Cuttings bed creation and removal when circulating field applied oil-based drilling fluids

Deviated well sections are common in modern well construction. In mature areas like the North Sea region, practically all producers or injector wells will have highly deviated sections. These wells must be drilled and completed in an optimal manner with respect to drill time, cost, risk and functionality. Most cuttings transport and hydraulic models are developed based on tests with model fluids and often in small diameter test sections. Hole cleaning and hydraulics with field fluids are different than the properties considered with most model fluids. Furthermore, results from small diameter tests with model fluids may not always be relevant for, nor scalable to, field applications due to time, length and other scales. Hence, there is a need for studies in controlled laboratory environments with various field application designed drilling fluids to improve engineering models and practices. In this paper, results from laboratory studies with geometries that permits comparison with field operations when oil-based drilling fluids from Norwegian offshore operations are applied are presented. The oil-based drilling fluids have densities and viscosities within the relevant shear rate range applied during drilling operations and in the tests. Previously published results with realistic fluids from this setup are with steady state conditions. Here characteristic transient behavior is presented and discussed. The experiments have been performed in a flow loop that consists of a 10 meters long test section with 2" OD freely rotating steel drill string inside a 4" ID wellbore made of cement. Sand particles were injected while circulating the drilling fluids through the test section. Experiments were performed at different wellbore inclinations. The applied flow loop dimensions are designed so that the results are scalable to field applications; especially for the 12,25" and 8,5" sections. The selected setup is designed to provide correct shear rate ranges and equal Reynolds number to the field application when the same fluids are applied. Results show that bed creation can be described as an alpha wave together with a bed thickness growth from start of drilling a highly deviated well section. The bed removal characteristics in presence of increased flow rate and/or increased drill string rotation are described.