Circulation & Well Control
Circulation and Well Control
Learning Objectives
You will learn the importance of pump rates and
pressures during well control operations.
You will learn pressure relationships.
You will learn basic calculations necessary in well
control for:
Capacity
Volume
Strokes
Circulation and Well Control
Overview
Pumps are one of the basic tools used in well control.
Used to circulate kick fluids out.
Used to circulate kill fluid throughout well.
Used to pump cement, pills, plugs, etc.
Usually measured in strokes per minute and output,
bbls/min.
Small changes in pump rate can greatly affect
pressures throughout well.
So, in well control, to keep bottom hole pressure in a specified
range, pump rate needs to be carefully controlled.
Kill Rate
Kill rate is a reduced circulating rate used during well
control operations.
Reduces circulating friction.
Allows kill fluid to be maintained during kill operations.
Reduces strain on pumps.
Allows more time to react to problems.
Allows adjustable chokes to work within orifice range.
Kill Rate
Pump speed is critical because it affects pump pressure.
Any change in speed may drastically affect circulating
pressure!
Usually taken at 1/4, 1/3 and 1/2 normal circulating rate.
Often 20, 30 and 40 stk/min are used.
Sometimes based on rate to reach a certain pressure.
Sometimes based on bbls/min (e.g., 2 5 bbls/min).
Kill Rate Pressure
Kill rate pressure is the circulating pressure at the kill
rate pump speed.
Kill rate pressures should be taken when:
There is a change in fluid density or fluid flow (rhelogical)
characteristics.
There is a change in bit and bit nozzles.
BHA, downhole tools and string changes.
When 500 or more feet of new hole is drilled.
Each tour.
After pump repair.
Kill Rate Pressure
Ideally, kill rate pressures should be taken through the kill
manifold and choke.
Have to flush kill manifold and choke.
Usually taken with BOP stack open instead.
A small change in rate can affect circulating pressure
greatly.
This can be mathematically calculated.
A pump pressure and rate must be known.
Fluid properties have to be the same.
Much better to get actual pressures than calculated.
P2 = P1 X (SPM22 SPM12)
Where:
P2 = original pump pressure at SPM1, psi
P1 = reduced or changed pump pressure at SPM2, psi
SPM1 = original pump rate, stks/min
SPM2 = reduced or changed pump rate, stks/min
Initial Circulating Pressure
Initial circulating pressure (ICP) - the combination of
shut-in drillpipe pressure plus the pressure needed to
circulate fluid at a given rate.
ICP = KRP + SIDPP
Where:
ICP = initial circulating pressure, psi
KRP = kill rate pressure, psi
SIDPP = shut in drill pipe pressure, psi
Initial Circulating Pressure
ICP is the circulating pressure used once the pump is
at kill rate speed.
If no kill fluid is pumped, then this is the circulating
pressure necessary to maintain BHP constant at or
slightly above FP.
If kill fluid is pumped ICP must be allowed to drop as the
kill fluid fills the string increasing hydrostatic killing the
pressure deficit.
Final Circulating Pressure
Once kill fluid fills the string the circulating pressure is
commonly referred to as Final Circulating Pressure (FCP).
This change in circulating pressure can be calculated by:
FCP = KRP X (KWM OMW)
Where:
FCP = final circulating pressure, psi
KRP = kill rate pressure, psi
KWM = kill weight mud (fluid), ppg
OMW = old mud weight, ppg
Surface to Bit, Pump Strokes and Time
The number of strokes to pump a fluid, such as a kill
fluid, from the surface to the end of the string is critical
for successful well control and maintaining BHP
constant.
Also, the time to pump from the surface to the end of
the string is important.
The volume within the string must be calculated. This is
the combined total of:
Volume of tubing or drill pipe.
Volume of BHA.
Additionally, the amount of strokes/time to clear the
surface equipment should be known.
Surface to Bit, Pump Strokes and Time
To calculate a pipe/string volume:
Volumebbls = Capacity bbl/ft X Length ft
This is calculated for each section of the string (i.e. pipe, HWDP, BHA)
The volumes are totaled for the entire string.
To calculate the amount of strokes to displace the string
volume:
Strokes = Total String Volume bbls Pump Output bbl/stk
Note: Capacity of pipe may be looked up in charts and tables.
Surface to Bit, Pump Strokes and Time
If lookup tables are not available, the following formula
can be used to calculate the capacity of a cylindrical
object such as an open hole, casing, tubing, drill pipe,
BHA or choke/kill lines in bbls/ft :
Capacity bbls/ft = Diameter2 1029.4
Annular Volumes
The calculation for annular volume is similar to that of
capacity, but subtracting out the displacement of the
tubulars in the well.
There may be several different bbl/ft annulus capacity
sections:
Based on well bore geometry and where different pipes
are located.
Each section must be calculated individually:
Tubing/DP in casing, liner, open hole and/or riser.
Hevi wate in casing, liner, open hole and/or riser.
BHA/DC in casing, open hole and/or riser.
Annular Volumes
If lookup tables are not available, the capacity for each
section can be calculated by:
Capacity bbls/ft = (OD2 - ID2) 1029.4
Where:
ID = ID of open or cased hole, inches
OD = OD of tubular in section, inches
Note: 1029.4 is the conversion factor between cylindrical area and volume
Once the capacities are known, the volumes for each
section and total volume can be calculated.
Strokes to surface and time to surface for bottoms up
can then be easily calculated.
Total Circulating Strokes and Time
Simply by adding the volumes for each section, a total
volume, strokes and circulating time down the string
and back to surface can be calculated.
By adding separate volumes from surface, or the bottom
of the string we can also calculate volume, strokes and
time to circulate for:
End of string or bit to casing shoe
Choke and kill line
Any individual or combined lengths and sections
