Basically, all formations penetrated during drilling are porous and permeable to some degree. Fluids contained in pore spaces are under pressure that is overbalanced by the drilling fluid pressure in the well bore. The borehole pressure is equal to the hydrostatic pressure plus the friction pressure loss in the annulus. If for some reason the borehole pressure falls below the formation fluid pressure, the formation fluids can enter the well. Such an event is known as a kick. This name is associated with a rather sudden
flowrate increase observed at the surface.
A formation fluid influx (a kick) may result from one of the following reasons:
- abnormally high formation pressure is encountered
- lost circulation
- mud weight too low
- swabbing in during tripping operations
- not filling up the hole while pulling out the drillstring
- recirculating gas or oil cut mud.
If a kick is not controlled properly, a blowout will occur.A blowout may develop for one or more of the following causes:
- lack of analysis of data obtained from offset wells
- lack or misunderstanding of data during drilling
- malfunction or even lack of adequate well control equipment
A formation gas or fluid kick can be efficiently and safely controlled if the proper equipment is installed at the surface. One of several possible arrangement of pressure control equipment is shown in Figure. The blowout preventer (BOP) stack consists of a spherical preventer (i.e.,Hydril) and ram type BOPs with blind rams in one and pipe rams in another with a drilling spool placed in the stack.
A spherical preventer contains a packing element that seals the space around the outside of the drill pipe. This preventer is not designed to shut off the well when the drill pipe is out of the hole. The spherical preventer allows stripping operations and some limited pipe rotation.
Hydril Corporation, Shaffer, and other manufactures provide several models with differing packing element designs for specific types of service. The ram type preventer uses two concentric halves to close and seal around the pipe, called pipe rams or blind rams, which seal against the opposing half when there is no pipe in the hole. Some pipe rams will only seal on a single size pipe; 5 in. pipe rams only seal around 5 in. drill pipe. There are also variable bore rams, which cover a specific size range such as 3½ in. to 5 in. that seal on any size pipe in their range.
Care must be taken before closing the blind rams. If pipe is in the hole and the blind rams are closed, the pipe may be damaged or cut. A special type of blind rams that will sever the pipe are called shear blind rams.
These rams will seal against themselves when there is no pipe in the hole, or, in the case of pipe in the hole, the rams will first shear the pipe and then continue to close until they seal the well.
A drilling spool is the element of the BOP stack to which choke and kill lines are attached. The pressure rating of the drilling spool and its side outlets should be consistent with BOP stack. The kill line allows pumping mud into the annulus of the well in the case that is required. The choke line side is connected to a manifold to enable circulation of drilling and formation fluids out of the hole in a controlled manner.
A degasser is installed on the mud return line to remove any small amounts of entrained gas in the returning drilling fluids. Samples of gas are analyzed using the gas chromatograph.
If for some reason the well cannot be shut in, and thus prevents implementation of regular kick killing procedure, a diverter type stack is used rather, the BOP stack described above. The diverter stack is furnished with a blow-down line to allow the well to vent wellbore gas or fluids a safe distance away from the rig. Figure bellow shows a diverter stack arrangement.
Read also about Drilling Bits
WHEN AND HOW TO CLOSE THE WELL
While drilling, there are certain warning signals that, if properly analyzed, can lead to early detection of gas or formation fluid entry into the wellbore.
- Drilling break. A relatively sudden increase in the drilling rate is called a drilling break. The drilling break may occur due to a decrease in the difference between borehole pressure and formation pressure. When a drilling break is observed, the pumps should be stopped and the well watched for flow at the mud line. If the well does not flow, it probably means that the overbalance is not lost or simply that a softer formation has be encountered.
- Decrease in pump pressure. When less dense formation fluid enters the
borehole, the hydrostatic head in the annulus is decreased. Although reduction in pump pressure may be caused by several other factors, drilling personnel should consider a formation fluid influx into the wellbore as one possible cause. The pumps should be stopped and the return flow mud line watched carefully.
- Increase in pit level. This is a definite signal of formation fluid invasion into the wellbore. The well must be shut in as soon as possible.
- Gas-cut mud.Whendrilling through gas-bearing formations, small quantities
of gas occur in the cuttings. As these cuttings are circulated up, the annulus, the gas expands. The resulting reduction in mud weight is observed at surface. Stopping the pumps and observing the mud return line help determine whether the overbalance is lost.
If the kick is gained while tripping, the only warning signal we have is an increase in fluid volume at the surface (pit gain). Once it is determined that the pressure overbalance is lost, the well must be closed as quickly as possible. The sequence of operations in closing a well is as follows:
- Shut off the mud pumps.
- Raise the Kelly above the BOP stack.
- Open the choke line
- Close the spherical preventer.
- Close the choke slowly.
- Record the pit level increase.
- Record the stabilized pressure on the drill pipe (Stand Pipe) and annulus pressure gauges.
- Notify the company personnel.
- Prepare the kill procedure.
If the well kicks while tripping, the sequence of necessary steps can be given below:
- Close the safety valve (Kelly cock) on the drill pipe.
- Pick up and install the Kelly or top drive.
- Open the safety valve (Kelly cock).
- Open the choke line.
- Close the annular (spherical) preventer.
- Record the pit gain along with the shut in drill pipe pressure (SIDPP) and shut in casing pressure (SICP).
- Notify the company personnel.
- Prepare the kill procedure.
Depending on the type of drilling rig and company policy, this sequence of operations may be changed.
Read also Drilling Rotating Equipment
KICK CONTROL PROCEDURES
There are several techniques available for kick control (kick-killing procedures).
In this section only three methods will be addressed.
- Driller’s method. First the kick fluid is circulated out of the hole and hen the drilling fluid density is raised up to the proper density (kill mud density) to replace the original mud. An alternate name for this procedure is the two circulation method.
- Engineer’s method. The drilling fluid is weighted up to kill density while the formation fluid is being circulated out of the hole. Sometimes this technique is known as the one circulation method.
- Volumetric method. This method is applied if the drillstring is off the bottom.
The guiding principle of all these techniques is that bottomhole pressure is held constant and slightly above the formation pressure at any stage of the process. To choose the most suitable technique one ought to consider
(a) complexity of the method,
(b) drilling crew experience and training,
(c) maximum expected surface and borehole pressure.
(d) Time needed to reestablish pressure overbalance and resume normal drilling operations.