Technical Paper: Rock strength parameters from annular pressure while drilling and dipole sonic dispersion analysis

Society: SPWLA
Paper Number: 2004o
Presentation Date: 2004


The characterization of rock strength is perhaps the most important task in choosing the optimal type of completion in soft-sediment reservoirs. While two strategies have commonly been used to guide this decision, both have significant shortcomings. A new technique is presented that estimates formation strength parameters from annular pressure and dipole shear measurements.

The highest quality rock strength properties are normally determined in a rock mechanics laboratory by stress-loading actual formation material to yield and failure. Seldom will an operator dispense with this step in fields prone to sand production. However, fewer and fewer wells are cored, and the type of completion for different zones in different wells is increasingly based on a small number of laboratory measurements in a single well. Logging measurements yielding lithology, porosity, and elastic moduli are often transformed into rock-strength parameters and used to supplement the high quality but sparse laboratory data. However, while this strategy of log calibration to laboratory data can yield the correct trend over large porosity ranges, it generally fails in predicting the strength variations in high porosity (> 27%), soft-sediment reservoirs.

The new technique monitors and captures the drilling process for what it is, an in-situ stress test. As the driller penetrates each formation, the borehole magnifies the far-field stresses by an amount dependent on the wellbore pressure. This increase in loading near the wellbore causes softsediment reservoirs to yield and fail. The depth of this yielded material can be quantified using broadband dipole shear measurements. The magnitude of stress causing this radially varying damage can be estimated with a mechanical earth model and the variation in annular pressure measured between the time of drilling and logging. The methodology for this new technique is illustrated with a recent deepwater discovery.