Concurrent method.

Since most kicks require only a small density increase, this method is usually the fastest means by which to kill a well safely.

Company requires that enough barite be stockpiled on location to allow mixing at one sack/min. until more barite is available. A minimum of 50 tonnes is required.

Prior to completing the well control worksheet, isolate as many mud tanks as possible by bypassing down the mud ditch, making the active mud system as small as practical, thus minimising the required mixing times.

The calculations required to allow the kick to be circulated out and the well killed can be done graphically or manually.

CAUTION:

Care must be taken to ensure that the proper graph is used. Graphs exist for mud densities of 1.0-1.3 S.G. and mud densities of 1.3-1.6 S.G.

• Information Required.

To use the well control nomograph properly the following information is required:

− Reduced Speed Pump Pressure previously recorded: RSPP (bar)

− Stabilised Shut-In Drillpipe Pressure: SIDPP (bar)

− True Vertical Depth: (m)

− Reduced Speed Pump Rate previously recorded: RSPR (m3/min)

− Mud Density: (S.G.)

− Maximum Allowable Casing Pressure previously recorded: MACP (bar)

NOTE: The Initial Circulation Pressure (ICP) and Final Circulating Pressure (FCP)

DO NOT INCLUDE ANY OVERKILL.

• Graphical mixing rate calculations (see Figures 2.1 and 2.2 *units=kPa and kg/m3). NOTE: Graphical Well Control Worksheets, see form 2.1 at the back of this section.

One Circulation STEP 1

Plot the reduced speed pump pressure (RSPP in bar previously recorded) on the vertical RSPP line.

STEP 2

Plot the stabilised shut-in drillpipe pressure (SIDPP) on the left hand axis of the lower graph.

Join the RSPP and SIDPP points with a straight line. From this line the initial circulating pressure (ICP in bar) can be read from the intersection with the vertical ICP line.

STEP 3

From point 2 (SIDPP) draw a horizontal line to the right until it intersects with the line representing the present true vertical depth of the well.

If a vertical line is projected downward from this point the density increase required to kill the well can be read on the lower graph’s horizontal axis. This density increase required to kill the well does not have any overkill built into it.

STEP 4

From the intersection with the well depth line (Point 3) project a vertical line upwards until it intersects the previously recorded speed pump rate line (RSPR in m3/min.) on the upper graph.

STEP 5

From point 4 (intersection with the RSPR), project a horizontal line to the left until it intersects with the left axis of the upper graph. From this intersection point the barite mix rate (sxs/min), required to kill the well in one circulation, can be read.

The density required to kill the well (S.G.) can be determined by adding the density increase required to kill the well to the original mud density.

STEP 6

If the required mix rate to kill the well is less than 1sx/min., or the rig is capable of mixing at greater than 1sx/min., then project a vertical line downward from the original Point 4 until it intersects with the well depth line. When the required barite mix rate to kill the well is less than 1sx/min. (40 kg/sx), Point 6 will be the same as the original Point 3.

STEP 7

From Point 6 project a horizontal line to the left until it intersects with the vertical Delta P line.

STEP 8

From Point 7 draw a straight line through the intersection on the ICP line. Continue to project this line until it intersects with the vertical final circulating pressure (FCP) line on the nomograph. The final circulating pressure (FCP in bar) can be recorded from this point (Point 8).

NOTE: If the required mix rate is greater than 1sx/min. or greater than the rig capacity,

then the well will require more than one circulation to kill. Immediately begin mixing 1sx/min. at the previously plotted ICP and RSPR.

Two Circulations Required STEP 9

From Point 4 if a line is projected horizontally to the left, the required barite mix rate (sxs/min.) to kill the well in one circulation can be read from the left-hand axis of the upper graph (Point 5).

When the required mixing rate to kill the well is greater than the mixing capability of the rig (i.e., >1sx/min.) then the well will require more than one circulation to kill. Immediately start mixing at 1sx/min. at the previously plotted ICP and RSPR.

From point 4 follow the RSPR line back until it intersects with the horizontal 1sx/min mixing rate line (Point 6). From Point 6 project a vertical line downward until it intersects with the well depth line on the lower graph (Point 7).

NOTE: It is Company policy that all rigs working for the Company are capable of mixing

barite at a minimum rate of 1sx/min. STEP 10

From Point 7 project a horizontal line to the left until it intersects with the vertical Delta P line on the nomograph.

NOTE: From Point 7 the density increase (S.G.) which will be achieved on this

circulation can be read by projecting a vertical line down to the horizontal axis of the lower graph.

