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Drilling Engineering Week


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Name the three classifications of rock.
1. Igneous rock (molten rock that cools and crystalizes)
2. Sedimentary rock (rock fragments joined together)
3. Metamorphic (high temp/high pressure)
State what is meant by clastic rock and list the processes involved in forming clastic rocks.
Clastic rock is formed from rock particles and skeletal remains. The process is as follows:

1. Erosion
2. Transport
3. Deposition
4. Compaction
5. Cementation
6. Diagenesis
Name the three types of sedimentary rock and how they are formed.
1. Clastic (formed from rock particles and skeletal remains)
2. Chemical (deposited by evaporation of water)
3. Biological (Remains of plant and animal life)
Name problems associated with drilling shales.
1. Reactive shales hydrate and destabilize
2. Fractured or stressed shales can be very unstable
Name problems with drilling limestone
1. Fractured and vugular limestone may cause total losses and stuck pipe
2. Bentonite based mud systems may flocculate with drilled limestone
3. Chert inclusions can destroy most bits
Name problems with drilling coal
1. Brittle and often fractured; if not naturally fractured, drilling stress may fracture it
2. Stuck pipe due to blocks of coal falling in
3. Gas might be present
4. Overgauge hole and ledges
5. Total losses in fractured coal
5. Can ignite if drilled with air
Name problems with drilling salts
1. Very high pressure kicks possible in or just below salts
2. Muds and cements must be designed for the salt present
3. Stuck at bit while drilling
4. Casing must be designed for a mobile formation
Name or describe one major process occuring at divergent, convergent, and sliding plate boundaries.
Divergent (crust moving apart): earthquakes, new crust formation
Convergent (moving together): earthquakes, volcanoes, mountain formation
Sliding (sliding next to each other): earthquakes
List the four types of rock deformation arising from stresses in the rock
Changes in porosity
Name the three types of fault and in each case state what sort of crust movement is the cause
Normal fault (crust stretches)
Thrust fault (crust compacts)
Strike-slip fault (crust slides)
How do you determine the principle stressed (sigma 1,2,3) in a tectonically relaxed area?
sigma one: Integrate a density log
sigma two: Determined from well offset instability
sigma three: leak off test or Eaton equation
Define normal compaction
When fluid is able to leave the pore spaces during burial and the rock compacts without abnormal pore pressure
State the effective stress law
overburden = effective stress + pore pressure
Describe how sonic log measurements deviate from the normal compaction trend in undercompacted zones
Under normal compaction, the sonic velocity approximates a straight line on a semi-log plot against depth; in undercompacted zones, DT does not reduce as quickly as in a normal trend.
Define overburden and describe how it is calculated
Overburden is the vertical "weight" of the rock; it is calculated as the sum of effective stress and pore pressure
List signs of increasing pore pressure while drilling.
1. ROP changes
2. Drilling torque and drag
3. Changes in volume or appearance of cuttings
4. Gas dissolved in mud
5. Mud temperature at the flowline
6. Mud resistivity/salinity
What are the two types of formation integrity test (FIT)? What is a FIT?
An FIT sets a maximum pressure that should be imposed at the casing shoe to avoid fracturing the formation. The two types are a leak off test and a limit test.
What are the two methods of FIT?
Hesitation and continuous
What are the limitations to the 'd' exponent?
1. Works only in softer rock
2. Needs longer transition zones
3. Cannot account for lithology changes
4. Does not work well with PDC bits
Why shouldnt the buoyancy factor be used to calculate the axial force in a submerged object?
Buoyancy factor should not be used when calculating axial force in a submerged pipe because buoyancy forces arise from hydraulic loads acting at specific points.
Define a kick
A kick occurs when the formation pore fluid flows into the wellbore due to insufficient mud hydrostatic pressure
What might cause a kick?
1. Drilling into an overpressure formation
2. Not keeping the hole full of mud
3. Swabbing
4. Reduction of the mud gradient due to contamination or poor maintenance
What do you do when you detect a kick?
1. Close BOP
2. Remove influx from annulus
3. Adjust the mud hydrostatic to balance the formation pressure
What are the two main methods of kick control?
1. wait and weight
2. driller's method
What are the differences between the wait and weight method and the drillers method?
