Abstract

This work demonstrates the potential of using the hybrid finite-discrete element method (FDEM) to model thermal and mechanical coupling mechanisms around a wellbore in shale formations. The simulated thermal and mechanical stresses were first validated in separate models by comparing them to closed-form solutions. Then, the stability of a borehole in Opalinus Clay (Mont Terri, Switzerland) was analyzed. The prominent mechanism affecting the wellbore stability was found to be the mud pressure applied to the excavation boundary. At lower in-situ stresses, the extent of fracturing due to tensile stresses from cooling the rock by -100 °C was greater than at higher stress regimes. This was due to the greater stress differential between the mud pressure and the stress field in the higher stress regime. The higher mud pressure counteracts the thermal stresses and thus less fractures develop. It was also observed that the choice of constitutive model (i.e., isotropic versus anisotropic) had little effect on the temperature change required for fracture initiation, but it did influence the fracture pattern due to preferential planes of weakness present in the bedded Opalinus Clay.

1. INTRODUCTION

Wellbore instability has long been an issue in the oil and gas industry which has resulted in estimated monetary losses of 8 billion US$ per year worldwide [1]. Instability issues have more often been observed when drilling through shale formations. As the hole is being excavated, drilling fluids have an integral role in stabilizing the hole since they cool the drill bit, remove drill cuttings, prevent formation fluids from entering the well and stabilize the borehole walls [2].

Inherently, the drilling fluids interact with the rock formation through thermal exchanges and chemical interactions. It is typical to encounter reservoir temperatures which are greater than the temperature of the drilling fluids [3]. As a result of cooling the formation, the rock tends to contract and a reduction of hoop stress is generated, effectively lowering the fracture gradient [4]; thus increasing the likelihood of developing tensile fractures at the hole wall. This mechanism may cause fractures to nucleate which in turn may result in wellbore instability.

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