Characterising the Uncertainty of Rock Stress and Strength Estimates

Musolino, Matthew1 Holford, Simon1 King, Rosalind1, Hillis, Richard1

1The University of Adelaide, Australian School of Petroleum and Energy Resources, Adelaide, Australia

Accurate estimates of in-situ stresses and rock strengths are required for multiple practical applications during petroleum exploration and development, such as ensuring wellbore stability, minimising breakouts, and the design of hydraulic fracturing treatments. Issues regarding the aforementioned practices cost operators about US$8 billion globally each year. However, when predictive geomechanical models are constructed, both rock stress and strength are typically estimates rather than fully quantified, and their uncertainties are rarely examined. Based on data from the Cooper Basin in South Australia, this presentation examines uncertainty relating to estimates of the three principal stresses (vertical stress and minimum and maximum horizontal stress) and rock strength as a result of (a) available data inputs and (b) methodological approaches. We show that the magnitude of vertical stress at depths of ~3 km can vary up to 6 MPa depending on the methodologies employed when integrating and processing density logs and sonic transit time data. Minimum horizontal stress magnitudes are commonly estimated using leak-off tests (LOTs). We demonstrate that accurate interpretation of LOTs is challenging due to factors such as the presence of pre-existing fractures, cement channelling, non-linear pressure build-up, and plastic and elastic leak off. Based on the choice of interpretation technique and calculations based on the assumption of tensile or shear failure, estimates of minimum horizontal stress may also vary by up to 6 MPa at depths of 3 km. The maximum horizontal stress magnitude is typically considered to be the most difficult to quantify, and we compare five methods (frictional limits, presence of drilling induces tensile fractures, presence of wellbore breakouts, wellbore breakout width, and shear plane failure). We show that at depths of ~2.5 km, estimates of maximum horizontal stress magnitude determined using these approaches can vary between 18-40 MPa. Finally, we compared empirical approaches for estimating rock strength using sonic velocity data with laboratory uniaxial compressive tests for a variety of stratigraphic units in the Cooper Basin. We show that, depending on the empirical correlation being used, rock strengths may be underestimated by 25-43% when comparing sonic velocity log derived rock strength to physical compression testing. In summary, our investigation into the uncertainty in principal stresses and rock strength estimation leads us to proposes enhancements to methodologies concerning density log preparation and filtering, check-shot calibration, low data density leak-off interpretation, and rock strength-sonic transit time relationships. Our results provide end-users with a better understanding of the uncertainties associated with stress and strength estimates that can be factored into geomechanical risk assessments.


Matthew is a final year PhD student at the Australian School of Petroleum and Energy Resources. Matthew is a 3-year ASEG research grant awardee with experience in potential field geophysics exploration. Matthews’ current goal is to see practical applications of his research to industry through optimised geomechanical workflows.

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