Unit 6 - Well Control Flashcards
What is well- control?
Control of formation pressure (primary- and secondary control)
What could be the consequences of a blowout?
- loss of human life
- loss of rig and equipment
- loss of reservoir fluids
- damage to the environment
- huge cost of bringing well under control again
On what depends the severity of a kick (fluid amount, which enters the wellbore)?
- type of formation → (porosity/permeability)
- pressure → the greater the negative pressure differential, the easier for formation fluids to enter wellbore
- nature of influx
What is primary well control?
- maintenance of positive differential pressure or overblance on formation pressure
When is secondary control necessary?
- necessary, when primary control has failed → formation fluids are flowing into well bore
What is the goal of secondary control ?
- stop flow of fluids into wellbore
- allow influx to be circulated to surface and safely discharged, while preventing further influx downhole
How come, that you loose primary control even if you calculated it before?
- formation pressure in penetrated zone is higher than predicted → because of that , mud weight is too low → bottom hole pressure is less than formation pressure
- Pressure due to column of mud decreases for some reason → bottom hole pressure drops below formation pressure
What are reasons, why the mudweight would reduce?
- solids removal
- excessive dilution of mud ( due to watering back)
- gas cutting of mud
How can the mudweight fall due to solids removal?
- cuttings are removed and if equipment is not properly designed, big amount of weighting solids (Barite) may also be removed → if barite is removed, you have to replace it, before circulating it down again
How can the mudweight fall due to dilution?
- To improve viscosity of mud , it is usually diluted with water (called water-back)
- or water is added for very deep drilling operations, where evaporation might be significant
What is gas cutting?
gas seeps from formation into mud → reduces density of mud
What are reason to why the mud column height may be reduced?
- tripping
- swabbing
- lost circulation
What will happen to the mud column while tripping?
- top of mud column will fall as drillpipe is pulled from borehole
→ therefore any point in wellbore has now less mud above itself → therefore bottom hole pressure will be reduced → hole must be filled up during tripping with mud
What is swabbing?
- process by which fluids are sucked into borehole from formation, when drillstring is pulled out of hole
How can swabbing happen?
- When bit has become covered in drilled material and drillstring acts like a giant piston when moving upwards → creates low pressure region below bit → formation fluids are sucked into borehole
How do you call the opposite effect of swabbing?
surging
The amount of swabbing will increase with ?
- adhesion of mud to drillpipe
- speed at which pipe is pulled
- use of muds with high gel strength and viscosity
- thick mud cake
- inefficient cleaning of bit to remove cutting
When does lost circulation occur?
- when a fractured, or very high permeability, formation is drilled → mud is lost to formation → reduction of column height in borehole
- another option: mudweight is too choosen too high and formation fracture gradient is exceeded
How can you minimise fluid losses to formation?
- use lowest practicable mud weight
- avoid pressure surges when running pipe in hole
- avoid small annular clearances between drillstring and hole
What is a possum belly tank/
- A trip tank, whit a small diameter to height ratio → it is therefore used to measure the amount of mud that is used to fill , or is returned from the hole, when the pipe is pulled from, or run intothe hole respectively
What are primary indicators of a kick?
- Increase of flow rate
- pit volume increase
- flowing well with pumps shut off
- inproper hole fillup during trip
what can be the reasons, that the well is flowing, when pumps are shut off?
- fluid is pushed out of annulus by some force → could be formation pressure, which is higher than hydrostatic pressure due to column of mud and therefore that an influx of fluid is taking place
- mud in borehole expands as it heats up → expansion results in small amount of flow when pumps are shut off
- if small amount of heavy mud has accidentally been pumped in and therefore mud in annulus is being displaced
What are secondary indicators for a kick?
- drilling break
- gascut mud
- changes in pump pressure
What is a drilling break ? (kick indicator)
-abrupt increase in rate of penetration → may indicate that higher pressure formation has been entered → therefore the chip hold down effect has been reduched and/or a higher porosity formation (for example due to under-compaction and therefore indicative of high pressures) has been entered
→ but an increase in drilling rate could also be simply due to a change from one formation type to another
What is gas cut mud?
when gas enters mud from formations being drilled → then the mud is said to be gas cut
what are reasons, why you receive a gas influx ?
- drilling into gas bearing formation
- swabbing when making a connection during trips
- influx due to negative pressure differential
What is the first thing you do if you see indicators of an influx into the wellbore?
- carry out a flow check
1. pick up kelly until a tooljoint appears above rotary table
2. shut down the mud pumps
3. set slips to support the drillstring
4. observe the flowline and check for flow from annulus
5. - If the well is flowing → close the BOP - If well is not flowing, resume drilling and check for further indications of a kick
What is the first step you need to do if primary control of a well is lost?
- close the BOP valves and seal off drillstring to wellhead annulus at surface
Why is it not necessary to close off the valves inside the drillpipe when primary control is lost?
Because since it is connected to the mudpumps, the drillpipe pressure can be controlled
What is the shut in procedure step by step?
- raise kelly above rotary table until a tooljoint appears
- stop the mud pumps
- close the annular preventer
- read the shut in drill pipe pressure, annulus pressure and pit gain
Why do you have to fully open the choke line before closing in the annular preventer while experiencing a kick?
