Characterization of Weakly Cemented Sands from Geophysical Logs /

Abstract

Shear strength is a critical parameter in evaluating the sanding potential of weakly cemented sands in oil-bearing formations. The current state-of-the-practice is to use empirical correlations (e.g., relating unconfined compressive strength to velocity logs) developed almost entirely for high-strength rocks that grossly overestimate the strength of weakly cemented sands. The objective of this study was to develop a methodology for estimating the strength of weakly cemented sands from commonly measured geophysical logs. An extensive laboratory testing program was first performed on a simulated weakly cemented sand from an oil well. An empirical model was developed that links porosity, horizontal effective stress, and the stresses at failure to synthetic compressional and shear wave velocities calculated using a modified Biot-Gassmann theory. A procedure was then established to infer unconfined compressive strength (qu), real cohesion (c'r) and tangential friction angle (j't) from the estimated stresses at failure. The model and the procedure were then applied to wireline log data from an oil-bearing formation, and there was reasonable agreement between estimated and measured values of strength. This methodology has the potential to estimate strength parameters of weakly cemented oil bearing sand directly from density/porosity logs and compression wave velocity logs.