Grayson, Audrey A]

Masters Thesis Defense
Abhishek Arya
Studying the Effect of Pre-Consolidation of Stress-Strain Behavior of Unconsolidated Sands

Date: May 3rd, 2018

Location: Energy Research Park Building 9, Room 123

Time: 10 AM-12 PM



      Committee Chair:         Dr. Michael T. Myers

      Committee Members: Dr. Lori Hathon, Dr. George Wong


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.  To measure the compressibility in laboratories, its required to stress condition the samples to represent in-situ samples and perform a uniaxial test.  We propose an alternative protocol to stress condition the sample and compared it to the standard protocol currently being used.
We compare the resulting uniaxial compressibility curves, as well as pre- and post- test CT scans of the samples. We found that in higher compressibility samples (larger grain size, higher angularity, less quartzose mineralogy) stress cycling at 1250 psi resulted in substantial brittle grain failure compared to the 24-hour hold testing protocol.  At low depletion stresses, the measured stress-cycled sample pore volume compressibility was as much as 23% lower than using the stress and hold protocol.  However, for more quartz rich sands, compressibility curves from the two methods repeated within experimental error over a range of depletion of up to 8000 psi.  Also, far less grain fracturing was observed due to stress cycling for this lithology. For unconsolidated sand reservoirs that are rich in feldspar or ductile lithic fragments, laboratory test protocols can result in dramatically different estimates of pore volume compressibility at low depletion stresses.  Additional work and modeling is required to understand which conditioning is appropriate to model in-situ measurements of compressibility.

In the second section of the thesis, we conduct a series of tests, where different samples are taken to 3 different pre-consolidation stresses.  We then perform triaxial tests reaching different mean stress levels and explore the effect of pre-consolidation on the stress strain behavior of sand packs. The results are compared with Modified Cam-Clay models, and we find the necessity for a double yield surface model which is discussed.

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