Experimental Study: Investigating Effect of Injection on Yield Surface, Stress Path Coefficients, and Associated Hysteresis in Unconsolidated Sands

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Injection geomechanics is an integral part of sub-surface work. Material behavior is stress magnitude and stress path dependent. Many researchers have studied effects of depletion on rock mechanical behavior and corresponding changes in in-situ stresses. Most literature regarding study of injection effects in unconsolidated sands is primarily focused on Canadian Oil sands where the effective stress magnitudes are different than in deep-water Gulf of Mexico. This thesis investigates the effects of depletion and injection on changing the minimum in-situ stress magnitude, yield behavior, and associated hysteresis in unconsolidated sands. A novel sample preparation technique is presented to make consistent and representative samples of the target reservoir in deep-water. The variation of minimum in-situ stresses with pore pressure is quantified using horizontal stress path coefficients. Far field reservoir boundary conditions are simulated through uniaxial strain boundary condition and with constant vertical stress. Results show material is altered during initial depletion and is associated with large plastic strains and hysteresis energy. Under repeat injection and depletion, material is elastic with minimal hysteresis energy. Yield behavior of geo-materials under unloading stress path direction (injection) has not been studied in unconsolidated sands with high effective stresses. This thesis focuses on understanding the dependence of yield surfaces on stress path directions and stress magnitudes. Three different stress path directions were studied to determine individual yield surfaces. Results show yield surfaces for loading and unloading paths were different and parallel within the range of stress magnitudes studied. A global minimum yield surface model is proposed to predict the location of the minimum yield surface and its stress path. Using this model, the upper and lower bounds of yield surfaces can be determined with two tests. A yield function is generated to predict yield stresses with an initial stress condition and a stress path direction.

injection, unconsolidated sands, unloading, yield behavior, stress path coefficients