- to prevent surging effects on the openhole formations
What do you have to do if you detect a kick while tripping?
- set top tool joint on slips
- install a safety valve (open) on top of the string
- close safety valve and annular preventer
- make up the kelly
- open safety valve
- read shut in pressures and the pit gain
We do we assume in a calculation, that the influx goes into the annulus?
- because you would expect it to follow the flow of fluids through the system when they enter the wellbore
Pressure gradient ranges of gas, oil and seawater?
- Gas: 0.075-0.150 psi/ft.
- Oil: 0.3-0.4 psi/ft.
- Seawater: 0.470-0.520 psi/ft.
What are the factors, that affect the annulus pressure?
- Size of the influx
- gas buoyancy effect
What can happen if you detect the influx late and have therefore a big volume of influx in the annulus?
- You have a higher pressure on the annulus (PAnn) and in general the whole pressure along the entire wellbore will increase → two things can happen:
1. At some point fracture pressure of one of the formations in open hole section may be exceeded → may lead to an underground blowout → formation fluid can enter the wellbore → leaving wellbore at some shallower depth
Explain the gas buoyancy effect! Why does it affect the annulus pressure?
- because of large density difference between gas and mud → gas bubble will be subjected to a large buoyancy effect → it will rise to the annulus and while it rises, it will expand and if the well is open it will displace mud from annulus
BUT: if well is not open→ mud cannot be displaced→gas can’t expand→gas influx will rise but will maintain its high pressure since it can’t expand
Result: PAnn will increase and higher pressures will be exerted all down to the well bore
How can you identify the fact, that gas is rising in the annulus with a very high pressure?
-by a simultaneous rise in drillpipe and annulus pressure → in this case the pressure must be bled off from annulus by opening the choke in very small doses
What is the MAASP?
- maximum allowable annular surface pressure → the maximum pressure, which can be allowed at surface before the fracture pressure of the formation just below the casing shoe is exceeded
Where is normally the weakest point in a drilled well?
- the highest point in the open hole section → that is at the previous casing shoe
Describe the volumetric method to circulate out an influx when there is no drillstring in the hole!
- uses expansion from gas to maintain bottomhole pressure greater than formation pressure
- pressures are adjusted by bleeding off at choke in small amounts
- once gas reaches surface, it is gradually bled off, whilst heavier mud must be circulated → can be done with a snubbing unit (equipment, which allows a small diameter pipe to get into hole through closed BOPs
What is stripping?
process, where pipe moves through closed BOPs under its own weight
What is snubbing?
- process, where pipe is forced through BOP mechanically
What are the two methods to kill a well with the drillstring inside the well?
- one circulation method
- drillers method
Describe quickly the one circulation method + the advantages of this method?
- circulate influx out and circulate in the heavier mud simultaneously
- influx is first of all removed with original mud
- then well is displaced to heavier mud during a second circulation
- safer, simpler and quicker than drillers method
Main disadvantage of the one circulation method?
- time taken to mix heavier mud which may allow a gas bubble to migrate
What kind of ram type preventers are there?
- Blind rams - completely close off the wellbore when there is no pipe in hole
- pipe rams - seal off around a specific size of pipe thus sealing off the annulus
there are also variable rams - close and seal off on a range off drillpipe sizes
- shear rams - same as blind rams but can cut through drillpipe for emergency shut in → only used as last resort
Where do you use a casing spool?
- wellhead, from which casing strings are suspended are made up of casing spools
→ casing spool will be installed after each casing string has been set → BOP stack is placed on top of casing spool and connected to it
What is a diverter system and when do you use it?
- diverter is a large, low pressure, annular preventer, equipped with large bore discharge flowlines → this type of BOP is usually used when drilling at shallow depths below the conductor
Why do you usually use a diverter system in shallow depths?
- because if the well was to kick at a shallow depth and you would try to close it in and attempt to contain the downhole pressure, it would probably result in fracturing the formations belwo the conductor →purpose of a diverter is it to allow the well to flow to surface safely →there it can be expelled safely through a pipeline leading away from the rig
What is the task of the choke line?
- to carry mud and influx from the BOP stack to the choke manifold
What is a kill line and what is the task of a kill line?
- a high pressure pipeline between side outlet, opposite of the choke line outlet on the BOP stack, and the mud pumps
- provides the option of pumping fluids downhole when normal method of circulating down the drillstring is not possible
What is a choke manifold?
- arrangement of valves, pipelines and chokes, designed to control the flow from the annulus of the well during a well killing operation
choke manifold must be capable of?
- controlling pressures by using manually operated chokes or chokes operated from a remote location
- diverting flow to a burning pit, flare or mud pits
- have enough back up lines if any part of the manifold would fail
- a working pressure equal to the BOP stack
What is a choke devicee?
Example of an internal blowout preventer?
- float valve: will prevent upward flow but allow normal circulation to continue → often used to reduce backflow during connections
What is a disadvantage of a float valve?
- you can not read the drillpipe pressure at the surface
What is ram to ram stripping?
- When the hydril fails and the pipe is stripped into the well using the pipe rams
- when the tooljoint reaches the upper pipe ram, the upper pipe ram is opened and then the tooljoint is allowed to pass → then upper pipe ram is closed and the lower opened to allow tooljoint to pass
