Most shale rocks which contain an appreciable fraction of reactive clays (e.g. Montmorillonite) will adsorb drilling mud filtrate (water+ions) and cause unstable drilling conditions. When contacted with the mud filtrate, these shales will swell, creating a soft, swollen zone around the wellbore, therefore, the natural mechanical properties or the strength of the swollen shales will decrease causing serious hole problems such as undergauge hole, stuck pipe, overpull on trips, and several other problems. Thus swelling stresses and rock strength reduction must be included in any attempt to effectively model shale mechanical properties after interaction with drilling fluid filtrate. In this study shale swelling stresses were integrated into the prominent Mohr-Coulomb failure criterion and therefore a new form of this criterion has been introduced which combined the natural mechanical properties with swelling stresses to predict the in-situ strength of shales when invaded by the drilling fluid filtrate. The modified failure criterion was verified experimentally.

1.1 Introduction

The swollen zone created around the wellbore during drilling in shale sections will be driven inwardly by high overburden stresses and requires that higher than usual mud weights be used to counteract this inward displacement (Erling et al., 1992). When shale contains high native water content, even before it is exposed to drilling fluid, it is abnormally weak and unable to withstand the differential stress imposed by drilling out the surrounding rock (support).

The mode of failure, when the stress differential created by the relief of lateral stress exceeds the yield strength of the formation, is plastic deformation of the wellbore. If the shale is under abnormally high pore pressure, spalling will be the result (Darley, 1969). Swelling stresses generated due to the interaction between the shale and water based drilling fluids must be taken into account when predicting the effect of swelling on borehole stability and failure criterion (Onaisi et al., 1994).

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