Stress, porosity, and failure-dependent compressional and shear velocity ratio and its application to wellbore stability

Abstract

The ratio of compressional to shear wave velocities strongly depends on the rock lithology, porosity and mechanical integrity under the in-situ state of stress. When the rock properties or in-situ stresses are changed due to drilling or geologic perturbations, the compressional and shear wave velocities exhibit certain correlations with the generation of fractures and stress redistribution. We analyze laboratory experiments where acoustic measurements are made of rock samples under triaxial compressive testing. The relationships of lithology, porosity, rock failure, and the ratio of compressional to shear velocities ( V p /V s) are established based on the experimental tests. The results indicate that the ratio of V p /V s increases significantly when the rock approaches its strength limit and after the rock fails. Due to the redistribution of stresses around the wellbore and the presence of fractures, the V p /V s ratio can reach several times its initial value. The abnormally high V p /V s ratio is primarily caused by deterioration of the rock's mechanical integrity brought on by geologic movement or borehole excavation while drilling. The creation of new fractures also increases permeability and facilitates fluid movement which can increase the rock porosity and further slow the propagation of shear waves. In both cases, the change in rock properties causes a significantly slower propagation of shear velocity and the resulting V p /V s ratio can be observed to detect drilling induced failure and naturally fractured rock zones. In this paper, logging while drilling and wireline measurements of sonic velocities from the Gulf of Mexico and Offshore Africa are examined to determine the relationship between the V p /V s ratio and wellbore failure. The analyses obtain the V p /V s ratio in a normal stress state for different lithologies, compare the normal velocity ratio to the in-situ observed velocity ratio, and then analyze drilling induced wellbore failure and pre-existing fractures from the normal/observed variations. Analysis of the well log data demonstrates that an over-gauge wellbore, or one with drilling induced or pre-existing fractures, corresponds to an abnormally high V p /V s ratio. The correlation between the increase in the V p /V s ratio and wellbore state demonstrates a new method to detect zones of highly fractured rocks and wellbore failures.