Paul Sonnemann
Consultant
The Woodlands, Tex.
The circulate and weight method, a combination of the two standard well control procedures, can kill a well as quickly as the wait and weight method yet with the practical advantages of the driller's method.
The use of one well control method, rather than a choice of two, call simplify the procedures and decisions made by a field supervisor under stressful well control conditions.
The driller's method and the wait and weight method, as described in American Petroleum Institute RP59, are firmly established procedures for well control and kill operations in the oil field. These methods essentially accomplish the same results: The existing well fluid with a as or liquid influx is replaced with a higher density fluid while the bottom hole pressure is kept relatively constant.
Although both methods have similar procedures and follow similar principles, they have sufficiently different advantages and disadvantages to warrant inclusion of both methods in most well control training manuals and classes.
The use of hand held calculators and portable computers on rigs simplifies the calculations required for the wait and weight kill mud displacement schedule; thus, many, companies have revised the steps in the driller's method to include some of the steps from the wait-and weight method. In this revised method, the first steps of the second circulation of the driller's method use the wait and weight drill pipe pressure schedule while the drillstring is displaced with the kill mud. This substitution results in several significant, practical benefits, but these benefits have not been widely recognized.
CIRCULATE AND WEIGHT
This modified driller's method may therefore be reasonably labeled the circulate and weight method. (Note that it is not the concurrent method, which used the same name in the past.)
A review of the derivation of the circulate and weight method from the two standard well control methods reveals that this method is not a radical or questionable concept. The three accompanying boxes list the basic steps in the driller's method, the wait and weight method, and the circulate and-weight method. Note that the only new terms added in the circulate and weight method are Step 5 and the terms in parentheses in Steps 3 and 4.
Steps 1 3 from the driller's method are used as Steps 1 3 in the circulate and weight method. Steps 1, 5, and 6 from the wait and weight method are used as Steps 4, 6, and 7, respectively, in the circulate and weight method.
A major disadvantage of the conventional driller's method is that Step 7, the displacement of the drillstring with the kill mud, cannot be undertaken unless Step 3 is fully completed. All of the gas must be removed from the annulus prior to using the surface casing (choke) pressure as a longterm indicator of bottom hole pressure.
The most reliable way to ensure no influx remains in the annulus is to displace the annulus from bottom to top with clean mud. Thus, the driller's method is time consuming because it is essentially a two circulation method.
The circulate and weight method avoids this problem by using the wait and-weight drill pipe pressure kill weight mud displacement schedule for monitoring bottom hole pressure while the drillstring is displaced (Step 6). This procedure works even if the annulus still contains part of the gas influx.
The decision as to when to switch from pumping original weight mud to pumping kill weight mud is based on factors such as mud availability, estimated position of the influx with respect to the last casing seat, and estimated time of arrival of gas influx at the choke. Generally, the kill weight mud should be used as early as possible, but in no case should it be used until the initial circulating pressure has been satisfactorily established and the pump/choke operation is going smoothly.
The circulate and weight method is thus a combined method:
- Its duration is variable, requiring no more time, and in many applications relatively few additional pump strokes, than the wait and-weight method.
- It uses well known principles accepted in the field.
- It permits circulation before, and possibly during, mud pit weight up activities.
- It can virtually duplicate the features of the driller's method and the wait and-weight method, depending on the control of one simple variable the time at which kill mud is introduced into the drill pipe.
Many companies use this method (sometimes called the modified driller's method) in place of the conventional driller's method. (In some cases, it has been suggested to begin a kill using the driller's method and then switch to the wait and-weight method once the kill mud is available.)
The ability to reduce the first circulation period to as little as the time required to establish the initial circulating pressure suggests that this method could replace both conventional methods (and could be accurately called the modified wait-and weight method).
ADVANTAGES
The circulate and weight method has several advantages similar to those of the drillers' method. These methods:
- Reduce the likelihood of stuck pipe or a plugged circulation path while the well is shut in.
- Eliminate the need to use volumetric methods in the event of gas migration during shut in.
- Permit the drillstring to be filled with fluid with a known density before the kill mud weight is calculated.
- Increase the likelihood of correctly establishing the initial circulating pressure (because no recalculated theoretical values are available instead).
- Permit worker concentration to be focused on the practical aspects of circulation through the choke, well response, and influx disposal rather than on theoretical pressures or calculations.
- Simplify pump start up by limiting variables such as changing mud weights, drill pipe pressures, and pump strokes.
The circulate and weight method, unlike the driller's method, also minimizes the chance of procedural error because it does not require different uses for some of the pressure gauges at different times. The circulate-and weight method and the wait and weight method have several common advantages. These methods:
- Can usually kill a well in minimal time if the kill mud is selected as soon as it becomes available.
- Minimize casing shoe pressures (unless the gas influx reaches the shoe before a significant volume of kill weight mud can be pumped through the bit).
- Minimize surface casing pressures (unless the gas influx reaches the surface before a significant volume of kill weight mud can be pumped through the bit).
The circulate and weight method, unlike the wait-and weight method, can also help eliminate errors relating to incorrect slow pump rate pressures, a lack of shut in drill pipe pressure because of a float in the drillstring, incorrect shut in drill pipe pressure because of an influx in the drillstring, incorrect shut in drill pipe pressure because of a low permeability formation, a failure to observe a second kick because of acceptance of the "gas expansion" explanation, and poorly executed pump start and stop procedures.
DISADVANTAGES
The circulate and weight method has several disadvantages:
- It requires calculations equivalent to those in the wait and weight method.
- It requires the ability to circulate through the choke with original weight mud while the drill pipe pressure schedule is being prepared and the kill weight mud is being mixed.
- It has not been clearly defined in the oil industry.
