Some of the control methods to follow are based on the same constant bottomhole approach as the Driller’s method and the Wait and Weight method, and some are not.
Every well control operation is different, and has specific issues to be dealt with, and there is not always just one correct way of solving a problem. Some procedures are better suited for certain operations than others, and a careful evaluation is necessary to make the right choice.
4.1.1 Reverse Circulation Method
As implied by the name, reverse circulation is opposite of the conventional circulation down the drillpipe with return up the annulus. The mud is pumped down the annulus on the casing side, and reversed up the string.29 This type of circulation is most commonly used in completion and workover operations, as opposed to the drilling phase.3, 25 In forward circulation, the friction pressure loss of the annulus is very small, and often ignored in calculating the effects on the bottomhole pressure. However, when the circulation is turned around, the effects of the friction pressure loss in the
drillpipe/tubing is experienced at the bottomhole. Since the restriction of tubing causes higher pressure losses, these could have a big effect on the bottomhole pressure.25 This needs to be taken into consideration so that the formation is not fractured due to excessive pressures. This is one of the main disadvantages of the procedure, and often makes it unsuitable for use in the drilling phase. There is also a greater chance of plugging the bit, or getting the pipe stuck.3, 13
In a producing well, the reverse circulation method can be advantageous in that the kick would be contained in the drillpipe/tubing, which would protect the rest of the wellbore form the increasing pressures, and gas kicks can be removed relatively quickly.3
4.1.2 Dynamic Kill Technique
This type of procedure uses friction pressures to its advantage. The principle is to pump fluids at higher rates, so that the high annular friction pressures increase the ECD, hence bringing the well back to a balanced situation. Most often this technique is used on a well that is already blowing out, but it may also be applied to gas kicks in shallow formations.8, 25 The fear of causing broaching to the surface when dealing with a shallow gas kick is eminent, and dynamic kills may be useful when dealing with these kicks.
4.1.3 Bullheading
The two previous methods discussed are examples of constant bottomhole pressure methods.25 With bullheading, however, the well fluids are pumped back into the wellbore with the intent of fracturing a formation for the kick to flow into.3, 8, 25, 29 This procedure is sometimes called deadheading, and is based on pumping the fluids back down into the well with enough force to reverse the flow, and prevent the kick from reaching the surface. The technique is most often used in cased holes, and it is very simple to perform. Although it is not recommended for drilling operations, as it might actually cause greater harm than good, there are instances where it might be useful to bullhead in the drilling phase.3, 8, 29 One such situation might be in the instance of an H2S kick. This gas is very poisonous and lethal if inhaled at high enough concentrations.
Other reasons for using bullheading might be if circulation is not possible, or if conventional methods would cause pressures which the well was unable to tolerate.3
4.1.4 Volumetric Method
The Volumetric method is a way of handling pressures, if killing the kick is not possible right away. If circulation is not possible for any reason, be it rig power failure or a plugged drillstring, the Volumetric method offers a means to let the gas kick expand on its way up the well, thus preventing surface pressure to increase to undesired levels.25, 29 As the method does not involve circulation, the driving method for the gas migration is buoyancy rather than pump pressures.3
Fig. 4.1 shows an example of the Volumetric method: As gas migrates up the well, the casing pressure starts increasing. It is allowed to do so until it reaches a predetermined limit called the safety margin. Then an additional working margin is allowed, before the choke operator bleeds off a pre-calculated volume of mud from the well. This reduces the casing pressure to the safety margin again, and the process is repeated until the first
gas is noticed at the surface. This method controls the hydrostatic pressure (HSP) in intervals, as shown in Fig. 4.2, and helps constrain surface pressures. Casing pressure is monitored after the kick has reached the surface, and a way to kill the well is sought next.
Fig 4.1 – Casing pressure response during Volumetric method.14
Fig. 4.2 – Bottomhole pressure response during Volumetric method.14
4.1.5 Lubricate and Bleed Method
As a follow-up to the Volumetric method, the Lubricate and Bleed method is often used.
It is a procedure for handling the free gas that has reached the surface. The shut-in gas is to be replaced by mud in a safe way, and Lubrication is one way to accomplish this.3 The principle is to pump a predetermined volume of kill fluid into the well, and let it fall through the free gas pocket. A calculated volume of gas is then bled from the well, and the process is repeated until the previously pumped mud starts exiting the well.25 By replacing the gas with weighted mud, the well is brought back under hydrostatic control.
As indicated above, this method is useful when gas has migrated to the surface of a shut-in well, as after the Volumetric method has been employed, or otherwise. Also, when surface pressures are getting close to the limits of the wellhead equipment, it is sometimes used “to lower the surface pressure to allow bullheading”.25
4.1.6 Staging the Hole
An example of a method that is most commonly used when the pipe is off-bottom, is staging the hole. This method aims to place a calculated volume of heavy mud on top of the original mud, thus creating added hydrostatic pressure in a well where the pipe is above the kick. As the pipe is run in the hole, however, the original mud tends to be displaced, and the heavy mud must compensate for this in order to still maintain pressure balance in the well. The process is repeated, with heavy mud added, and pipe run in the hole, until the pipe is back at the depth of the kick.3, 25, 29
This practice is not often recommended as many complications may occur.29 One risk is that of the kick migrating and displacing fluids. This would complicate the process severely. If any indication of kick migration is detected, staging in the hole should not be considered.3