16

TIME, sec.

Figure 5-60b. Typical pipe velocities and accelerations measured as a joint of casing was lowered into well bore. Note: Maximum surge pressure, point b in Figure 5-60a, occurred at peak pipe velocity, point B in Figure 5-60b. (From Burkhardt.74 Copyright 1961 by SPE-AIME.)

strength as measured at the surface for t0 in his field tests. In practice, downhole gel strengths will vary quite considerably from this value, depending on the depth of the hole, the temperature gradient, and the thixotropic behavior of the mud. When the drill pipe is started back in the hole after a trip, the mud will have been at rest for several hours. Since the mud at the bottom of the hole will have been undisturbed for much longer than that near the surface, and further more, will have been subjected to higher temperatures, there will be a gradual increase in gel strength with depth.

Each time a stand is run in, the column of mud above the bit is disturbed for some fifteen to thirty seconds, but the gel partially rebuilds while the next stand is being made up. When a large number of stands have been run, the gel strength of the mud near the surface will have been reduced considerably, whereas the gel strength of the undisturbed mud at the bit will be that which has developed since the bit was pulled past the point on the way out of the hole. Thus, when the bit reaches bottom, the average gel strength of the mud column will be much greater than the gel strength of the mud coming out of the hole at the surface. Indeed, even after circulation is started, the average effective viscosity remains high until the mud from the bottom of the hole reaches the surface. For example, Figure 5-63 shows that the annular pressure loss, as measured by Carlton and Chenevert59 in field tests, remained above normal until returns from bottom reached the surface, marked by the rise in gel strength about 95 minutes after circulation started.

The maximum positive surge pressure to which a formation at a given depth will be subjected occurs when the bit reaches that depth. The value of r0 for use in Equation 5-64 will lie somewhere between the actual gel strength of the mud at that depth and the initial gel strength of the mud coming out of the hole. The undisturbed gel strength is best evaluated by heating a sample of the mud in a closed container for the appropriate time and at the appropriate temperature, and then determining the gel strength with a shearometer.

Swab pressures are lower than positive surge pressures, partly because pipe speeds are lower when pipe is being pulled, and partly because r0 is at a minimum when starting out of the hole. Nevertheless, field tests by Cannon77 showed that gel strength as measured by a bottom-hole pressure gauge was a more important factor than viscosity in determining swab pressures (see Figure 5-64).

Equation 5-65 shows that under turbulent flow conditions the mud properties will have little effect on surge pressures, which will depend mainly on pipe speed. Figure 5-62 indicates that with typical mud and well conditions, turbulent flow will commence at pipe speeds a little above 200 ft/min (60 m/min). This value for the critical pipe speed is supported by the results of field tests by Goins et ai,78 which showed a marked increase in positive surge pressures at pipe velocities above about 200 feet per minute, as shown in Figure 5-65.

Recently, Lai80,81 postulated a flow model for swab and surge pressures based on unsteady flow and a compressible fluid. A computer program enables the

The Importance of Hole Stability

The primary objective of the drilling engineer must be to maintain hole stability, because a gauge hole can be cleaned with a low viscosity mud, in which case progress will be rapid and problems will be few. If the hole enlarges, as it inevitably will in many formations, viscosity and gel strengths will have to be increased in order to clean the hole, but the higher viscosities and gel structures will decrease penetration rates and cause high swabbing and surge pressures, gas cutting, etc. The conflicting rheological requirements will be minimized by using a shear-thinning mud, which sets to a gel which is sufficient to suspend cuttings when circulation is stopped, but which breaks up quickly to a thin fluid when disturbed. Such a mud will have a high yield point/plastic viscosity ratio, and a iow flow-behavior index, n.

Techniques for controlling the rheological requirements in the field are beyond the scope of this chapter, but it may be said that high YP/PV ratios are best obtained by lowering the plastic viscosity rather than by increasing the yield point. As a general rule, therefore, maintain the lowest possible plastic viscosity by mechanical removal of drilled solids at the surface, and keep the yield point no higher than required to provide adequate carrying capacity. The yield point is controlled by adding or withholding thinners when drilling in colloidal clays, and by adding bentonite when drilling in other formations.

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