• Always maximize pipe motion in open hole
• Maintain circulation on connections for as log as possible
• Always break circulation on the up stroke
• Monitor and record depths of higher torque and/or stick slip
• Always monitor the shakers to ensure adequate cuttings return and check for the presence of or increase in the volume of cavings.
• If pump repairs are necessary, stop drilling if adequate hole cleaning cannot be guaranteed with one pump
• Be prepared to limit ROP if hole cleaning is poor
• Follow connection procedure for bringing the pumps slowly to avoid shocking the formation.
• If hole conditions dictate perform a wiper trip
• Maintain correct mud specifications 8.2 Flow Rates
Ensure flow rates are adequate to clean the hole with worst-case scenarios in mind including mud properties and no rotation. Flow rates required to clean the hole are specified at the start of each hole section.
8.3 Connection Procedure
The connection procedure below is a guideline to what practice is required to minimise ECD surges during operations. The procedures can be varied according to operational requirements, and field experience. The main priority for ECD management remains slow initiation of circulation.
Key issues
• ECD peak when resuming circulation
• Swab / surge risk
The following connection procedure has proven successful in preventing pressure spikes following connections.
1. Drill down stand. Pull ± 2m off bottom 2. Take torque and drag readings
3. Stop rotation
4. Work torque out of string if necessary 5. Activate the brake (keep string still)
6. Decrease flow below lower tool operational limit (approx 500 gpm) 7. Increase flow to above lower operational limit (approx 600 gpm)
8. Continue pumping until survey is registered at surface reduce pump rate to below the activating flow for the under-reamer, pull up a maximum of 2.5 joints, turn off the pumps, run down and make connection (this exercise can help to level out cutting beds and help to give a better picture of the hole condition). In the 14” hole, do not pull the entire stand up to avoid pulling the Autotrak ribs into the 14” hole.
9. Set string in slips (Do not make connection if gas levels increasing significantly) 10. Make connection
11. Fill pipe until pressure shows string is full then reduce to 10 spm 12. Pick pipe out of slips
13. Start rotating the drillstring (30 - 50 rpm) (if in rotary mode with motor in the 12 ¼”
hole)
14. Observe return flow
15. Increase to 35 spm while observing increase in return flow. Do this slowly over the course of 30 seconds.
16. Increase flow rate to drilling rate over a further 30 seconds while observing increased return flow.
17. Wait another 15 seconds before moving string to bottom.
18. Bring up to full rotation while bedding in bit. Resume drilling.
The drilling personnel became familiar with connection procedures quickly, but required continued DOE monitoring to ensure connections were made without error.
Anadrill MWD service requires a different connection procedure to minimize pressure surges was as outlined below:
1. Drill down stand. Pull +/- 2 m off bottom 2. Wipe stand with full drilling circulation rate
3. Run the string back to +/- 2 m off bottom, take off bottom torque and drag measurements 4. Work torque out of string if necessary
5. Set string in slip
6. Turn off pumps 7. Make connection 8. Pick pipe out of slips 9. Start filling pipe 10spm
10. Start rotating the drillstring at 60 rpm. If orientation is required after connection and the initial pressure required to break the gels is over 150 psi above SCR at 10spm then rotation should be employed to assist in breaking gels. If breaking pressure is less than 150 psi above SCR then no rotation is necessary.
11. Reduce rpm to 0 before returns are gained.
12. Stage up pumps to 50 spm, at a rate that well conditions allow. Extra time may be used here if well conditions dictate
13. From 50 spm to 90 spm increase pumps as quickly as conditions allow. As fast as reasonably practical to allow survey to be taken.
14. MWD will call when data is being transmitted.
15. Continue staging up pumps to full rate.
16. Bring up to full rotation while bedding in bit. Resume drilling.
Plot 5: Increasing pump rate too quickly resulted in a pressure spike of 0.4 ppg, this could be enough to cause severe wellbore stability problems and possible loss of the section.
Plot 6 Connection procedure good, with a very small pressure increases, initial flow is 55 – 60 gpm for 1 minute. Followed by a slow increase to full flow rate over 3 minutes
Plot 7 Connection procedure initially good, initial flow is 55 – 60gpm for 1 minute.
