Driller’s Method vs Wait and Weight Method

Driller’s Method vs Wait and Weight Method: One offers distinct well

control advantages

Posted on 29 October 2009

By Rana S Roy, Conroy James Nini, Paul Sonnemann, Berton Thomas Gillis, Chevron

The two widely used constant bottomhole circulating methods are the Driller’s Method

and the Wait and Weight (W&W) Method. Well control experts are often strongly

opinionated on selecting the better method to circulate an influx out of the wellbore. The

purpose of this article is to highlight the major advantages and disadvantages of the two

methods.

The basic principle of both methods is to keep bottomhole pressure (BHP) constant at or,

preferably, slightly above the formation pressure.

The Driller’s Method requires two circulations. During the first circulation, the influx is

circulated out with the original mud weight. Constant BHP is maintained by holding

circulating drill pipe pressure constant through the first circulation. If the original mud

weight is insufficient to balance the formation pressure, the well is killed by circulating a

heavier mud (kill mud) in a second circulation.

To hold constant BHP during the second circulation, one of two procedures is employed.

Casing pressure is held constant while pumping kill mud from surface to bit, and drill

pipe pressure is held constant thereafter until kill mud is observed returning to the surface.

Alternately, during second circulation, a drill pipe pressure schedule can be calculated

and followed while pumping kill mud from surface to bit, and drill pipe pressure is held

constant thereafter.

The W&W Method involves only one circulation. The influx is circulated out, and the

kill mud is pumped in one circulation. While pumping kill mud from surface to bit, a drill

pipe pressure schedule has to be calculated and followed. The drill pipe pressure is held

constant thereafter until kill mud is observed returning to the surface.

The W&W Method is sometimes called the Engineer’s Method because it involves more

calculations compared with the Driller’s Method. There is a widespread misconception

that the Driller’s Method is preferred only because it is simple

. We will discuss various

reasons why the Driller’s Method could be better for circulating an influx in many or

even most wells drilled.

Any drilling organization or company can adopt a policy of recommending just one well

control method so that everybody in the organization can be competent in at least one

method. This may help in avoiding confusion in the field and promote understanding of

how to efficiently circulate a kick out of the wellbore without creating major well control

problems.

There is a shortage of experienced personnel in the drilling industry, and ensuring

competency in one method could lead to fewer disasters. We have to keep in mind,

however, that even experienced personnel do not routinely kill wells. They may kill only

a few wells in their entire career. We can achieve operational excellence by ensuring their

competence in one method. It may be better to use a good method expertly than a slightly

better method inexpertly.

COMPARISON

We will compare the advantages and disadvantages of the two methods under specific

conditions. The different applications are various types of wells and their geometry.

Deviated hole / tapered drill string:

The drill pipe pressure schedule for the W&W

Method is fairly simple to calculate if the wellbore is vertical and there is one size of drill

pipe. The schedule becomes complicated and difficult for rig personnel in complex

deviated well geometries and/or with multiple sizes of drill pipe. If the proper drill pipe

pressure schedule is not calculated while performing the W&W Method, BHP pressure

may not be held constant.

Figure 1 shows two drill pipe pressure schedules for a horizontal well. The first schedule

does not compensate for the hole deviation while the second one does. As per this

example, if we do not compensate for hole deviation, we will have approximately a 200-

psi overbalance when the kill mud gets to the end of build inside the drill string.

This overbalance of 200 psi could be detrimental to weak formations and could increase

shoe and surface pressures. One often-mentioned advantages of the W&W Method is

lower pressure at the casing shoe. But if a proper schedule is not calculated for the W&W

Method, we may expose the casing shoe or weak formations to higher pressures

compared with the Driller’s

Method.

Hole problems:

Many wells are drilled in areas with significant hole instability problems.

If the drill string is kept static with no mud circulation, the drill string may get stuck in

the hole due to pack-off problems. If it is decided to kill the well with the W&W Method,

kill mud may have to be mixed before circulation can be established. This long period of

non-circulation with little or no pipe movement may not be desirable in problematic hole

sections.

The Driller’s Method has some obvious adv

antages under these circumstances.

Circulation can be started as soon as a stabilized shut-in casing pressure (SICP) and shut-

in drill pipe pressure (SIDPP) are established. The first circulation of the Driller’s

Method is done with the original mud in the hole. If the method is understood and

followed correctly, non- circulating time in the well is minimized, and any further hole

problems may be minimized.

Fluid mixing capability of rigs:

While we are building new rigs and modifying existing

ones to drill wells more efficiently, a vast majority of wells are still drilled all over the

world using older rigs with limited capabilities. Kill- weight mud may not be quickly

prepared and/or pumped at a desired rate if the W&W Method is employed. The Driller’s

Method may be preferred under these circumstances to avoid excessive increase in

surface and shoe pressures due to gas migration.

We acknowledge that on some rigs, kill mud can be mixed at a fast rate without problem.

But simultaneous mixing and pumping of kill mud may make pit volume gain and loss

difficult to track and lead to confusion, particularly in the event of complications.

Drilling in formations with ballooning potential:

Ballooning is a phenomenon

occasionally encountered in some formations. Ballooning can be defined as flowback

from the well after shutting the pumps off, which is preceded by losses while the pumps

were running. Losses in the well can be attributed to extra BHP due to equivalent

circulation density (ECD).

After the pumps are shut down, the ECD does not exist anymore, resulting in a drop in

BHP, and mud is forced back into the wellbore. It appears the well is flowing and is

referred to as ballooning.

Ballooning is often misinterpreted as a kick. If it is decided to kill the well with the

W&W Method, mud weight may be increased due to incorrect measurement of formation

pressure. Due to the additional mud weight, BHP increases even further. This can induce

more losses and worsen the ballooning problem.

For the above reasons, it is commonly recommended that the Driller’s Method be

followed in ballooning formations. Since the Driller’s Method does not require any

increase in mud weight during the first circulation, no additional BHP is exerted on the

formation. After the first circulation of the Driller’s Method, the situation can be assessed

and further course of action can be decided (i.e., drilling ahead with no mud weight

increase if ballooning continues).

Complications and friction changes during well control:

While a well is being killed,

complications may occur during the process. When killing a well with the W&W Method,

if one or more of the bit nozzles plug while drill pipe pressure schedule is followed, the

pressure schedule must be recalculated immediately.

The failure to notice the change and to recalculate the proper drill pipe pressure schedule

may result in underbalance. On-the-spot recalculation of the drill pipe pressure schedule

may be difficult for highly deviated wells and/or with tapered drill strings. Furthermore,

when a kick is taken, it is normal that the rig crew become nervous. If any complications

arise while killing the well, rig personnel may panic and make poor decisions.

If th

e kick is circulated with the Driller’s Method and one or more of the bit nozzles plug,

the response by the choke operator is fairly simple. The circulating drill pipe pressure