1. No. of circulations
2. Time to start job
3. Total job length
4. Difficulty of calculation
Name three ways to avoid gas entry and migration in setting cement?
1. Right angle set
2. latex additives
3. Minimize free water and fluid loss
Why do we generally use the driller's method for kicks that occur when running casing?
1. Kick likely due to swabbed influx, so same mud gradient needed
2. Not enough info for wait and weight method
3. No improvement in open hole pressures
What is the volumetric method?
Choke off the well, releasing influx as the annulus pressure reaches the desired choke pressure. This does not kill the well, just allows the influx to safely migrate to the surface.
What is a kick tolerance? What two things define a kick tolerance?
A kick tolerance is a measure of teh volume and pressure of a gas influx that can be safely circulated out of the well. It must state the assumed pore pressure of the kicking formation and the acceptable influx volume.
List cases where the pore pressure of a kicking formation might be higher than normal.
1. wildcat well
2. where high pore pressures might be present, eg salt diapirs
3. where moderate to highly overpressured zones might be present in development wells
What risk factors might increase the volume of an acceptable kick?
1. high expected permeability
2. short transition zones
3. drilling from a floating rig
3. kick detection equipment is to a low standard
4. crews are not trained in well control
When shouldnt standard kick tolerance policies be used?
1. slim hole sections
2. HPHT hole sections
3. when oil-based muds are used
4. where the BOP is not nippled up
Define stress. What are the units?
load/cross sectional area
Define strain. What are the units?
Stretch/original length
No units
What is Hooke's law?
E=stress/strain, within the elastic limit
What are three factors that reduce the strength of downhole tubulars?
1. corrosion
2. wear
3. Increased temperature
What is the worst form of corrosion?
How do you reduce wear?
1. Reduce the side force exerted by tooljoints
2. Use drilling fluids with lubricating solids
3. Use sharp tong dies
4. Minimize rotating hours
In non-corrosive environments, what stress limit should you use to reduce fatigue damage?
1/2 of the UTS
What is the difference between a safety factor and a design factor?
A safety factor uses a rating based on catastrophic failure, a design factor uses a rating which can be based on almost anything
What is the difference between uniaxial and triaxial stress?
Uniaxial -- one force, one direction
Triaxial -- maximum combined stress from all forces
List the five major steps in the casing design process, in order
1. data collection
2. preliminary design
3. detailed design
4. triaxial analysis
5. documentation of design decisions
What three types of load cases are used to evaluate designs?
1. burst and collapse loads
2. installation axial loads
3. service loads
Define a "connection"
A mechanical device used to join lengths of pipe, downhole devices, and/or accessories to form a tubular (pipe) string of specific geometric and performance characteristics
What are the four types of API connections?
short round, long round, buttress, and extreme line
What's the rule of thumb in buckling?
Casing is unlikely to buckle unless the TOC is deeper than 4000 ft, the uncemented annulus is vertical, and the internal pressure/temperature is increasing after cementing
Changes in buckling conditions come from?
1. temperature changes after production starts
2. internal pressure changes due to tubing leaks
3. external pressures changes from mud solids settlings out
Define lost circulation
Lost circulation is when whole mud leaves the circulating annulus downhole
What two conditions are both necessary for losses to occur?
1. the pressure in the wellbore exceeds pore pressure
2. there is a pathway for the fluid to flow out of the wellbore
List pressure related loss prevention techniques
1. keep mud gradient/gel strength low
2. RIH at moderate speeds, circulate slowly
3. Run casing slowly
4. Use shear thinning rheology
5. Use the hesitation method for FIT
6. Ensure sufficient kick tolerance is maintained
What are the four pathways for losses?
natural fractures, cavernous spaces or 'vugs', very high permeability, or induced fractures
What are the four catagories of lost circulation?
Seepage (up to 10 bbl/hr)
partial (10-50 bbl/hr)
severe (over 50 bbl/hr)
total (mud stays below the surface whether circulating or not)
What formation is associated with seepage, partial, severe, and total losses?
seepage- hard rock wtih small fractures
partial- gravel beds or other coarse, unconsolidated formations, small or induced fractures that are only slightly open
severe- long sections of gravel or coarse sand, natural or induced fractures
total- cavernous formations, large fractures
How do you cure seepage or partial losses?
