Date of Award

5-1-2002

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Geological Engineering

Abstract

This thesis is concerned with the stability of inclined boreholes. It addresses the mechanical stability of a wellbore as a function of time while also considering the impacts of chemical osmosis, temperature differences between the drilling mud and formation, and the introduction of an impermeable filter cake on the state-of-stress near the wellbore. The stress and pore pressure expressions are derived for the general case of an arbitrarily inclined wellbore subjected to three, unequal, in-situ stresses. It is assumed that the material is the linearly poroelastic and the stress and pore pressure expressions are found by solving the three-dimensional problem using the concept of generalized plane strain. This allows the problem to be decomposed into a poroelastic plane strain, elastic antiplane, and elastic uniaxial problem. There are three sets of equations developed; namely the poroelastic, chemo-poroelastic, and poro-thermoelastic equations. The poroelastic equations are representative of drilling in a fluid-saturated, porous, chemically-inert rock under isothermal conditions. The fully-coupled chemo-poroelastic expressions are also valid under isothermal conditions but take into account the differences in the activities of the water phases of the formation and drilling mud. The poro-thermoelastic expressions are developed by coupling the effects of pore fluid expansion (contraction) to rock deformation. The poro-thermoelastic expressions are applicable when there is a temperature difference between the rock and drilling mud and when the effects of thermal osmosis can be neglected.

The stress and pore pressure expressions are derived (for both a permeable and impermeable wall) and are used to develop a wellbore stability design code. The model evaluates for tensile and shear failure by applying either the poroelastic, chemo-poroelastic, or poro-thermoelastic solution (chosen by user) at a specific borehole orientation. The model enables the user to choose either the Mohr-Coulomb or Drucker-Prager shear failure criteria and is capable of analyzing for failure inside the rock. The output generated by the model represents a safe operating zone, which corresponds to the range of mud weights that can be used to avoid shear and tensile failure at a given wellbore trajectory. Several numerical examples are used to examine the effects of time, chemical osmosis, and thermal loading on wellbore failure at the wall and inside the rock. The results indicate that, in general, as time increases the potential for shear and tensile failure also increase. In addition, radial spalling is initiated inside the rock (near the wellbore) at low mud weights that requires a minimum mud weight to overcome. The numerical examples further indicate that chemical osmosis and thermal loading significantly alter the near wellbore pore pressure and stresses which may lead to time-delayed failure.

A parametric analysis is also conducted to ascertain the sensitivity of the near wellbore stress and pore pressure to various input coefficients. The parametric study reveals that determining the Poisson’s ratio, undrained Poisson’s ratio, Biot’s constant, Skempton’s pore pressure constant, intrinsic formation permeability, water activity of both the drilling mud and pore fluid, and the thermal diffusivity are critical to determining the mud weight window. The remaining input parameters did not affect the safe operating zone.

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