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Well control and blowout prevention have become particularly important topics in the hydrocarbon production industry for many reasons. Among these reasons are higher drilling costs, waste of natural resources, and the possible loss of human life when kicks and blowouts occur.
Well-control means methods used to minimize the potential for the well to flow or kick and to maintain control of the well in the event of flow or a kick. Well-control applies to drilling, well-completion, well-workover, abandonment, and well-servicing operations. It includes measures, practices, procedures and equipment, such as fluid flow monitoring, to ensure safe and environmentally protective drilling, completion, abandonment, and workover operations as well as the installation, repair, maintenance, and operation of surface and subsea well-control equipment.
One concern is the increasing number of governmental regulations and restrictions placed on the hydrocarbon industry, partially as a result of recent, much-publicized well-control incidents. For these and other reasons, it is important that drilling personnel understand well-control principles and the procedures to follow to properly control potential blowouts.
The key elements that can be used to control kicks and prevent blowouts are based on the work of a blowout specialist and are briefly presented below:
- Quickly shut in the well.
- When in doubt, shut down and get help. Kicks occur as frequently while drilling as they do while tripping out of the hole. Many small kicks turn into big blowouts because of improper handling.
- Act cautiously to avoid mistakes—take your time to get it right the first time. You may not have another opportunity to do it correctly.
Many well-control procedures have been developed over the years. Some have used systematic approaches, while others are based on logical, but perhaps unsound, principles. The systematic approaches will be presented here.
With the constant-bottomhole-pressure concept, the total pressures (e.g., mud hydrostatic pressure and casing pressure) at the hole bottom are maintained at a value slightly greater than the formation pressures to prevent further influxes of formation fluids into the wellbore. And, because the pressure is only slightly greater than the formation pressure, the possibility of inducing a fracture and an underground blowout is minimized. This concept can be implemented in three ways:
- One-Circulation, or Wait-and-Weight, Method. After the kick is shut in, weight the mud to kill density and then pump out the kick fluid in one circulation using the kill mud. (Another name often applied to this method is “the engineer’s method.”)
- Two-Circulation, or Driller’s, Method. After the kick is shut in, the kick fluid is pumped out of the hole before the mud density is increased.
- Concurrent Method. Pumping begins immediately after the kick is shut in and pressures are recorded. The mud density is increased as rapidly as possible while pumping the kick fluid out of the well.
If applied properly, each method achieves constant pressure at the hole bottom and will not allow additional influx into the well. Procedural and theoretical differences make one procedure more desirable than the others.
Fig. 1 depicts the one-circulation method. At Point 1, the shut-in drillpipe pressure is used to calculate the kill-weight mud. The mud weight is increased to kill density in the suction pit. As the kill mud is pumped down the drillpipe, the static drillpipe pressure is controlled to decrease linearly until at Point 2, the drillpipe pressure is zero. The heavy mud has killed the drillpipe pressure. Point 3 shows that the initial pumping pressure on the drillpipe is the total of psidp plus the kill-rate pressure. While pumping kill mud down the pipe, the circulating pressure decreases until, at Point 4, only the pumping pressure remains. From the time kill mud is at the bit until it reaches the flow line, the choke is used to control the drillpipe pressure at the final circulating pressure. The driller ensures the pump remains at the kill speed.
In the two-circulation method, the circulation is started immediately. Kill mud is not added in the first circulation. As seen in Fig. 2, the drillpipe pressure will not decrease during the first circulation. The purpose is to remove the kick fluid from the annulus.
In the second circulation, the mud weight increases, but causes a decrease from the initial pumping pressure at Point 1, to the final circulating pressure at Point 2. This pressure is held constant while the annulus is displaced with kill mud.
This method is the most difficult to execute properly (see Fig. 3). As soon as the kick is shut in and the pressures are read, pumping immediately begins. The mud density is increased as rapidly as rig facilities will allow. The difficulty is determining the mud density being circulated and its relative position in the drillpipe. Because this position determines the drillpipe pressures, the rate of pressure decrease may not be as consistent as in the other two methods. As a new density arrives at the bit, or a predetermined depth, the drillpipe pressure is decreased by an amount equal to the hydrostatic pressure of the new mud-weight increment. When the drillpipe is displaced with kill mud, the pumping pressure is maintained constant until kill mud reaches the flow line.
Choosing the best method for well control
Determining the best well-control method for most situations involves several considerations including the time required to execute the kill procedure, the surface pressures from the kick, the complexity relative to the ease of implementation, and the downhole stresses applied to the formation during the kick-killing process. All points must be analyzed before a procedure can be selected. The following list briefly summarizes the general opinion in the industry regarding these methods:
- The one-circulation method should be used in most cases.
- The two-circulation method should be used if a good casing shoe exists and there is going to be a delay in weighting up the system.
The concurrent method should be used only in rare cases, such as for a severe (1.5 lbm/gal or greater) kick with a large influx and a potential problem with developing lost circulation. In this case, the pump rate should be kept to a minimum to allow the weight to be raised continuously. In an analysis of kick-killing procedures, emphasis is placed on the one- and two-circulation methods (i.e., the wait-and-weight method and the driller ’ s method, respectively). Inspection of the procedures will show that these are opposite approaches, while the concurrent method falls somewhere in between.
