In an ideal borehole and mud system, cuttings would be transported to surface with the same order and composition as they were cut, as in Figure 1. In reality, it is commonly observed that, no matter how sharp a drilling break or clearly defined a formation top on an electric log, the formation top as logged from cuttings will appear to be “transitional.” (Note: not transitional in the sense that the cuttings show a varying composition, but that cuttings of a new type appear in increasing abundance as more hole is made.)
The reason for this is that, in transit in the annulus, cuttings tend to travel at different net velocities, resulting in a relative “slippage” and sorting. That such a process can take place is demonstrated at surface. Mass sorting, a function of cuttings density and size, is seen to take place at the shale shaker and in the sieve while washing. A similar process is also seen to occur in centrifuge or hydroclone separation, which may be defined as having a specific “cut” or minimum particle size to be separated.
A device designed to remove a particular “cut” of clay, silt or sand particles will in fact remove much smaller particles of denser materials such as barite. Thus a supposed size separation process is in fact a separation process controlled by particle mass and shape.
Function Solids Control Equipment
The complex of flow regimes through which cuttings pass between the formation and the flowline (including the various breaks in circulation for connections) results in a major sorting of cuttings. This will be affected by the relative densities of various cuttings and mud, the rheological properties of the mud and, since crosssectional area affects the interrelationship, the size and shape of the cuttings. The combination of these factors causes highly variable and not always predictable results.
Very little quantitative research has been performed, but.the following examples illustrate the effects of changes in mud properties and flowrate on the recovery of particles of similar shape and density but different sizes. These are illustrated in Figure 2.
Mud Density
(Refer to Figure 1-2) Comparison of mud (c) and mud (d) shows that increasing mud density increases the carrying capacity, thus delivering the maximum quantity of cuttings in the minimum time. (Mud circulation time for this test system is approximately four minutes.) With similar rheology, mud (d) recovers 75 percent of the total material in under two circulations, while mud (c) requires more than three to achieve a similar result. The reason for this is readily apparent. Decreased density of fluid relative to that of the carried particles decreases the weight of the particles in the fluid and therefore decreases the capacity of the fluid to both suspend and carry them.
VISCOSITY
Intuition suggests that increasing the viscosity would improve the mud’s capability for carrying cuttings. However, comparison of (a), which is water, with muds (b) and (c) shows that increasing viscosity, even when accompanied by increased fluid density, actually decreases the efficiency of cuttings recovery. Mud (f) with a viscosity of 30 centipoise shows a recovery rate of the same order as water, but this appears to be a result of its 12.4-lb/gal density since the recovery rate is less than mud (d) with its lower density and viscosity. The reason appears to be that, at higher viscosities, the fluid is more likely to be in laminar flow which is less efficient for cuttings transport.
When a solids particle is carried by a fluid up an annulus, the particle moves upward with the fluid, but the effect of gravity on the denser particle retards the upward movement.
As this difference becomes increasingly positive, particle recovery becomes more efficient. If it is negative, that is, if the downward slip velocity is greater than the upward mud velocity, particle recovery does not take place. However, not all of the fluid in the annulus is moving at the same velocity (see Figure 3). In turbulent now the nuid elements move in countless eddies, swirls or “turbs.” In laminar now the nuid elements follow the streamlines or “laminae.” The overall result is that the statistical vector average of all “turbs” at any point falls close to the average velocity of the nuid as a whole, while velocity distribution throughout the streamlines shows a much wider variation from the average.
The behavior of carried particles under the two flow types is also very different. Under turbulent now the particles are carried in an even manner and, although disturbed by the turbulence, they tend to maintain the maximum surface area perpendicular to the mean direction of flow. As would be expected, mass sorting takes place, resulting in recovery in the following order: small, medium, large size particles (see Figure 2, a and b).
Under conditions of laminar now the behavior of suspended particles varies according to their dimensions. In the case of the aluminum disks, the large disks moved up the annulus in a manner similar to turbulent now. The medium and small disks were turned on edge and moved to the outer wall (and to the pipe when it was not rotated) and slipped down before beginning to rise again.
Again, due to their greater mass, the medium disks tended to slip back further than the small. In some cases these disks were held against the borehole wall and not t’ecovered, without increasing annular velocity. The result is recovery in the order large, small and medium sized particles, with notably poorer (less than 50 percent) total recovery of medium sized particles (see Figure 2, e).
Due to the overall mass sorting effect in the annulus, in denser muds (Figure 2, d and f) small and large particles were recovered at approximately the same rate, with medium sized particles less efficiently recovered – the turning of the particle being the most important factor but particle mass being a secondary innuence.
