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Drilling Engineering > Unit 5 - Formation Pressure > Flashcards

Unit 5 - Formation Pressure Flashcards

1
Q

What is the formation pressure

A

the magnitude of pressure in pores of a formation

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2
Q

What is the overbalance?

A

the differential between mud pressure and pore pressure → mud pressure > pore pressure

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3
Q

How is a fluid containing salt called ?

A

Brine

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4
Q

What is the pressure gradient of pure water ?

A

0.433 psi/ft

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5
Q

What is the normal pressure gradient?

A

0.45 psi/ft. → (80000ppm salt content)

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6
Q

What is the overburden pressure or geostatic pressure?

A
  • the vertical pressure at any point in the earth → function of rock mass and fluid above point of interest
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7
Q

What is the bulk density ?

A

the average density of a rock, which includes the density of the rock matrix and the density of the fluid in the pore space

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8
Q

Does bulk density vary with depth? If yes, why?

A

yes, because rock type, porosity and fluid content vary with depth

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9
Q

What is a subnormal pressure?

A

pressure less than 0.45 psi/ft.

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10
Q

What is an overpressure?

A

Pressure > 0.45 psi/ft.

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11
Q

What is the most common mechanism for generating overpressure?

A

Undercompaction

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12
Q

What is necessary to creat abnormal pressures?

A

a seal

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13
Q

What kind of seals exist?

A
  • Physical seal → formed by gravity faulting during deposition or deposition of a fine grained material
  • Chemical Seal → may be due to calcium carbonate being deposited → therefore restricting permeability
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14
Q

What are the major mechanisms that create subnormal formation pressures?

A
  • Thermal expansion
  • Formation Foreshortening
  • Depletion
  • Precipitation
  • Potentiometric Surface
  • Epeirogenic Movements
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15
Q

Why is thermal expansion an origin for subnormal formation pressure?

A
  • when sediments and pore fluids are buried → temperature rise → if fluid can expand, density will decrease and pressure will reduce
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16
Q

Why is formation foreshortening an origin for subnormal formation pressure?

A
  • during compression process → bending of strata leads to upper beds being bend upwards, lower beds being bend downwards (will both be overpressured) → intermediate beds muss expand to fill void (subnormal pressured)
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17
Q

Why is depletion an origin for subnormal formation pressure?

A
  • when HC/Water are produced from competent formation and no subsidence occurs → subnormally zone could result
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18
Q

Why is precipitation (rain) an origin for subnormal formation pressure?

A
  • in arid areas water table may be located deep below surface → reducing hydrostatic pressure
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19
Q

What is the most common mechanism for overpressures?

A

Undercompaction

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20
Q

What is undercompaction?

A
  • the rapid burial of sand enclosed by impermeable barriers → no time for fluids to escape → trapped fluids help support the overburden
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21
Q

What are the major mechanisms to create overpressured formations?

A
  • Undercompaction
  • Faulting
  • Phase Changes during Compaction
  • Massive Rock Salt deposition
  • Salt Diaperism
  • Tectonic Compression
  • repressuring from deeper levels
  • generation of HC
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22
Q

What are drilling problems that are associated with abnormal formation pressures?

A
  • if the overbalance of the mud is too great, it can lead to:
  • reduced penetration rates → chip hold down effect
  • breakdown of the formation (exceeding fracture gradient) → subsequent lost circulation
  • excessive differential pressure, causing stuck pipe
23
Q

What is the transition zone?

A
  • Zone between normally pressured yone and an overpressured zone
24
Q

When will a transition zone be abrupt?

A

When the seal is of crystalline rock → no permeability at all → it is not possible to detect an increase in the pore pressure across the seal

25
Q

How can you detect transition zones?

A

Based on three forms of data:

  • drilling parameters (e.g. ROP)
  • drilling mud (monitoring)
  • drilled cuttings - examining cuttings, trying to identify cuttings from a sealing zone
26
Q

Why will the ROP decrease with depth if parameters stay constant?

A

Because the compaction of formations increases with the depth

27
Q

Why will you have a higher ROP once you drill into the the transition zone?

A

Because the transition zone is more porous than the normally compacted formation

28
Q

The “d” exponent equation is based on which assumptions?

A
  • rock, which is drilled will not change (a =1)
  • rotary speed exponent (e) is equal to one

this means: → equation can only be applied to one type of lithology and theoretically only at one single rotary speed

29
Q

Formula for d-exponent?

A

R= penetration rate (ft/hr)

N= Rotary Speed (rpm)

W = WOB (lb)

B = bit diameter (in.)

30
Q

What happens to the d-exponent when you drill into an overpressured zone?