Followed by an increase to full flow rate over 2 minutes. This small difference of 1 minute in the build up to full flow was enough to cause a significant pressure spike
8.4 Tripping
8.4.1 Circulating Prior to Tripping Out Of Hole
• Circulate at least 2 ½ times bottoms up at the minimum rate required to clean the hole
• Use an RPM of 130 – 140 to optimise hole cleaning
• Reciprocate string slowly while circulating and rotating to avoid cutting drillstring across stringers
• Be aware that under-reamer is likely to continue to generate cuttings 8.4.2 Tripping Speed
On reviewing the post well data from the memory of pressure subs, the effect of tripping speed on swab and surge has been concluded as follows:
• Tripping speeds of 2 mins per stand should be adhered to across the majority of the hole section
• Tripping speeds of 3 mins per stand should be applied in the following circumstances:
When running the BHA through the casing shoes.
When running the last 5 stands to TD
Particular attention should be given to “tight” zones or stringers while tripping to limit mechanical damage
8.4.3 Tripping In Hole
Shear the mud in the active pit and aim to keep mud temperature above 120 deg F. When running in hole, break circulation at least once prior to entering open hole and at least every 500 m in open hole. It is recommended to break circulation in 250 m intervals in the open hole and then adjust up to a maximum of 500 m depending on ECD values seen while circulating.
Break circulation as follows:
1. Fill pipe until pressure shows string is full then reduce to 10 spm.
2. Break circulation on the upstroke
3. Start rotating the drillstring (30 - 50 rpm) 4. Observe return flow
5. Increase to 35 spm while observing increase in return flow. Do this slowly over the course of 30 seconds.
6. Increase flow rate to drilling rate over a further 30 seconds while observing increased return flow
7. Gradually increase rotation to 100 rpm while carefully monitoring pressures from ECD sub to help break gels.
Plot 8 A high surge pressure resulted from running the BHA through the 13 3/8” casing shoe on A3D too quickly. A running speed of 3 min per stand should be used entering the casing shoe and rat-hole.
Plot 9: A increasing surge pressure is observed on A3d running the last 5 stands to bottom. A running speed of 3 min per stand should be used for the last five stands above bottom.
8.4.4 Tripping Out of Hole
1. If hole conditions are good and swabbing is not expected (particularly above 1500 m TVD), attempt to trip out conventionally. Trip first 5 stands wet, slug pipe and continue to POOH at 2 mins/stand. If swabbing is recorded, pump out of hole as per procedures in step 2 below. If tight spots are seen, work string down and work over tight zone.
Never pull more than 30 klbs overpull. If there are stringers in the BHA area, attempt to ream through with slow pipe rotation (30-50 rpm) with no circulation. Run back down and attempt to pull through obstruction again. Do not rotate in the same place for more than 5 - 10 mins. If the hole is still tight, RIH a minimum of 1 stand, start rotating at 30 - 50 rpm and break circulation as per tripping in procedure. Circulate hole clean prior to continuing trip out of hole.
2. If unable to pull out of the hole without swabbing, pump out of hole with a flow rate of 150 gpm and a typical tripping speed of 2 mins/stand. When inside casing, attempt to trip out conventionally. If tight spots or pack offs are recorded, shut pumps off and work string down into a free area. Start rotation at 30 - 50 rpm without circulation and attempt to ream through obstruction. When through the obstruction, wipe the area once.
Stage-up pumps and wipe the area once more while closely watching drillpipe pressure.
3. If steps 1 & 2 are unsuccessful, back ream out of hole. If backreaming out of the hole, the objective is to both work through the obstruction and move cuttings beds up the hole. Steps 1 & 2 are designed to trip through cuttings beds. Back ream out at full circulating rate used for drilling and 130 - 140 rpm. Stage up pumps and RPM as per normal procedures for each stand. Backream out till stand is ± 3m above rotary table, circulate for ± 5 mins, reduce rotary and turn off pumps. Break connection. Back ream out at 3 - 4 mins per stand. Carefully monitor ECD while backreaming to avoid overloading the annulus. If ECD is approaching limit, reduce RPM while maintaining circulation rate. Monitor returns over shakers and if cuttings volume increasing, circulate bottoms up every 5 stands. Consider going back to step 2 if hole conditions improve.
Plot 10 Running pipe in too fast causing pressure surges
8.4.5 Backreaming
Back reaming is not anticipated. Backreaming should not be necessary with the use of a rotary steerable system (RSS) as the continual rotation does not produce the “ledges”
created by conventional motors. Backreaming will be at the discretion of the directional drillers, depending on hole conditions, angle, and available surface RPMs of the motor.
8.4.6 Wiper Trip Guideline
No wiper trip is planned for at TD of the hole sections or over intermediate depths.
However, wiper trips may be performed if hole conditions dictate.