Two important considerations relative to time are required for the kill procedure: initial wait time and overall time required. The first concern with time is the amount required to increase the mud density from the original weight to the final kill-weight mud. Because some operators are very concerned with pipe sticking during this time, the well-control procedure that minimizes the initial wait time is often chosen. These are the concurrent method and the two-circulation method. In both procedures, pumping begins immediately after the shut-in pressures are recorded.
The other important time consideration is the overall time required for the complete procedure to be implemented. Fig. 1 shows that the one-circulation method requires one complete fluid displacement (i.e., within the drillpipe and the annulus), while the two-circulation method (Fig. 2) requires the annulus to be displaced twice, in addition to the drillpipe displacement. In certain situations, extra time for the two-circulation method may be extensive with respect to hole stability or preventer wear.
During the course of well killing, surface pressures may approach alarming heights. This may be a problem in gas-volume expansion near the surface. The kill procedure with the least surface pressure required to balance the bottomhole formation pressure is important.
Figs. 4 and 5 show the different surface-pressure requirements for several kick situations. The first major difference is noted immediately after the drillpipe is displaced with kill mud. The amount of casing pressure required begins to decrease because of the increased kill-mud hydrostatic pressure during the one-circulation procedure. This decrease is not seen in the two-circulation method because this procedure does not circulate kill mud initially. In fact, in the two-circulation method, the casing pressure increases as the gas-bubble expansion displaces mud from the hole.
The second difference in pressure occurs as the gas approaches the surface. The two-circulation procedure has higher pressures resulting from the lower-density original mud weight. It is interesting to note these high casing pressures that are necessary to suppress the gas expansion to a small degree result in a later arrival of gas at the surface.
Process suitability partially depends on the ease with which the procedure can be executed. The same principle holds true for well control. If a kick-killing procedure is difficult to comprehend and implement, its reliability diminishes.
The concurrent method is less reliable because of its complexity. To perform this procedure properly, the drillpipe pressure must be reduced according to the mud weight being circulated and its position in the pipe. This implies that the crew will inform the operator when a new mud weight is being pumped, that the rig facilities can maintain this increased mud-weight increment, and that the mud-weight position in the pipe can be determined by counting pump strokes. Many operators have stopped using this complex method entirely.
One- and two-circulation methods are used more prominently because of their ease of application. In both procedures, the drillpipe pressure remains constant for long intervals of time. In addition, while displacing the drillpipe with kill mud, the drillpipe pressure decrease is virtually a straight-line relationship, not staggered, as in the concurrent method (Fig. 3).
Although all considerations for choosing the best method are important, the primary concern should always be the stresses imposed on the borehole wall. If the kick-imposed stresses are greater than the formation can withstand, an induced fracture occurs, creating the possibility of an underground blowout. The procedure that imposes the least downhole stress while maintaining constant pressures on the kicking zone is considered the most conducive to safe kick killing.
One way to measure downhole stresses is by use of "equivalent mud weights," or the total pressures to a depth converted to lbm/gal mud weight. For example,
where ρe = equivalent mud weight, lbm/gal.
The equivalent mud weights for the systems in Figs. 4 and 5 are presented in Figs. 6 and 7. The one-circulation method has consistently lower equivalent mud weights throughout the killing process after the drillpipe has been displaced. The procedures generally exhibit the same maximum equivalent mud weights. They occur from the time the well is shut in until the drillpipe is displaced.
Figs. 6 and 7 illustrate an important principle: maximum stresses occur very early in circulation for the deeper depth, not at the maximum casing pressure intervals. The maximum lost-circulation possibilities will not occur at the gas-to-surface conditions, as might seem logical. If a fracture is not created at shut-in, it probably will not occur throughout the remainder of the process. A full understanding of this behavior may calm operators ’ concerns about formation fracturing as the gas approaches the surface.
Nonconventional well control procedures
Many attempts have been made to develop well-control procedures based on principles other than the constant-bottomhole-pressure concept. These procedures may be based on specific problems peculiar to a geological area. One example is low-permeability, high-pressured formations contiguous to structurally weak rocks that cannot withstand hydrostatic kill pressures. Often, nonconventional procedures are used to overcome problem situations that result from poor well design.
|De||= depth equivalent, ft|
|ρe||= equivalent mud weight, lbm/gal|
|pΣ||= total pressure, psi|
- "Definition of Well Control - IADC Lexicon." IADC Lexicon. N.p., 23 Apr. 2013.
Noteworthy papers in OnePetro
A.C.V. Martins Lage, E.Y. Nakagawa, A.G.D.P. Cordovil 1994. Well Control Procedures in Deep Water, SPE Latin America/Caribbean Petroleum Engineering Conference, 27-29 April 1994, Buenos Aires, Argentina, 26952-MS, http://dx.doi.org/10.2118/26952-MS
Bode, D.J., Noffke, R.B., Nickens, H.V 1991. Well-Control Methods and Practices in Small-Diameter Wellbores, Journal of Petroleum Technology Volume 43, Number 11, 19526-PA, http://dx.doi.org/10.2118/19526-PA