Abstract

The objective of this work is to model the influence of shear stresses induced by viscous fluid flow on wellbore spalling. We simulated a drop of stress and pore pressure at the wall of a meter-scale borehole with a plane strain Finite Element model. The rock mass was modeled as a jointed continuum. Block sliding was predicted from the tangential displacements in the joint after the shear failure criterion was reached. Simulations show that: (1) Higher far field stresses induce more normal stress in the joints, which prevents the occurrence of shear plastic strains in the joints and reduces block sliding at the wall; (2) Shear stresses and consequent shear plastic strains that are induced by viscous fluid flow in the joints are higher for higher fluid viscosities, and decrease over time as the blocks on each side of the joint slide on each other; (3) In joints that are in contact with the borehole, a change of one order of magnitude in the fluid viscosity results in a change in joint shear stress by a factor of 2. Results suggest that if drainage had been simulated over a longer period of time or for a smaller borehole diameter, the failure criterion would have been reached on a larger zone around the borehole, which could have a critical impact on the risk of borehole spalling. The numerical approach proposed in this work is expected to be useful to recommend wellbore operation modes so as to avoid excessive spalling and clogging.

1. INTRODUCTION

Fractures existing prior to, or induced by drilling play a determinant role in spalling and block detachment during the stages of fluid injection and withdrawal in quasibrittle rocks (e.g., shale). Both numerical simulation and in situ investigation have proved that the mechanical instability resulting in block detachment is affected by the orientation of fractures in reference to the principal direction of in situ stress [1, 2] and to the borehole axis [3, 4]. The fluid pressure difference induced by fluid flow around the borehole - especially in the fractures - also plays a primary role on the stability of blocks [5, 6]. As a matter of fact, the fluid present in the fractures applies a normal force on blocks faces, which disturbs the mechanical equilibrium of these blocks. In addition, viscous fluid flow generates shear stresses on fracture and block faces, which can also affect the mechanical stability of the blocks. To our best knowledge, these shear effects have not been estimated or taken into account in previous studies. The objective of this work is to model the influence of shear stresses induced by viscous fluid flow on wellbore spalling. We model the rock mass as a jointed continuum, in plane strain, with POROFIS Finite Element program [7]. Constitutive laws used for the bulk and joint materials are explained in Section 2. Joint elements are assigned a plastic model coupled to a failure criterion, which allows predicting the relative displacements of the blocks. We simulate a pore pressure drop at the borehole wall. Parametric studies on the ratio far field stress to initial pore pressure and on fluid viscosity are presented in Section 3. Conclusions on the risks of borehole spalling are drawn in Section 4.

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