Efficient bottomhole cleaning has been shown to significantly contribute to the increase in ROP. A detailed investigation into cutting dislodgment and movement may give insight on how to potentially increase the bottomhole cleaning ability of a well. This cleaning ability can be predicted by the use of a Scouring Model.
The hydrodynamic (scouring) model consists of a progression of mathematical relationships relating the drilling fluid hydrodynamic forces to a well cutting resting on a well bottom. These relationships are made assuming ideal conditions in order to decrease the complexity of the fluid hydrodynamics of the well bottom. The forces considered that act upon a well cutting are the hydrodynamic drag lift forces, and the submerged weight of the cutting. These forces can be related to the angle of repose which is dependent upon the cutting type (Sandstone. Shale. etc.). These forces can be described as functions of the cutting size and density, fluid density. and fluid velocity. The relationship between the hydrodynamic forces and the angle of repose will thus give an equation predicting the critical incipient velocity. This critical velocity is the velocity required by the fluid to initiate movement of the well cutting and thus inherently predicts the cleaning ability of the well bottom by the drilling fluid.
The removal of cuttings from the bottom of a well hole is similar to the removal of sediment from stream beds. The movement of sediment lin alluvial streams is such a complex problem that it may never be completely subject to rational solution. It represents, in fact. the most extreme degree of unsteady, non-uniform flow, since the stream bed as well as the water's surface may be continuously changing in form. With the current information an approximate understanding of the general transport mechanism can be obtained only by isolating particular details or by so simplifying the boundary conditions that only the most significant variables need to be considering.
The scouring criteria can be analyze2 as follows: Consider a plain) stationary bed of cuttings consisting of loose particles of uniform size with liquid flowing over it. As soon as the liquid starts flowing, hydrodynamic forces are exerted Upon the solid particles. A further increase in the flow intensity causes an increase in the magnitude of these forces. Hence, for a particular stationary bed, a condition is eventually reached at which particles In the movable bed are unable to resist the hydrodynamic forces and, thus, begin to be dislodged and eventually to start to move. This movement is not an instantaneous one for all particles of a given size resting in the top layer. In fact, at any given hydrodynamic condition, some move and some do not. This is due to the statistical nature of the problem which implicitly brings out the fact that the flow regime is usually turbulent. The condition if the "initial movement of the bed" is determined by observation; therefore, its definition is a very subjective one.