TAMU Pemex Well Control

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TAMU - PemexWell Control

• Lesson 7B
• Other Abnormal Pressure Detection Methods

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Contents

• Moores Equation - Drilling rate
• Gas in the Drilling Fluid
• Rock Sample Characteristics
• Use of Surge and Swab Pressure to determine
  Overbalance
• Changes in Drilling Fluid Properties
• Temperature Indications
• Hole Conditions

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Moores Equation

• Moore proposed a practical method for maintaining
  pore pressure overbalance while drilling into a
  transition.
• If drilling parameters are kept constant while
  drilling into an abnormal pressure zone, the
  drilling rate will increase.

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Moores Equation

• Moore suggests that we increase the mud weight
  sufficiently to keep the drilling rate from
  increasing.
• The increase in mud weight will then be a measure
  of the abnormal pore pressure.
• But how much do we increase the mud weight?

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Moores Equation
Transition zone
Begin weighting up Weight up complete
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Ex. 2.10
?
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Example 2.10

• Bit parameters prior to transition were
• Bit Weight 4,700 lbf/in
• Rotary Speed 80 rpm
• Transition detected at 9,100 ft and the operator
  immediately reduced the bit weight to 2,900 lbf/in

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Example 2.10

• Determine the extrapolated normal penetration
  rate at a depth of 9,250 ft
• if the bit weight is reduced from its current
  value of 4,700 to 2,900 lbf.
• Use the data in Fig. 2.46 and Moores penetration
  rate model.

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Solution
Fig. 2.46

• The extrapolated normal penetration rate at
  9,250 is 15.7 ft/hr, at 4,700 lbf bit weight.
• This would have been the target rate had the bit
  weight remained constant.

9,250
15.7
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Solution contd

• The target penetration rate at the reduced bit
  weight of 2,900 lbf is calculated below

(assumes R a W)

• The target rate would revert back to 15.7 ft/hr
  if the operator resumes drilling at 4,700 lbf/in.

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Example 2.11 - Fig. 2.46
How much should we increase the mud weight?
(Moore)

• At 8,300 ft (under normal conditions) increase
  the ECD from 9.6 to 10.1 ppg.
• In response, the drilling rate decreases from
  20.5 ft/hr to 18.5 ft/hr
• What is the shale compaction coefficient, c?

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Solution

• ECD changes from r1 9.6 to r2 10.1 ppg
• Calculate c, the shale compaction coefficient
• (9.6)c log 20.5 (10.1)c log 18.5
• (10.1/9.6)c log 20.5 / log 18.5 1.035
• c log 1.052 log 1.035
• c 0.679

Now use Eq. 2.24 to calculate the required change
in mud weight
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Example 2.12
Fig. 2.46

• At 9,090 ft the normal penetration rate is 16.5
  ft/hr
• Actual penetration rate is 18.0 ft/hr, using a
  mud weight of 9.6 ppg
• Normal MW 8.3 ppg

9,090
Dr (10.0 - 9.6) 0.4 ppg
rp 8.3 0.4 8.7 ppg
16.5
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Other predictors of abnormal pressure

• Drilling rate is not the only available predictor
  of abnormal pressure.
• Properties of shale cuttings can be used

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Shale density
Offshore Nigeria
D_shale density from Boatman
Transition
rn ro - g/cm3
Density - g/cm3
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Example 2.15
pp_14,000 ?
rn 2.54
ro 2.44
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Solution

• At 14,000, rn 2.54 and ro 2.44 g/cm3
• so, Dr 0.1 g/cm3
• From Fig. 2.48
• p14,000 0.05214.614,000
• p14,000 10,629 psig

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Dshale density from Boatman
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Shale density measurement

• 1. Fill a standard API mud balance with shale
  cuttings (wash and dry with a towel) until
  balance reads 8.33 ppg.
• 2. Fill the cup to top with water and record
  reading (e.g. 13.3 ppg).

8.33
Calculate S.G. of shale cuttings S.G.
8.33/(16.66 - 13.3) S.G. 2.48
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Possible Sources of Gas in a Drilling Fluid

• Drilled gas,
• Produced gas
• Recycled gas
• Contamination gas

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Possible Sources of Gas in a Drilling Fluid

• Drilled gas, cuttings gas, or liberated gas
  refers to gas released from rock cuttings
  generated by the bit. Usually small volumes.
  Increasing MW will not help.
• Produced gas refers to gas which enters the
  wellbore from the walls of the hole. Increasing
  MW will reduce the quantity.
• Recycled gas is any wellbore gas that remains in
  the mud after at least one pass through the
  surface equipment.
• Contamination gas is gas released from any
  volatile hydrocarbons intentionally added to the
  system (mud additives).

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Gas in Mud

• Connection gas - gas that has entered the
  wellbore when pumps are shut down to make a
  connection, can be detected in a gas trap.
• Trip gas - gas that entered the wellbore during a
  trip can also be detected.
• Background gas - gas baseline concentration in
  the mud usually small.