The following are intended as a guideline if wiper trips should be considered:
a) Does the theoretical vs. actual cuttings-removal / mud-loss plots, provided by mudloggers, suggest the hole has been cleaned?
b) Is there persistent drag, over and above, the normal trend during connections and backreaming?
c) Has “spotty” torque been recorded over the lower hole section suggesting cavings in the annulus?
d) Does the ECD sub confirm the hole is clean?
8.5 Running Liners - WIP 9-5/8” Liner Running and Cementing Operations
11 ¾” liners have been run on Valhall as a contingency in the event of unforeseen hole problems. Due to the weak formation surge pressures must be kept as low as possible to avoid major losses/ borehole collapse while running in. Due to the small clearance when running in the 11 ¾” liner, flow diverter tools have been run which reduce surge pressures considerably. The following objectives are required on WIP which will run a 9-5/8” liner.
Objectives
• Run 9-5/8” liner to 50 m TVD above the Intra Late Eocene with minimum surge damage to formation.
• Provide a seal with cement at the shoe that is sufficient to drill the next section without creating any formation stability problems below the shoe. After cementing allow the mud weight to be increased from 14.6 ppg to 15.0 ppg without any losses.
• Set an integral liner hanger packer inside the 13-3/8” casing to provide a seal against any potential flow of formation fluids - no cement in liner lap.
General
The 9-5/8” liner is set just above the Intra Late Eocene to cover the weak zone below the 13 3/8” shoe, the gas cloud and potential troublesome faults.
1. It is of utmost importance that the hole is 100 % clean prior to run casing.
2. Liner overlap is planned to be ±50 m into the 13–3/8” casing.
3. Liner is usually run slick.
4. In order to minimise surge pressures while running and avoid taking returns up the drill string, the liner can be run with a flow diverter tool (Baker Oil Tools Hyflo valve or with a 4” bore running tool Type: BIG BORE and a flow diverter tool (Weatherford/Allomon)) installed 1 stand above the liner running tool. This tool when open diverts the flow coming up the liner through ports to the 5 ½” DP x 13 3/8”
casing annulus. The tool is closed by a 2 ¼” brass ball that is allowed to free-fall to the tool (same ball that is used for activating the setting of the hanger).
The following actions can be applied to reduce surging of the formation while running the liner.
1. A float less liner with surge reducing diverter will be run 2. Running speeds will be optimised based on returns observed 3. Pick-up and set-down in the slips will be consistent and slow
4. Actual hookload will be plotted vs. theoretical to determine cuttings build up/hole condition. (See example plot 11)
5. No attempts will be made to circulate in order to break circulation, stage in or to warm up mud, this is to prevent formation damage and losses. (Ref F3A).
Plot 11: pick and slack off weights running 11 ¾” liner contingency F19
2/8-F-19, M. Guardian
Running in with 11 3/4" 65.0ppf Liner w/250m 5 1/2" HWDP
& 5 1/2" DP to Surface (78000 lbs Block weight included.)
150000
600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 2100 2200
Measured Depth [m] 13 3/8" Csg at 1585mMD
Torque [ft-lbs]
Start to pull the string a few metres.
Hookloads taken from mudloggers charts - full hookload not neccesarily required to
pick string up from slips
8.6 Limestone Stringers
Limestone stringers increase in frequency towards the end of the overburden section. A record of all limestone stringers is kept on the drill floor to assist the driller during trips.
Limestone stringers, when pulling out of hole, have frequently given mechanical overpull and packoff problems on Valhall wells. On F-19 the only problems when pumping out of the 12 ¼”
hole were associated with limestone stringers. Fortunately, the frequency and thickness of the limestone stringer intervals on F-19 were much less than on F-3A, and they did not prove a serious problem.
What was noted on F-19 was that after pumping slowly though troublesome stringers (with overpull and pack-off tendency), the zone was then wiped and the problem fully alleviated. For future wells, with a greater frequency of stringers, the wiping of such zones prior to drilling to TD should be considered seriously, with the implementation of wiper trips should it be necessary.
Packing off has occurred across the limestone stringers where cuttings beds may build up, this is more commonly seen when an under reamer is in the string.
Hard limestone stringers in relation to sliding directional activities
From the outset it was planned to rotate through limestone stringers. From a practical viewpoint, if sporadic thin stringers of indeterminate zonal thickness are encountered during a steering interval, the tendency of the directional driller is, if the toolface remains true and ROP tolerable, to proceed steering.
On one occasion during the 12¼” section of F19, when dropping through a thin stringer in steering mode the motor immediately stalled giving a pressure increase of 500 psi above normal drilling pressure. This pressure was only held momentarily before the motor became free. The release of the drillpipe pressure gave a pressure surge in the annulus, measured at 15.15 ppge EMW (MW 14.6 ppg, drilling EMW 14.85 ppge).