STEP 11

From Point 8 project a line through the intersection of the vertical ICP line on the nomograph.

STEP 12

Continue projecting the straight line from Point 9 until it intersects with the vertical final circulating pressure (FCP) line on the nomograph (Point 10). The FCP (bar) at the end of this circulation can be read from this point.

At the end of this circulation the well should be shut in and the pressure recorded again. The nomograph must be filled out again and another circulation must be performed. This process must be repeated until the well can be killed with a circulation requiring 1sx/min. or less of barite.

NOTE: During this circulation the ICP must be gradually reduced to the FCP, as the new

density mud is circulated from the surface to the bit. From a practical standpoint there is usually very little difference between the ICP and FCP. As long as minimal overkill is being used (i.e., 7bar) the entire circulation can be made at the ICP. If there is more than a 5bar difference between the calculated ICP and FCP, then the circulating pressures should be adjusted as above.

Figure 4.2 Well control worksheets (steps 4 through 8)

STEP 1

1. Plot the initial circulating pressure (ICP in bar obtained from the well control nomograph or calculated manually) on the left-hand axis of the graph. 2. Plot the final circulating pressure (FCP in bar obtained from the well control

nomograph or calculated manually) at the time/strokes to circulate at the reduced- speed pump rate (RSPR in m3/min.) from surface to the bit.

3. Join the ICP and FCP points with a straight line.

4. Crack open the choke. Start the pump and bring it up to the RSPR. Maintain constant casing pressure until the pump pressure has stabilised. Adjust the choke until the drillpipe pressure is at the plotted ICP. Start mixing barite at the required mix rate (sxs/min.).

STEP 2

Circulate the kill density mud from the surface to the bit. Slowly reduce the drillpipe pressure from the ICP to the FCP.

NOTE: If there is not a great deal of difference between the ICP and FCP (i.e., <5bar)

then it is acceptable to circulate the kill density mud to the bit by holding the drillpipe pressure constant at the ICP. If there is a large difference between the ICP and FCP, then circulate the kill density mud to the bit in 5bar increments, as shown in the well kill procedure example.

The ICP and FCP which are obtained from the well control nomograph or manual calculations do not include any overkill. Approximately 7-14bar in the overkill margin should be added to the calculated values when plotting the circulation pressures to be followed during the well control operation.

STEP 3

1. Once the kill density mud is at the bit, hold the drillpipe pressure constant at the FCP until the kill density mud reaches the surface. Once the kill density mud reaches surface, the casing pressure should be equal to the amount of overkill being used, if the well was capable of being killed in one circulation (i.e., a mixing rate of <1sx/min. was required).

2. Stop the pump and allow the overkill to bleed off. Shut in the well. Both the drill pipe and casing pressures should read zero pressure.

NOTE: If there is pressure remaining after shutting the well in and the drillpipe pressure

and casing pressure are equal, it may be due to trapped pressure in the wellbore during the shut in. Bleed off the pressure and shut in the well again.

If more than one circulation is required to kill the well then the SIDPP and SICP must be recorded again, and the well control problem solved for another

Figure 4.3 Modified concurrent method

Manual Well Kill Calculations.

Although the graphical method of determining the required barite mixing rate (40kg sxs/min.) is less complicated, there may be situations where more accuracy is required or a check on the graphical solution is desired. The following steps are required to solve the mixing rate required to kill the well.

STEP 1

Calculate the circulating volume/time in minutes and strokes.

Calculate the circulating volume and time (strokes). The volume/strokes must be segregated into 3 components:

− Surface to bit (based on measured depth) − Bit to surface (based on measured depth) − Surface tank volume

1. Surface to Bit:

DP capacity (m3/m) x length (m) + Collar capacity (m3/m) x length (m)

= Volume surface to bit (m3)

Strokes = Volume (m3)

Pump Capacity (m3/stroke)

2. Bit to Surface (Annulus):

Annular Capacity Collars/OH (m3/m) x length (m) + Annular Capacity DP/OH (m3/m) x length (m) + Annular Capacity DP/Csg (m3/m) x length (m) = Volume bit to surface (m3)

Strokes = Volume (m3)

3. Surface Tanks:

Only use the active tank volume (m3). Do not include any by-passed tank compartments.

