SIMPLIFIED EQUATIONS IMPROVE "LUBRICATION" FOR WELL CONTROL

Gary W. Nance, Randy Smith
Well Control Schools
Lafayette, La.

Several assumptions and two easy-to-use equations can improve "lubrication," a well control procedure used to displace gas with drilling fluid or completion fluid.

The equations are used to make a pressure schedule for monitoring the initial pressure, the pressure increase from pumping kill fluid, and the final pressure after bleeding for each lubrication cycle.

These equations are simple to use in the field. There is no need to measure tank volumes or to make rigorous calculations. This method of lubricating gas out of a well is more accurate than the standard lubricating procedure if the well is taking fluid. Comparing the actual and modeled pressures helps quickly spot common sources of error that complicate standard lubrication procedures.

There are two common lubrication procedures:

Pumping a kill fluid, waiting, and bleeding off pressure
Continued circulation across the top of the well (the dynamic lubrication procedure).

The pump, wait, and bleed method is most commonly used and may require numerous cycles to remove the gas completely. Both methods involve the coordinated use of a pump, a choke, and an accurate measuring tank.

CONVENTIONAL LUBRICATION

The pump, wait, and bleed lubrication procedure involves the following steps:

Pump fluid into a closed-in well (the surface pressure increases because of fluid compression).
Allow sufficient time for the fluid to lubricate (fall or slip through the column of gas).
Determine the hydrostatic pressure of the fluid volume lubricated for the particular cycle. The following formulas can be used to determine the hydrostatic pressure increase after each lubrication cycle: Hydrostatic increase = (hydrostatic head per barrel of lubricating fluid) x (volume lubricated per cycle). Hydrostatic head per barrel of lubricating fluid = (fluid -gradient)/(capacity). The capacity is the linear volume in bbl/ft of the annulus, tubing, or casing, depending on the location of the gas.
Bleed gas through the choke to reduce bottom-hole pressure by the increase from the compression of the gas and the hydrostatic increase from the lubricating fluid.

The use of this procedure can be tiresome and requires complicated calculations that sometimes prove difficult for field use. The bottom-hole pressure before and after each cycle should be nearly the same. Several sources of error, however, may contribute to differences in bottom hole pressures:

The volume measurements are inaccurate because of measuring tank design (excessive surface area), gas-cut fluids bled off, and insufficient time for fluid to lubricate through the gas.
The well bore is accepting fluids. For example, the fluid can enter open perforations in completed wells, or the fluid can be squeezed into weak zones or below the casing shoe in open well bores.
The calculations are field dependent.

NEW PROCEDURE

For a gas well, one of the first decisions is whether to bullhead or lubricate for the kill. Fig. 1 is a general schematic of a well type that was often bullheaded dead by the operator. These wells often required several tubing volumes for the kill operation. Because the bullheading operation was thought to damage the formation, a lubrication procedure was later used. The sources of error with the standard lubrication procedure made the process difficult, and excessive amounts of completion brine were still lost to the formation.

The lubrication procedure can be easier and more accurate to use if the following three pressures are properly accounted for during each lubrication cycle: the initial pressure prior to lubrication (P1), the compression pressure increase from pumping in the lubricating fluid (P2), and the final pressure reached after the gas is vented (P3).

Given the following assumptions, Equations 1 and 2 define the lubricating pressure relationships:

The product of the pressure times the volume of gas remains constant (P1V1 P2V2).
The lubricating fluid density, once the gas has been displaced, will balance (or overbalance) the formation pressure.
Only gas is bled from the well.

Equation 1 is for vertical or constant-drift directional wells, and Equation 2 is for deviated wells. The simplicity of Equation 1 makes it well-suited to most lubrication applications. Most often, lubrication is required to remove a vertical column of gas.

EXAMPLE PROBLEM

The following example well, however, requires the use of Equation 2; the well has an average inclination of 53 and a kick-off depth of 2,000 ft (Fig. 1). Equation 2 is used until the remaining gas is only in the vertical section of the tubing.

Table 1 is a lubrication schedule with about a 200-psi pressure increment for each lubrication cycle.

The pressure changes for open well bores may be chosen based on the ability of the well bore to sustain the pressure increase (shoe leak off, etc.) from compression. Pressure increments for well bores with open perforations may be chosen in the field (that is, the actual pressure increase sustained by the well bore).

Equation 2 predicts the lubrication pressures in Table 1 until 2,000 ft of gas remains or until the tubing pressure is reduced to 694 psi or less (694 psi = (0.447 psi/ft - 0.1 psi/ft) x 2,000 ft). With a fluid gradient of 0.447 psi/ft and a gas gradient of 0.1 psi/ft, the product of H x (1 - cos 0) x (fg - gg) remains constant at about 278 psi.

The sources of error can be identified by comparing the pressures predicted by the model to the actual pressures observed during the lubrication procedure:

Gas-cut lubricating fluid is bled off during the operation. As with the standard lubricating procedure, bleeding gas-cut fluid from the well creates difficulty in accounting for the hydrostatic pressure increase.
In addition to the gas, other fluids (oil or underbalanced drilling or completion fluid) enter the well bore and contribute to the underbalance.

If the pressure to be killed by lubrication is not entirely caused by the underbalance of the gas column, the model will predict lower bleeddown pressures. Using this pressure schedule, the well will become underbalanced. This error will be obvious because the bleed-down pressure (P3) will not remain at the predicted value. This pressure will increase because of the additional influx in the underbalanced well.

The migration of an additional gas influx below the column being lubricated is controlled using the standard volumetric procedure.

Although this method is not a definitive solution to all lubricating problems, the model should improve well control procedures for many field application