Introduction to Well Control

The formation fluids can flow into the well bore and eventually to the surface when the formation pressure becomes greater than the hydrostatic pressure exerted by the column of drilling fluid (Click Here). During operations, it is very important to keep the hydrostatic pressure greater than the pore pressure. The formation fluids influx or the kick has to be controlled, if no action is taken, then the formation fluids can reach the surface generating what is called as a blowout (Click Here). The blowout is an uncontrolled situation which has to be remedied rapidly, carefully and effectively. 
Blowout can lead to:
- Human life losses
- Rig and equipment losses
- Reservoir fluids losses
- Environment Damage
- High cost of getting the control again

The formation fluids are basically prevented to flow into the well bore by two types of control:

The primary control: it is the main method of control to prevent formation fluids influx. It is performed by maintaining the hydrostatic pressure applied by a column of drilling fluids greater than the pore pressure. The positive differential pressure is called the overbalance.

The secondary control: when the primary control failed, the secondary control is required. The main purpose of this type of control is stopping the flow of formation fluids into the well bore and circulating them out of the well in a controlled way. The secondary control is started by closing the blowout preventer (BOP), then displacing the formation fluids using heavy drilling fluids which are circulated into the drill string, down to the bottom then up to the surface (Click Here).

Fig 01- BOP Stack

I- Primary Barrier Affecting Parameters: 

Mud Weight Reduction

The mud weight is designed to provide a sufficient overbalance to stop formations fluids to flow into the well bore. Losing the overbalance for any reason can increase the risk of taking a kick. Monitoring the mud weight during operations is very essential to ensure that the adequate mud is being pumped into the well. Any disturbance in mud density should be remedied to the programmed value.

The mud weight can fall during operations for these reasons:

- Removal of solids: drilling operations generates drilled cuttings which have to be removed from the drilling fluids by removal equipment to keep the required properties. If the removal equipment are not operating or designed properly, weighting products (ex: Barite) can be removed with the drilling cuttings leading to mud weight decrease. 

- Excessive dilution: drilling fluids are treated continuously to keep the programmed properties. Dilution of mud by water is frequent treatment to adjust some properties or compensating the evaporate water while drilling deep water wells. Losing the control of water percentage in mud can lead to unstable properties. 

- Gas cut of drilling mud: The gas can seep from the formation into the circulated mud which can affect the mud weight. The gas can also expand before reaching the surface leading to a decrease in hydrostatic pressure exerted by a column of gas. The gas has to be removed from the mud before recirculating it back into the well.

Mud column Height Reduction:

During drilling operations, the volume of drilling fluids pumped into the wellbore has to be the same as the volume of fluids returned to the surface. Two situations can be faced after the mud pumps are stopped: 
- The flow out from the well bore can continue which indicates that a kick is taking a place.
- The level of the mud at the surface can decrease which indicates that the mud is lost into the formation leading to what it is called lost circulation situation (partial or total)
If the height of the mud column decreases, it affects the hydrostatic pressure which can become less than the pore pressure allowing the formation fluids to flow into the well bore. The mud column height can decrease for many reasons:

- Tripping: The height of mud column can decrease when pulling the drill pipe out of the hole which leads to bottom hole pressure decrease. The volume of the pulled pipe has to be replaced by filling the well with drilling fluids to keep the required bottom pressure and maintain the primary barrier (Click Here).

- Swabbing: The swabbing is the phenomenon when the formation fluids are sucked into the well bore. A low pressure region can be created below the bit when pulling the string allowing the formation fluids to flow into the well bore. The severity of swabbing can be related to the speed of pulling the drill pipe, high gel strength of mud or when getting bit baling which can affect the clearances between the wellbore and drill string. 

- Lost Circulation: It happens when drilling fluids flows into the drilled formation which can be fractured or with high permeability. The fracture pressure can be exceeded when pumping high mud weight or surging the well by running drill string at high speed. When the drilling fluids start to flow into the formations, the height of the mud column decreases and consequently the bottom pressure also decreases which allows the formation fluids to come into the well bore, flowing up to the surface and affecting the primary barrier. Lost circulation can be avoided by: using low mud weight, controlling drill string running speed and optimize the annular clearance between the hole and drill string (Click Here).

II- Warning Signs of a Kick:

When a kick occurs, many warning signs can be noticed at the surface that indicate that the formation fluids have been flowed into the wellbore. These signs have to be analyzed properly in order to avoid any uncontrolled situation. The warning signs or indicators are divided into two categories: primary and secondary.

Primary Signs of Kick (Click Here):

Pit volume increase: The mud volume which flows into the well has to be the same as the mud volume returning from the well and this can be recorded as stable level of the active mud pits. If the level of the active system increases that means that additional fluids volume has entered the system. Then, the operations have to be stopped and the well must be monitored by performing a flow check. If the drilling fluids continue to flow from well, this is a positive sign of taking a kick.

Flow Rate Increase: when the mud pumps are operating at a constant rate, normally, the flow rate out of the hole must equal to the flow rate of fluids pumped into the well. If the ''Out flow rate'' becomes greater than the '' In flow rate'', then it can be deduced that an additional fluid is flowing with the mud in the annulus up to the surface.

Flowing Well With Shut Off Pumps: When the mud pumps are stopped, the drilling fluids should not flow out of the well. If they continue to flow that means that they are pushed by another force. It can be explained that the pore pressure is higher than the hydrostatic pressure. The flowing well can be also explained by two other reasons: the expansion of the drilling fluids in deep wells or the U tube effect which add an additional force.

Improper Filling-up During Tripping: the well has to be filled up during tripping. The amount of the mud volume which has to be added it has to be the same as the volume of the pulled drill pipe. If it is less than the volume of the pulled drill pipe that means that another fluid has occupied the volume of drill pipe.

Secondary Signs:

Drilling Break: It is a sharp increase in rate of penetration. It indicates that high pressure zone is being drilled which has higher porosity.

Pump Pressure Changes: When the influx comes into the drilling fluids system, it makes the mud column lighter with low viscosity which requires less force to displace it out of the hole. This will be noticed as a significant drop into the required pump pressure. 

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