Wellbore stability evaluation method based on the continuous tangent envelope of a Mohr circle

Area of Science:

• Geotechnical Engineering
• Petroleum Engineering
• Rock Mechanics

Background:

• Accurate formation collapse pressure is vital for selecting drilling fluid density and ensuring wellbore stability.
• Traditional methods for determining rock strength parameters may not be suitable for high-pressure environments.

Purpose of the Study:

• To develop and validate a new method for calculating formation collapse pressure using a continuous tangent envelope.
• To compare the effectiveness of the new method against traditional approaches for wellbore stability analysis.

Main Methods:

• Conducted large-scale, high-density confining pressure triaxial experiments on tight sandstone.
• Determined a mathematical expression for the continuous tangent envelope of a nonlinear Mohr circle.
• Analyzed the variation of cohesion and internal friction angle with confining pressure.
• Used MathCAD for simulation and comparison of different methods.

Main Results:

• The parabolic curve method is unsuitable for high confining pressure triaxial tests.
• The continuous tangent envelope method accurately captures the relationship between rock strength parameters and confining pressure.
• This method provides a more realistic assessment of rock mechanics parameters compared to linear or parabolic envelopes.
• The continuous tangent envelope method offers advantages in determining critical density for wellbore stability.

Conclusions:

• The continuous tangent envelope method offers a more accurate approach to calculating sandstone formation collapse pressure.
• This method provides a stronger theoretical basis for optimizing drilling fluid density and enhancing wellbore stability in deep formations.
• The findings guide the study of wellbore stability in challenging geological conditions.