Poroelastic effect on borehole stability analysis using hollow cylinder model

  • Zili Qi

Student thesis: Master's Thesis

Abstract

Linear elastic theory is the most widely used theory to be applied to wellbore stability analysis for its simplicity and convenience. It assumes that rocks behave as a linear elastic material and ignores the pore pressure effect, what is not in agreement with the mechanical properties of real formations. Therefore, the stresses and critical pressures predicted by this theory will be always seriously inaccurate. Coupled poroelastic theory could efficiently prevent the above deficiency of linear elastic theory. This poroelastic theory takes into consideration of the coupled interaction between formation fluids and rock matrix. Pore pressure redistribution and time-dependent effect of wellbore stability could be estimated using this theory. However, due to the complexity of this non-linear poroelastic theory, it is more challenging in application and theoretical work than the classic theory of linear elasticity. The objectives of this dissertation were focused on the investigations into the wellbore stresses and stability analysis using these two theories and also on the systematic comparisons of the theoretical solutions obtained from these two theories. Some recommendations and guidelines to the practical petroleum engineering problems of wellbore stability have been achieved through these analyses. The primary approach adopted in this work was the theoretical problem solving. For the linear elastic theory, the stress distribution and critical collapse and fracturing pressures were estimated using one set of given data. After that, parameter sensitivity analyses and impact of failure criteria were conducted. For the coupled poroelastic theory, the solution of this theory in time domain using Laplace transforms was obtained first. The pore pressure redistribution and its time dependent evolution were subsequently calculated using one set of given data. Combining pore pressure alteration induced stresses and elastic effect induced stresses, the wellbore stability analyses were performed and critical collapse and fracturing pressures were estimated. Furthermore, the parameter sensitivity analyses and impact of failure criteria were also conducted for this theory. Moreover, the whole analysis processes above were conducted for two scenarios. The first one was that the initial pore pressure was higher than the wellbore pressure (which represented the overbalanced drilling) and the second one was that the pore pressure was lower than the wellbore pressure (which represented the underbalanced drilling). Different phenomena of this poroelastic effect on the wellbore stability were presented for these two scenarios. Finally, the systematic comparisons of the theoretical results from these two theories were presented. It was shown that the pore pressure redistribution after drilling and the pore pressure alteration induced stresses will exert great influence on the wellbore stability. They will not only change the critical collapse pressure and fracturing pressure, but also make it possible that the shear failure can initially occur inside the rock formation instead of the wellbore wall.
Date of AwardDec 2011
Original languageAmerican English
SupervisorPawel Nawrocki (Supervisor)

Keywords

  • Applied sciences
  • Borehole stability analyssis
  • Oil well drilling
  • Petroleum engineering
  • 0765:Petroleum engineering

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