Carrying Capacity

One important component of a good solids management philosophy is the drilling fluid carrying capacity. As drilled solids (sloughings and drill bit-generated) enter the wellbore, they should be brought to the surface as soon as possible. If these drilled solids are tumbled and slowly brought to the surface, they have time to disintegrate and increase the low-gravity solids content of a drilling fluid.

Suprisingly, drilled solids also affect the drilling fluid properties that control carrying capacity in vertical and nearly vertical (up to 35°) wells. An empirical relationship has been published relating a carrying capacity index (CCI) to the product of the mud weight (MW), annular velocity (AV), and a characteristic viscosity (K). This can be expressed as:

The "400,000" constant was empirically determined by observing hole cleaning conditions on many rigs over an 8- to 10-year period. A CCI value of "one" seems to indicate good hole cleaning in both water-based and oil-based drilling fluids. The constant is probably not accurate to more than one significant figure.

The "K" viscosity is the viscosity from the Power Law Rheological Model, expressed as equivalent centipoise. It can be related to the Plastic Viscosity (PV) and Yield Point (YP) through the equations:

At a constant yield point value, increasing the plastic viscosity decreases the value of the viscosity. This is clear from the curves in Figure 1-1.

If the cuttings on the shaker screen indicate that they are not arriving at the surface without considerable tumbling, the CCI equation can be used to calculate the yield point needed to clean the


10 20 Yield Point: lb/100 sq.ft.

FIGURE 1-1. K-viscosity.

wellbore. Because the mud weight and annular velocity will not usually be changed, the K-value needed can be calculated. Generally, the plastic viscosity will not change significantly when the yield point is increased. Consequently, the curve in Figure 1-1 (or a similar curve) can be used to determine the yield point needed to clean the hole.

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