1. Increase the plastering capability with LCM
2. Increase low shear rate viscosity
How do you cure severe or total losses?
1. Use cement to fill the loss zone
2. Drill blind
(only good for shallow sections, shouldnt be done with a diverter or when hydrocarbon zones are exposed)
What to do when you have losses on bottom and a kick higher up?
1. pump LCM slugs into annulus
2. pick up bit off bottom
3. pump mud into string
4. set casing ASAP
What to do when you have a kick on bottom and losses higher up?
1. Cannot kill well conventionally
2. Barite or cement plus should be set to try and stop the flow
3. Set the casing before drilling ahead
What are success factors when curing total losses with cement?
1. Drill completely through the total loss zone before spotting cement
2. Dont use LCM
3. Use sufficient cement
4. Ensure that cement is not displaced away from the wellbore
What are the problems with drilling blind?
stuck pipe
loss of surface samples of cuttings
cost of lost mud
should run a float in teh string
long flow check in casing if cant fill hole
what are the success factors with drilling blind?
1. must maintain enough AV to move cuttings to loss zone
2. closely monitor torque/drag
3. use water if no reactive clays present
4. use reduced WOB and RPM
Define wellbore instability
When the wellbore diameter doesnt stay the same as the diameter of the bit that drilled it
What are the mechanisms of wellbore instability?
shale hydration, pore pressure penetration, fractured rock, plastic deformation and creep, erosion, and effective stress exceeds rock strength
What is the rule of thumb for drilling in regards to principle stresses?
Wellbore is most stable if the principle stresses are as equal as possible. Avoid drilling parallel to sigma 2.
What are the symptoms of shale hydration?
soft cuttings, clay ball in flowline, bit and stabilizer balling, tight hole tripping, problems increase with time
How do you minimize shale hydration?
keep water away from the shales, use additives which inhibit hydration, minimize open hole time
What are symptoms of pore pressure penetration?
tensile cavings, cavings increase after wiper trips
How do you prevent pore pressure penetration? How do you mitigate PPP?
1. Use oil based muds or mud which plugs the pore throat (TAME or silicate muds)
2. Circulate to a specialty mud system, slowly and continuously increase the mud gradient, use good connection practices, avoid wiper trips, minimize open hole time
What are the symptoms of fracturing?
High level of cavings with angular sides, amount of cavings increase after wiper trips
How do you prevent fracturing? Mitigate it?
1. minimize pressure surges, slowly trip, use plugging mud additives, avoid OBM, minimize lateral vibrations
2. Same as prevention, minimize open hole time
What are the symptoms of creep? How do you prevent it?
Tripping through newly drilled formation can be tight, bit feels "grabbed". May not be possible to prevent.
How do you mitigate creep?
Spot fresh water around the stuck point, program wiper trips every 18 hours or so, ream out tight spots when running back in.
What is the main symptom of erosion?
Overgauge hole forms quickly
How to you prevent erosion?
Avoid high tricone bit HSI, optimize for jet impact instead of HHP, maximize ROP. There is no cure for erosion.
What are the symptoms of effective stress?
Large amount of blocky cavings, hole geometry large on one axis than the other.
How do you mitigate effective stress problems?
Adjust the mud density, minimize pressure fluctutations, avoid serious lateral drill string vibrations
Define stuck pipe.
When the tool or equipment in hole cannot be pulled out without exceeding the working load.
Major signs of a stuck pipe?
The drill string may not be free to rotate, may be unable to circulate, or it may not be possible to lower the drill string
What are the 2 methods of identifying the depth of the free point?
free point indicator tool & stretch data
What are the three minor stuck pipe mechanisms?
running into partially set cement, running into distorted casing, junk falling downhole
What are the three major mechanisms of stuck pipe?
problems due to whole geometry, problems due to solid buildup, differential sticking
Major mechanism of stuck pipe that prevents movement in any direction but allows circulation
hole geometry problems
Major mechanism of stuck pipe that prevents movement and circulation
solids build up
4 conditions of differential sticking
overbalance, filter cake, wall contact, stationary pipe

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