Physical and Mechanical characteristic relationships of Late-Cretaceous to Eocene reservoir rocks in the Maui, Maari and Manaia Fields, New Zealand

Constraint on the rock strength parameters within the subsurface is a fundamental requirement for accurate geomechanical modelling of aspects of reservoir stability and regional scale basin interactions. Insufficient rock strength data for offshore lithologies within the Taranaki Basin leads to a dependence on uncalibrated, empirical relationships applied in conjunction with wireline measurements for rock strength predictions. Use of these uncalibrated empirical relationships can lead to unreliable strength estimates, which reduces the confidence in geomechanical modelling and the subsequent solutions for the region.

We conducted uniaxial and triaxial experiments on cores from offshore Taranaki reservoir rocks from 2000 to 4000 m depth to develop the first, calibrated, empirical rock strength relationships for reservoir rocks of the Taranaki Basin, using both grain size and porosity as input parameters. We show that grain size and porosity can be used as predictive tools for determining Hoek-Brown and Mohr-Coulomb failure criterion parameters for petroleum geomechanics. As mean grain diameter and porosity are the dominant control on rock strength, we infer that rock strength parameters within the Taranaki Basin will follow a similar spatial distribution as the reservoir sandstone facies, which are dominantly defined by grain size.

We also show that the empirical relationships we developed with this dataset can be locally calibrated for other parts of the Taranaki basin, and indeed for other sandstones, such as the Buntsandstein from the Rhine Graben, France. Finally, we provide an approach by which grain size (+/− porosity) can be used to approximate the input parameters for the Hoek-Brown failure criterion in the absence of laboratory experiments. We propose, therefore, that the empirical relationships presented herein can be used to link facies descriptions with first-order estimates of mechanical properties at the basin scale.