Analysis of borehole breakout development using continuum damage mechanics

Damage distribution and evolution have a significant effect on borehole stress concentrations. To model the complex fracturing process and inelastic deformation in the development of the borehole breakout, we implement a continuum damage mechanics (CDM) concept that takes tensile and compressive failure mechanisms into account. The proposed approach explicitly models the dissipative behavior of the material due to cracking and its evolution, which leads to an inhomogeneous redistribution of material properties and stresses in the vicinity of the borehole wall. We apply a constitutive plastic model for Berea sandstone and compare our numerical results to laboratory experiments performed on Tablerock sandstone. We are able to reproduce several characteristics of the failure process during the breakout development as observed in experimental tests, e.g. localized crack distribution in the vicinity of the borehole wall, damage evolution, which exhibits a widening process in the beginning followed by subsequent growth in depth, and shear fracturing-dominated breakout growth in sandstone. A comparison of our results with laboratory experiments performed on a range of stress conditions shows a good agreement of the size of borehole breakouts. The importance of the constitutive damage law in defining the failure mechanisms of the damaging processes is discussed. We show that the depth and the width of breakouts are not independent of each other and no single linear relation can be found between the size of breakouts and the magnitude of the applied stress. Consequently, only one far field principal stress component can be estimated from breakout geometry, if the other two principal stresses are known and sufficient data on the plastic parameters are available

In Situ Stress Assessment Based on Width and Depth of Brittle Borehole Breakouts

Borehole breakouts, as well as breakouts in tunnels and shafts, are a common occurrence, especially under high in situ stresses or stress states with high deviatoric component. Though they can pose a risk to stability, often they are of use, especially in deep boreholes, as they can help to determine to a certain extent the primary in situ stress. Observations have shown that while their depth evolves, their width remains constant. Currently the width only is used in conjunction with the Kirsch analytical solution to establish a linear relationship between the two in plane principal primary stress components. The stress state cannot be fully determined since one equation is available (failure criterion) for two unknowns. A recently proposed numerical tool based on conformal mapping is used in this work to simulate the formation of shear breakouts and investigate the feasibility of the determination of both principal primary in situ stress components, by making use of both the depth and the width of the breakout. Concluding, recommendations are provided for the use of the proposed methodology and limitations of its applicability are discussed

Saturation overshoot and hysteresis for twophase flow in porous media

Saturation overshoot and hysteresis for two phase flow in porous media are briefly reviewed. Old and new challenges are discussed. It is widely accepted that the traditional Richards model for twophase flow in porous media does not support non-monotone travelling wave solutions for the saturation profile. As a concequence various extensions and generalizations have been recently discussed. The review highlights different limits within the traditional theory. It emphasizes the relevance of hysteresis in the Buckley–Leverett limit with jump-type hysteresis in the relative permeabilities. Reviewing the situation it emerges that the traditional theory may have been abandoned prematurely because of its inability to predict saturation overshoot in the Richards limit