Quanitfying Inaccuracy In Rock Stress/Strength Determination And Considering The Implications For Geomechanical Predictions

Abstract

The understanding of subsurface stress is critical to the efficient management and prediction of fluid behaviour in Earth’s shallow crust. However, uncertainties related to the accuracy of measurements and calculations of the vertical and horizontal stresses are rarely explored. The impacts of these uncertainties that are carried through to resulting subsurface models, may lead to significant engineering and thus economic implications.

This project is based around improving the accuracy of geomechanical predictions. To achieve this we explore the uncertainty associated with estimates of vertical and horizontal stresses, rock strength and pore fluid pressure. The starting point for this project is an assessment of the uncertainty of vertical stress measurements, which are generally considered to be the most accurately constrained of the three principal stresses. We use data from the Moomba gas field, in the Cooper Basin, a sedimentary basin of Permian-Triassic age situated in the far northeast of South Australia. The geology of the Moomba field is well constrained from several hundred wells, hence any variation in vertical stress measurements may well represent a measure of its uncertainty, as opposed to reflecting changes in vertical stress due solely to geological heterogeneity.

We started by evaluating the effects of removing spurious data from density logs, and the associated impacts of interpolating between the resultant voids in log data. We also evaluated the influence of extrapolating density log data to surface levels when large sections of shallow geological sections have not been analysed. Additionally, we contrasted the influence of the application of the Nafe-Drake and Gardner methods on uncertainties when determining vertical stress gradients.

From this project, we expect to develop more accurate geomechanical workflows that can be used to minimise exploration risk and provide improved data to allow the safe and efficient management of subsurface fluids.

AAPG Datapages/Search and Discovery Article #90324 © 2018 AAPG Asia Pacific Region GTW, Pore Pressure & Geomechanics: From Exploration to Abandonment, Perth, Australia, June 6-7, 2018