In this case, the pressure surge was held by the formation without consequences, however in weaker formations these short-lived pressure pulses may be sufficient for fracture generation and subsequent borehole stability problems.
An increase in drag is often associated with these limestone stringers and often an increase in torque.
8.7 Torque and Drag
The torque & drag plots are an essential tool for monitoring both hole cleaning and borehole stability.
It is essential that pick up and slack weights are recorded on each connection along with torque.
The theoretical torque & drag data, calculated with Baker Hughes Deapteq software using friction factors refined over several wells to accurately represent Valhall formations, are plotted prior to drilling and running casing. The data can then be used as a guideline, against which the actual values are plotted.
A plot is kept on the drillfloor, and regularly updated by hand after each connection, as well as on the DOE computer.
Close monitoring of these plots during F-15 and previous wells allowed the DOEs to accurately determine hole condition, with regards to cleaning and stability.
It was seen on F-15 and previous wells, that two phenomena give characteristic increases in hole drag;
1. Limestone stringers, which by their nature give increased mechanical drag. Experience has shown that this drag increase is little to do with borehole stability, but does have consequences when pulling out through the stringers.
2. The Balder Formation, which is typified by borehole instability.
The current learning is that any increase in hole drag on Valhall wells which is not related to the two examples above is almost certainly due to inadequate hole cleaning. In such cases it has been seen that circulation with rotation will clean the hole and reduce the drag.
.
Plot 12: torque and drag plot
2/8-F-19, M. Guardian
Drilling with 12 1/4" Speedreamer and 8" M1XL to TD at 3367m 11 3/4" Liner shoe at 2164mMD
(78000 lbs Block weight included.)
150000
2000 2100 2200 2300 2400 2500 2600 2700 2800 2900 3000 3100 3200 3300 3400 3500
Measured Depth [m] Calc. off bottom torque Actual off bottom torque
Friction Factor;
Ff_csg. : 0,12 Ff_ open Hole= 0.13
11 3/4" Liner at 2164mMD Torque [ft-lbs]
frequent limestone stringers circ BU at 3020m circ BU at 3251m & 3260m TD at 3367m. Circ clean, increase MW to 14.7 ppg
circ BU at 3191m circ BU at 3333m
circ BU at 2907m circ BU at 3077m & weigh up to 14.6 ppg
drill with 14.5 ppg mud up to 20% cavings (related to top Balder)
occasional limestone stringers
8.8 Cycling Pumps
If problems occur with MWD/ Autotrack / Powerdrive tools it is bad practice to cycle the pumps. Bringing the pumps up too quickly will cause pressure spikes. The procedures for pumping as mentioned in the connection procedure should be followed at all times.
Plot 13 On A3D the pumps were cycled which resulted in a pressure spike up to 15.18 ppg.
9.0 LOST CIRCULATION
Lost circulation has been common problem on Valhall. Losses can occur to faults, natural fracture or mechanically induced fractures,
Solution to Lost Circulation Problems
• Recognition of the loss
• Identification of the cause of the loss
• Location of the “thief” zone
• Proper remedial treatment to combat the loss
Recognition of Borehole Problems May be made by:
• Pit volume totalizer (PVT)
• Flow return sensor
• Pump pressure reduction
• Drilling break - often first indication
Mechanically induced fractures
One of the main causes of lost circulation on Valhall is mechanically induced fractures Causes
1. Running in too fast after trip producing high surge pressures. Also when running casing.
2. Excessive pump pressure to break circulation after trip.
3. Cuttings or sloughing shale close off annulus --pressure build up in system.
4. High gel strengths & viscosity -- requiring higher pump pressure to break circulation 5. Unbalanced mud column -- raising mud weight too rapidly.
6. Opening pumps too rapidly.
7. Pipe whipping.
8. Pressure developed while killing a well.
8. Effective circulating density greater than fracture gradient.
Preventative measure to avoid inducing fractures
1. Rotate drill pipe before starting pump -- a lower pressure will be required to break circulation
2. Break circulation slowly
3. When running in hole, do not run in too fast and break circulation at intervals 4. Avoid thick wall cake which reduces hole diameter and can thus increase surge pressures
5. Maintain the minimum mud weight necessary to control the formation pressure of the borehole
6. Avoid excessive annular velocities which result in high annular pressure losses, and thus high circulating pressures
7. Maintain the minimum mud viscosity, gel strength and yield point to prevent settling of the weight material
On Valhall, the commonly used type of LCM is the Norwegian blend
On Valhall, the commonly used type of LCM is the Norwegian blend