Viscous Rheology and State of Stress in Unconsolidated Sands

Abstract

Abstract Observations of triaxial creep and stress relaxation of unconsolidated reservoir sands imply markedly different static and dynamic modulii and may explain observations of very low differential stresses found in the Wilmington Field, CA, the Gulf of Mexico, and elsewhere. Triaxial testing performed on 1 inch plugs of reservoir sands under both stress- and strain-controlled conditions show transient stresses and strains. All tests were performed under dry or drained conditions to eliminate poroelastic effects. We previously observed creep under hydrostatic stress conditions in these unconsolidated sands. The resulting strain vs. time curves could be fit with a standard linear solid viscoelastic model that had a relaxation time of about 10 hours. The data demonstrates the rock's ability to dissipate stress through relaxation and creep. The reservoir material shows viscoelastic rheological behavior in both creep and relaxation tests which appears to be related to the presence of intergranular clay. Measurements of the static Poisson's ratio under large strains yielded values .29 which suggests that the particles in the matrix efficiently coupled the principle strains. P. 465