Abstract

Wellbore enlargement due to sloughing and caving is considered as one of the major wellbore instability issues encountered during drilling operations that can lead to substantial loss in drilling time. Amongst its many adverse effects, wellbore caving can result in loss of circulation, stuck drilling string, and, ultimately, loss of the bottom hole assembly. In general, stability during drilling operations in shale are controlled by both (i) physico-chemical interactions between the formation and drilling fluid, and (ii) geomechanical parameters. Sloughing and caving problems have been encountered when drilling of shale formations, such as the Buckinghorse, using water-based mud as the drilling fluid. To understand the geomechanical properties that can impact the drilling operations, a comprehensive testing campaign, comprising of uniaxial compressive, triaxial, Brazilian disc, and fracture toughness tests, was conducted on samples retrieved from depth from the Buckinghorse Formation. To evaluate the mechanical anisotropy the Formation, samples were tested at 0°, 45°, and 90° with respect to the bedding planes. Transverse isotropy of the mechanical properties is evident in the various sections of the Buckinghorse Formation. The results were comparable at the 0° and 90° orientation while the 45° orientation showed anomalous failure patterns prompted by the slippage/shearing along bedding planes, and demonstrated that there is a dominant change in the failure mechanisms when the load is applied at an angle to the bedding planes. Hence, anisotropy could play a major role in the wellbore stability and fracture enhancement within unconventional reservoirs.

Introduction

Entire field development economics may be quickly eroded by increased expenditures associated with downhole drilling complications, reduced rates of penetration, or fishing operations at the extreme end of the spectrum. In a more mature state of the field development, known as "Brownfield development", many of these risks are assumed to have been experienced during earlier drilling operations, with the root cause analysis being understood and mitigation procedures in place to improve wellbore stability and capture penetration rate efficiencies. In many mature developments, field boundaries may be expanded based on refined geological models or seismic interpretation. This was the case of the Buckinghorse development in Northeastern British Columbia, as the targeted Montney Formation boundaries were pushed Eastward to explore the viability of a more economic oil prone window. Facies change and rock mechanics properties in "step out wells" of this nature introduce unforeseen or disregarded risks in the core development area, not only in the Buckinghorse Formation, but also in other basins if not properly assessed. As mud systems are designed to reduce rock/fluid interactions, lateral facies changes in some of shallower formations may respond differently to the prescribed drilling fluids defined in the fairway of the field. As facies change laterally both mechanical and chemical interactions with the rock/fluid-fluid/rock interactions may result in risks such as wellbore enlargement due to sloughing and caving. This has been noted in other basins resulting in an upward of 25% increase in total drilling time from spud to total depth, if encountered (Fontana et al. 2007). Wellbore caving can create a plethora of cost overrides via the initialisation of a cascade of drilling issues including loss of circulation, mechanical or differential stuck drill string, loss of the bottom hole assembly, or ultimately the loss of well control.

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