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Technical Paper: Review of Methods for Uniaxial Compressive Rock Strength Estimation in Deepwater Formation With Uncertainty Quantification in Wellbore Stability Analysis in Absence of Core Measurements

Society: SPE
Paper Number: 150343
Presentation Date: 2012
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Abstract

With the increase in deepwater exploration and unconventional tight gas exploration, wellbore stability analysis has emerged as a pivotal part of finalizing drilling programs. It becomes more important when inclined wellbores are drilled where stable mud weight windows are narrow. Wellbore stability analysis is conducted to validate calculated stress and rock properties against drilling observations. Uniaxial compressive rock strength (UCS) is an integral part of geomechanics studies involving mechanical earth modeling. Absence of direct core measurement brings uncertainty in the model. Presently, the industry uses different empirical relationships for estimation of UCS depending on formation type. It is critical to use appropriate relationship with minimum error and known quantification effects on final mud weight recommendation. Depositional sequence brings formation under different compaction regime of grains with interstitial pore fluid. Sandstone and shale are two common lithology found in India deepwater exploration field with intermediate siltstone. Validation of different models depends on porosity, volume of clay, saturation and sonic slowness response of formation. The present paper reviews these methods with four case studies from India and discusses how different models respond in deepwater formation sequences.  Best method depending on range of volume of clay and porosity has been discussed in term of fuzzy logic. Mathematical model is build for uncertainty quantification of effect on mud weight optimization using different methods for UCS estimation is also discussed.

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