A

compaction and differential pressure will decrease → ROP will increase → d-exponent should decrease

31
Q

Generally said: how can you identify an overpressured zone with the d- exponent?

A

You plot the d-exponent against depth and check where it reduces

32
Q

If you want to have accurate results with the d-exponent, what do you have to do ?

A
  • keep WOB and RPM as constant as possible
  • to reduce the dependance on the lithology → the equation should be applied over small depth increments only (plot every 10’)
  • for a nice trendline you need a good, thick, shale!
33
Q

Why is there an equation with a modified d-exponent → dc ?

A

Because the d-exponent doesn’t take mudweight into consideration, which causes 1. chip hold down effect and 2. a lower ROP

34
Q

Formula for the modified d-exponent?

A
35
Q

What is the advantage of the dc exponent?

A

it gives a better definition of the transition (includes variations in mudweight)

36
Q

Until when do you usually use the d-exponent?

A

generally used to simply identify the top of the overpressured zone

37
Q

Why can an increased torque also be a sign of an overpressured zone?

A
  • Because if the overbalance decreases, the borehole wall can start to break down, this will lead to excess accumulation of material next to the drill cuttings in annulus which will increase the torque

-

38
Q

What are the main affects on the mud due to abnormal pressures?

A
  • Increasing gas cutting of mud
  • decrease in mud weight
  • increase in flowline temperature
39
Q

What is a general problem with the effects of abnormal pressured zones on mud?

A

the effects can only be measured when the mud is back on the surface, which can take hours → detection of overpressured zones is delayed → during that time the bit could have penetrated very far into the overpressured zone

40
Q

What are the two ways of “gas cutting of mud”?

A
  1. From shale cuttings→ if gas is present in drilled shale, gas may be released into annulus from cuttings
  2. Direct influx → if overbalance is reduced too much or can also be due to swabbing when pulling back drillstring
41
Q

How come that there can be a decrease in mud weight due to abnormal pressures?

A
  • if gas influx→ mud density will largely decrease if it is a gas influx
42
Q

Why can you have an effect on flowline temperature due to abnormal pressures?

A
  • under-compacted clays with relatively high fluid content have a higher temperature than other formations
  • normally flowline temperature decreases through normally pressured zones → increase in temp. by drilling through overpressured formation
43
Q

What is a shale factor?

A
  • a technique, which measures the reactive clay content in cuttings → uses dye test to determine the reactive clay present which indicate the degree of compaction
44
Q

Which tests will be examined to figure out when a formation will fracture?

A
  • leak-off test
  • limit test
  • formation breakdown test
45
Q

What is the basic principle of a leak-off test, limit test etc.?

A

to determine the weakest part of the system on the assumption, that this formation will be the weakest formation in the subsequent hole

46
Q

What is the only difference between the tests to measure formation strength?

A
  • only the point, at which the test is stopped
47
Q

Procedure of a leak off test?

A
  1. run and cement casing string
  2. run in drillstring and drillbit for next hole section and drill out casing shoe
  3. drill 5-10 ft. of new formation below casing shoe
  4. full drillbit back into casing shoe (avoid chance to become stuck in open hole)
  5. Close BOPs (generally pipe ram) at surface
  6. apply pressure to well (pump 0.5 barrels at a time) → stop pumping, record pressure at surface, → repeat those steps

→ plot the volume of mud and the corresponding pressure at the surface for each increment

48
Q

Why do you do a leak-off test?

A

to determine the pressure at which the rock in the open hole section of the well just starts to break down → the operation is done when pressure does not increase anymore linearly

49
Q

Why do you usually pump another 0.25 bbl into the well during the leak off test even though there is already a deviation from the line?

A

To confirm, that the deviation from the line is not simply an error

50
Q

Why do you do a limit test?

A

To determine if rock in open hole can withstand a specific predetermined pressure → this pressure determines the maximum pressure that the formation will be exposed to whilst drilling next wellbore section

51
Q

What can you do if you know the leak off pressure?

A
  • Calculate maximum mudweight
  • calculate maximum allowable annular surface pressure (MAASP)
52
Q

What is the Equivalent Circulating Density (ECD of a fluid)?

A
  • when drilling fluid is circulated through drillstring, borehole pressure ist bottom of annulus will be greater than hydrostatic pressure of mud

→ the extra pressure is there because of frictional pressure required to pump a fluid up the annulus

→ you need to add this frictional pressure to the hydrostatic pressure from the mud column to get a true representation of pressure acting against formation at bottom of well

53
Q

How are fracture pressures estimated without testing?

A

from leak-off tests on nearby wells

54
Q
A

Drilling Engineering (13 decks)

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