Modified Hoek-Brown Criterion for Rock Shear Failure Prediction
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Solution Overview
Problem
Current criteria for predicting shear failure in rock formations, such as the Mohr-Coulomb, Drucker-Prager, Modified Lade, and Hoek-Brown criteria, either underestimate or overestimate rock strength due to their failure to account for intermediate principal stress, leading to inefficiencies in drilling and wellbore stability in naturally fractured formations.
Innovation Solution
A modified Hoek-Brown failure criterion is introduced, which considers the effects of natural fractures and intermediate principal stress on rock strength enhancement, incorporating a weight term to differentiate the contributions of intermediate and minimal principal stresses, thereby improving the prediction of shear failure in rocks with fractures.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If traditional failure criteria (Mohr-Coulomb, Drucker-Prager, Hoek-Brown) are used to predict shear failure, then the prediction process is simple, but the accuracy of rock strength prediction deteriorates due to not accounting for intermediate principal stress
Solution Approach 1:
The patent modifies the Hoek-Brown failure criterion by introducing a new parameter ω (omega) that represents the contribution of intermediate principal stress. The modified criterion transforms from σ1 - σ3 = f(σc, mi, s, a) to σ1 - σ3 = f(σc, mi, s, a, ω), where ω ranges from 0 to 1. This parameter change allows the criterion to account for intermediate principal stress effects while maintaining the fundamental structure of the original Hoek-Brown equation, thus improving prediction accuracy without excessive complexity
Solution Approach 2:
The modified failure criterion combines multiple factors into a composite model: it integrates the original Hoek-Brown terms (rock strength σc, material constant mi, fracturing parameters s and a) with the new intermediate principal stress contribution term ω. This composite approach creates a more comprehensive failure criterion that incorporates multiple stress components and rock properties, improving the overall accuracy of shear failure prediction
2Reliability
If traditional failure criteria are applied to naturally fractured formations, then the calculation process is straightforward, but the reliability of wellbore stability prediction deteriorates due to underestimation or overestimation of rock strength
Solution Approach 1:
The patent introduces a controllable parameter ω that quantifies the intermediate principal stress effect, allowing the model to adjust between different stress conditions. When ω=0, the model reduces to the original Hoek-Brown criterion; when ω>0, it accounts for intermediate stress enhancement. This parameter adjustment mechanism improves reliability by enabling accurate prediction under various wellbore stability conditions without requiring a completely new complex model
Solution Approach 2:
The modified criterion provides a dynamic framework where the influence of intermediate principal stress can be adjusted based on specific formation conditions. The parameter ω can be calibrated according to the actual stress state and fracture characteristics of the rock formation, allowing the model to adapt to different geological conditions and improve prediction reliability across diverse wellbore scenarios
Data Source
AI summary
Systems and methods for determining shear failure of a rock formation are disclosed. The method includes receiving, by a processor, a plurality of parameters related to physical properties of the rock formation, applying the plurality of parameters to a predetermined failure criterion, and determining shear failure of the rock formation based on the failure criterion. In some embodiments the failure criterion is a modified Hoek-Brown failure criterion that takes into consideration an intermediate principal stress, and the difference between normal stresses and an average confining stress.