90

4. High mud densities enhance cuttings transport.

5. Drill string rotation helps cutting transport because it knocks cuttings from the layer back into the mud stream.

The minimum rising velocities reported by Martin et al are generally less than those deduced from data in the literature.

Release of Cuttings and Entrained Gas at the Surface

Structural viscosity impedes the separation of drilled solids and entrained gas at the surface. In a quiescent fluid, particles will not fall nor gas bubbles rise, unless the stress created by the difference in density between the particles or bubbles and the fluid is greater than the gel strength of the mud. The high shear rates prevailing in mechanical separators and degassers promotes the release of solids and gas by reducing structural viscosity.

Transient Borehole Pressures

So far, we have discussed pressure arising from steady state flow, but various transient pressures which must be minimized occur during the normal drilling cycle. These transient pressures affect the safety of the well. Figure 5-59 depicts typical transient pressures in terms of equivalent mud density of a mud whose hydrostatic density is 11.8 lb/gal (specific gravity 1.41).76 A positive pressure surge occurs each time a stand of drill pipe is run in the hole, because the pipe acts like a loose-fitting piston, forcing mud out of the hole. When the bit reaches bottom, the pressure required to break circulation causes another surge. The greatest surge in the drilling cycle occurs when reaming down rapidly with the pump on, prior to making a connection. Finally, negative surge pressures (or swab pressures as they are usually called) occur when pulling pipe, because of a swabbing effect. It is not necessary for fluid to be physically swabbed out of the annulus, or carried out inside the drill pipe, but the pressure reduction is greater when such actions occur.

In normal or moderately geopressured formations, such as those shown in Figure 5-59, transient pressures are not hazardous; but, in highly geopressured formations, pore pressures are high, allowing only a small operating margin between the mud density required to control formation fluids and that which would fracture the formation (see Chapter 9). Under this condition, swab pressures may be sufficient to cause blow-outs, and positive surges may cause induced fracturing with consequent loss of circulation. Correlations of swab pressures with actual blowouts, and surge pressures with loss of circulation, have been firmly established by studies of case histories.77 78

Burkhardt79 identified three factors contributing to pressure surges: the pressure required to break the gel; the acceleration or deceleration of the mud; and the viscous drag of the mud. In some field tests, he measured pipe velocities and corresponding subsurface pressures. His results (Figure 5-60) showed that the maximum positive surge pressure occurred at the maximum pipe velocity and, therefore, that viscous drag caused the peak surge pressures.

Burkhardt derived equations relating surge pressures to pipe velocity, based on the premise that the difference in pressure caused by moving a pipe through a stationary liquid is the same as that caused by flowing the liquid through a stationary pipe at the same velocity. Equations for pressure surges in a drilling well are complicated by the fact that both the pipe and the liquid move. Thus, the mud has one velocity with respect to the moving pipe, and another with respect to the

o TIME —^

Figure 5-60a. Typical pressure surge pattern measured as a joint of casing was lowered into well bore.

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