Strokes = Volume (m3)

Pump Capacity (m3/stroke)

Circulating Volume:

Volume Surface to Bit (m3) + Volume Bit to Surface (m3) + Surface Tank Volume (m3) = Circulating Volume (m3)

Circulating Time:

Circ. Time (min.) = Circulating Vol. (m3)

RSPR (m3/min.)

where RSPR = Reduced Speed Pump Rate

Circ. Time (strokes)= Circulating Vol. (m3)

Pump Capacity (m3/stroke)

STEP 2

Calculate the density increase necessary to kill the well in one circulation.

The density increase (DI) required to kill the well can be found by utilising the SIDPP. This DI does not include any excess. Once the well has been killed, additional barite should be added to provide a 14bar overbalance while drilling.

DI (S.G.) = SIDPP

0.1 x DI

where: DI = Density increase to kill well (S.G.)

SIDPP = Shut In Drillpipe Pressure (bar)

TVD = True vertical depth of well (m)

STEP 3

Calculate the barite mixing rate to kill the well in one circulation.

The barite mixing rate can be determined as follows: 1. Barite Required:

Barite Required (kg/m3)= 4.25 (S.G) x DI (S.G.)

4.25 (S.G.) - NMD(S.G.)

where: Barite Required (kg/m3) = Barite required per 1m3 of circulating

volume to kill the well

4.25 S.G. = Density of barite

DI (S.G.) = Density increase to kill well

2. Total Barite Required (TBR):

Barite Required (kg/m3) x Circulating Volume (m3)=Total Barite Required (kg) 3. Total Number of Barite Sacks Required:

# Sacks = TBR (kg)

40 kg/sack

4. Barite Mix Rate (sxs/min.):

Mix Rate (sxs/min.) = #Sacks

Circulation Time (min.)

STEP 4

If the mixing rate to kill the well is greater than 1sx/min. or the rig mixing capability, then calculate the density increase achieved with a circulation at 1sx/min.

When the mixing rate required to kill the well is greater than 1sx/min. or the rig's mixing capability, then more than one circulation will be required to kill the well. In this case the density increase that will be achieved with the present circulation at 1sx/min. or the rig's mixing capability will be required:

Density Increase (DI in S.G.):

DI = 40kg/sack x MR x (4.25 S.G. - MD)

(4.25 S.G. x RSPR) - (40kg/sack x MR)

where: DI = Density increase in S.G. that will be achieved this circulation.

MR = Barite mix rate in sacks/minute

MD = Mud density in S.G. at the beginning of the present

circulation.

RSPR = Reduced speed pump rate previously recorded (m3/min.)

NOTE: The above calculation has assumed 40kg sacks of barite. If the sack size is

different from this then the proper sack weight must be used. In addition, the density of barite has been assumed to be 4250 kg/m3.

STEP 5

Calculate the ICP and FCP.

The calculation of the initial circulating pressure (ICP in bar) and the final circulating pressure (FCP in bar) is dependent upon whether or not the well can be killed in one circulation or not.

1. Mix Rate of 1sx/min. or less (1 circulation to kill):

ICP = SIDPP + RSPP

FCP = RSPP x NMD

OMD

ICP = (SIDPP + RSPP) x OMD NMD

FCP = RSPP x NMD + [( KMD-NMD)xLx9.81]

OMD

where :

SIDPP = Stabilised shut-in drillpipe pressure (bar)

RSPP = Previously recorded reduced speed pump pressure (bar) NMD = New mud density at the end of present circulation (S.G.) OMD = Original mud density at the start of present circulation (S.G.) KMD = Mud density required to kill the well (S.G.)

L = True vertical depth of the well (m)

NOTE: The above calculations do not include any overkill. Approximately 7-14bar of

overkill should be held on the choke while circulating out the kick to prevent an influx of formation fluids during well control operations.

STEP 6

Prepare a well control procedure graph as shown in figure 2.3. It is best to prepare the graph without any overkill included in the actual pressure line. The overkill should be shown on the stepped down pressures that the driller will actually be holding on the choke. Complete the well control worksheet and control log at the back of this section (Forms 2.2 and 2.3).

• Crew Positions.

NOTE: The listed crew positions are applicable to alternate methods of well control. Driller - Controls the choke at the remote station on the floor. Ensures valve to degasser

is open.

Assistant Driller - Assists the driller.

Derrickhand - Mixes barite as required at mixing station. Floorhand 1 and 2 - Assists the derrickhand.

Rig Superintendent/Toolpusher - Ensures that all the crew members are performing

their duties as required. Ensures that all required equipment is operating properly and any required standby equipment is available.

Company Representative - Monitors all rig operations to ensure that well kill plan is

carried out correctly.

NOTE: Only essential personnel are to be present on the drilling floor.

In document Drilling Manual (Page 53-62)