Drilling long horizontal wells is common in development of unconventional reservoirs. Effective cuttings transportation for better hole cleaning during drilling operations can increase the rate of penetration (ROP) and mitigate various drilling associated problems such as high drag and torque and pipe sticking. A large-scale Slurry Loop Unit (SLU) was used in this study for simulation purposes. The objective of this study was to investigate the cuttings size, density, and fluid properties; coupled with wellbore deviation and circulation rate on hole cleaning efficiency. The analytical models used to predict critical velocities for lifting and rolling the cuttings particles were based on the equilibrium cuttings bed height model and forces acting on a cuttings bed. The analytical model results could predict, with some degree of accuracy, the effective injection rate to clean the annulus. Also, experimental results showed that at angles higher than the repose angle of the sand, only rolling and lifting mechanisms ensure the bed movement and effective hole cleaning. Similarly, at the range of 0° to 60° inclination, the only major forces acting on the cuttings is gravity which can be overwhelmed by increasing the fluid carrying capacity and/or flow rate.
One of the main functions of the drilling fluid is the efficient removal of the cuttings from the bottom hole to the surface. Poor hole cleaning results in the deposition of drill cuttings in different wellbore locations, possibly leading to several drawbacks in the drilling operation and well completion, such as stuck pipe, high torque and drag, and faulty cementing jobs.
Pigott (1942) pioneered the extensive study of hole cleaning in vertical and near-vertical wellbore geometries, which was followed by several other studies discussed later in this section with a focus on the particle's settling velocity of the cuttings as a major factor influencing the hole cleaning. The settling velocity is dependent upon cuttings density, size, and shape, as well as fluid rheology and flow rate. Chien (1993) has introduced a correlation between the settling velocity and irregularly shaped cuttings by introducing a factor to account for the non-sphericity and apply it to a fictive spherical particle with an equivalent volume. His findings can expand the work from spherical to non-spherical particles.