Fluid movement
Mud flow forced out of the well
Pipe
Fluid movement
Mud flow forced out of the well
Pipe
Fig, 18-12 Annular fluid flow resulting from pipe movement wellbore pressure below formation pressure. Surge pressures increase the total weIIbore pressure and can cause formation fracturing and lost circulation.
Computing swab and surge pressures is difficult because of the manner in which the fluid flows as pipe is moved in the well. As shown in Fig. 18-12, pipe moving down the annulus causes the mud adjacent to the pipe to be dragged downward. Since the annulus is a fixed volume, however, and the mud is considered incompressible, some mud must flow out of the annulus. The mechanics are different than pumping since the fluid flow is considered to be only one direction.
Burkhardt conducted iield studies to evaluate surge and swab pressures. His work involved running casing into a well that was equipped with pressure sensors. A typical pressure response to the pipe movement is shown on Fig. 18-13. Positive and negative pressures are shown for the cases of pipe being lifted off the slips and later being decelerated.
The complex fluid flow model shown in Fig. 18-10 was evaluated by Burkhardt. Using involved mathematics, Burkhardt developed a relationship between pipe and hole geometries and the effect of the dragging mud adjacent to the pipe. The term clinging constant represents this relationship (Fig. 18-14).
The mud velocity in the annulus must be computed before the clinging constant can be applied. The flow rate of steel for a closed drillstring into the well is given by Eq. 18.56:
Where:
Q = flow rate, gal/min Vp — pipe velocity, ft/sec
The velocity in the annulus is the quotient of flow rate and area:
TT dp2
If the pipe is open-ended, the flow velocity is solved in a similar manner:
Fig. 18-13 Typical pressure-surge pattern measured as a joint of casing was lowered into the wellbore
Applying the clinging constant, k, the effective annular velocity (VJ is as follows:
Burkhardt noted that k = 0.45 was a good assumption for most typical geometries.
In operation, a pipe velocity is used to compute the How of pipe into the well, which is assumed to be equal to flow out of the well. The clinging constant is applied to yield an effective velocity based on the complex flow patterns in
Dp/Dh = Ratio of pipe diameter to hole diameter
Fig. 18-14 Mud clinging constant, K, relative to annulus geometries the annulus. The surge or swab pressures are computed by substituting the effective velocity into any of the previously defined friction pressure equations. It is reasonable to use laminar How equations since normal pipe velocities seldom cause greater than critical velocities.
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