An investigation of mechanical, acoustic and failure properties in unconsolidated sands

Abstract

Abstract Prediction of sand production and associated casing and tubing problems require a good understanding of failure behavior in unconsolidated and poorly consolidated sands. An experimental investigation was conducted on several fully saturated unconsolidated sand samples to determine stress states that cause sand production. Biaxial stress cycling measurements were performed at several confining pressures to demonstrate the effect of horizontal stress on formation stability. Berea sandstone samples were also tested under the same stress conditions to investigate the role of consolidation and cementation on stability. Ultrasonic wave velocity, porosity and permeability variations under biaxial stress conditions were simultaneously measured to obtain a complete comperative analysis. Failure envelopes obtained from the biaxial experiments were analyzed using Drucker-Prager failure criteria. Experimental data showed that state of stress has significant influence on the elastic and plastic deformation behavior of unconsolidated sand formations. Samples experienced inelastic and plastic deformation almost immediately with the initiation of loading. On contrary, Berea sandstone behaved almost elastically in spite of its nonlinear hysteretic behavior. It was also observed that unconsolidated sands behaved as strain hardening materials at high strain rates. The results presented here demonstrate that stability and/or failure conditions for a given formation can be predicted at all in situ stress states with limited number of experiments. The results can also be extended for fracturing unconsolidated and friable formations to inject drill cuttings into these formations for complying with the environmental regulations in an economic manner.