Well Control Preliminary Calculations


Controlling a kick involves taking many actions to evacuate the influx and getting to balance by pumping a killing mud with required density. The control has to be performed without allowing a new influx or formation fracturing . 
A Kick can a occur at any moment during drilling operations ( Click Here ), for this reason many parameters have to be chosen, measured or calculated before facing well control situation. These parameters have to be also updated regularly. Other parameters are calculated using some important information after closing the well on an influx ( Click Here for Shut in Procedures ) .

Slow pump Rate (Slow Circulating Rate SCR)

The influx is in general circulated with reduced flow rate compared to drilling flow rate. The slow pump rate is estimated between the quarter to half of drilling flow rate. There are many benefits of selecting such reduced rate:

- Limited pressure on bottom by reducing the friction losses in the annular space ( Click Here
- Give the choke operator the ability to adjust the pressures and react effectively in case of facing any problem
- Take in consideration the mud gas separator capacity to treat the kick
- Reduce the risk to damage or wash out the choke.

Measurement of friction losses at reduced flow rate ( Pscr )

The friction losses can be taken straight forward by reading the pressure on the gauges on the choke panel. The reading has to be taken from the choke panel because it will be used in case of well control.
The readings has to be updated regularly after:
- Crew change
- Changing BHA
- When changing mud density or mud rheology
- After drilling long section (ex: 300 ft or more)

Fig 01- Choke Panel

Maximum Allowable Surface Pressure MASP

The maximum allowable pressure in the annular space is an important information which has to be estimated accurately to avoid fracturing the formation when controlling a kick. This pressure is determined from the results got after performing a leak off test.


Where
Ptest : Pressure measured at the leak off test
MWin: Mud weight used when performing the leak off test
TVDsh: True vertical depth of the last casing shoe

Gain Record 

It is considered as the influx volume. It is estimated to be the difference in mud tanks volume before and after shutting the well in.

Shut in pressures Recording and Observation

Just after closing the well, the drill pipe and annular pressure have to be recorded closely till getting to steady values. The timing to get to the steady pressures depends on:

- The differential pressure at the bottom of the hole 
- The reservoir permeability
- The nature of the influx

The shut in drill pipe pressure is considered as the difference between the pore pressure and the hydrostatic pressure of the mud column but the shut in casing pressure depends on the pore pressure, influx nature and volume. 
In most cases, the shut in casing pressure is greater than the drill pipe pressure. The difference between these two pressures will increase when the volume of influx increase.
If the drill string is fitted with float valve, the reading of the drill pipe pressure become difficult. In this situation, the pressure on drill pipe can be read as follows:

- Start mud pump with very low flow rate and note any change in casing pressure
- Stop pumping after noticing an increase in casing pressure.

The shut in drill pipe pressure can be considered as the pressure stabilized when noticing the casing pressure increase.

Kill Mud Density

The kill mud density is defined as the required density to deliver a hydrostatic pressure to balance the pore pressure. After shutting the well, the pore pressure can be estimated as the sum of the pressures into the drill string or the sum of pressures into the annular space. The density and the height of the influx can be difficult to be determined, so for this reason, the pore pressure can be calculated accurately by using the drill pipe pressure because of the presence of known fluid density into the drill sting.

Where
Ppore  : Pore pressure
MWin : Mud weight in the well
TVD   : True vertical depth

and the kill mud density can be calculated as follows

Where:
MWkill :  kill mud weight
SIDPP   : Shut in drill pipe
TVD      : True vertical depth

Initial Circulating Pressure ICP

While performing well control, the flow rate has to be steady to keep a constant bottom hole pressure which is equals to pore pore pressure. The initial circulating pressure is determined as:

Where
ICP: Initial Circulating Pressure
Pscr: Pressure at slow circulating rate

Final Circulating Pressure FCP

In order to keep a constant bottom hole pressure when circulating kill mud into the drill string, the discharge pressure has to decrease from the initial circulating pressure to the final circulating pressure which is the represented by the friction losses into the drill string when the kill mud reaches the drill bit. It can be estimated by


NB: In addition to the calculations mentioned above, the volume of many section has to be calculated to be introduced into the kill sheet like string volume and annular volume.




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