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Example 2.17

• Determine the density of the gas-cut mud returns
  from a well at a depth of 2 ft below flowline
  outlet if
• Clean MW 12.0 ppg
• Flowline MW 7.0 ppg
• Atmospheric press 14.7 psia
• Sample temperature 100 deg F
• Gas gravity 0.6

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Solution At the Surface

• From Eq. 1.22,
• rg gg p/(2.77 Z T)
• rg,surface 0.614.7/(2.771560)
• rg,surface 0.00569 ppg

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Solution At the Surface

• From Eq. 2.26,
• rgm rm (1 - fg) rg fg
• fg (rm - rgm)/ (rm - rg)
• (12.0 7.0/(12.0 - 0.00569) 0.417
• This is the gas fraction at the surface, but fg
  varies with depth.

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Solution contd

• By definition, fg Vg /(1Vg)
• so, Vg fg / (1- fg), but, pV ZnRT
• n fg p / ZRT(1- fg )
• n 0.000234 lb-moles/gal of mud
• This parameter stays constant with depth provided
  the downhole gas entry rate remains constant.

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Solution 2 ft down

• Assuming the density of the mud-gas mixture does
  not change appreciably over two ft of depth.
• p2ft 14.7 0.052 7.0 2 15.43
  psia
• rg,2ft 0.6 15.43 / (2.77 1 560)
  .00597 lbm/gal

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Example 2.17 contd

• fg,2ft (1 0.000234 80.275 560) / 15.43
• 1(1 0.000234 80.275 560) / 15.43
• fg,2ft 0.405 (down from 0.417 at the
  surface)
• r2ft 12 (1 - 0.405) 0.00597 0.405
• r2ft 7.14 ppg
• This is an increase of 0.14 ppg in just 2.
• See Fig. 2.51 for plot of entire range

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Clearly most of the gas expansion is near the top
of the wellbore. At 10,000, MW 11.9
ppg. What is the resulting reduction in BHP due
to the gas?
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Example 2.18

• What is the total change in HSP at the bottom of
  the well described in Ex. 2.17? Average
  temperature is 150 deg F.
• From Eq. 2.28

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Example 2.18 contd

• In the annulus, without, gas
• BHP 12 10,000 / 19.25
  6,233.8 psig
• BHP 6,248 psia
• Average pressure (14.7 6,248) / 2
  3,131 psia
• From Fig. 1.6, Zavg 0.868

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Example 2.18 contd

• If pgm 6,248 psia, then
• Dpred 60 psi
• pgm 6,248 - 60 6,188 psia
• EMW (6,188 14.7) / (0.052 10,000)
  11.87 ppg

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Gas cut mud

• A second iteration is generally not necessary if
  the assumed value for pgm is reasonably close to
  the calculated value.
• Furthermore, adding gas to a drilling fluid will
  increase viscosity, so the annular friction drop
  will increase, partially off-setting any
  reduction in BHP due to gas.

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Gas cut mud

• Another factor that will tend to offset the
  reduction in mud density is drilled cuttings.
• At a moderate to high drilling rate, the quantity
  of cuttings present in the mud at any time, may
  be significant.

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Gas in mud

• Gas in mud is monitored as the mud exits the
  flowline. A gas trap is placed to sample the gas
  before the mud passes over the shale shaker.
• The gas concentration is recorded in arbitrary
  gas units.
• Look for relative changes.

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Gas detection unit
Gas detector located in the shale shakers possum
belly. BBG Background gas This is the baseline
gas concentration in the mud, and is usually in
the order of a few gas units. CG Connection Gas
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CG constant BGG constant Overbalanced
CG increases BGG increases Underbalanced
CG increases BGG constant ?
CG increases BGG increases Transition zone
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Measuring Surge Pressure
Swab pressure is hard to measure, but surge is
not. Run one stand of pipe in hole at constant
velocity. Repeat at different velocities. Plot
surge pressure vs. pipe velocity.
Flowline
Mud Level
Closed Safety Valve
Pressure Recorder Sub
Drillpipe
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Measuring Surge Pressure
By assuming surge swab, we can predict
the swab pressure at different pipe pulling
speeds.
Surge/Swab Pressure, psi
Pipe Velocity, ft/sec
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Example
67 sec/std 59 sec/std 48 sec/std
452 min-units 1,036 min-units 2,132 min-units
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Example

• Estimate the pore pressure at TD if MW 11.7
  ppg The length of each stand is 90 ft.
• V1 90 ft / 48 sec 1.88 ft/sec
• V2 90 / 59 1.53 ft/sec
• V3 90 / 67 1.34 ft/sec

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Example

• From Figure
• Dp1 405 psi
• Dp2 300 psi
• Dp3 242 psi

Surge/Swab Pressure, psi
Pipe Velocity, ft/sec
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Example

• From Figure, plot of gas units vs. swab pressure,
  
• when line is extrapolated to zero velocity (zero
  gas), overbalance is found to be 197 psi

Gas Units
0
197
Pressure, psi
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Example

• With an overbalance of 197 psi
• Pore pressure MW - (overbalance)
• 0.052 TD
• Pore pressure 11.7 - (197 / 0.052 13,600)
• pp 11.4 ppg.

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Changes in drilling fluid properties

• Gas in mud
• reduced density
• increased viscosity
• Salt water inflow
• chloride content

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Changes in drilling fluid properties

• Salt water inflow
• Chloride content
• Flocculation of sodium bentonite clay
• increases yield point
• increases gel strength
• increases water loss
• poor filter cake
• pH change

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Changes in drilling fluid properties

• Drilled rock salt can have similar effect
• CO2 and H2S may reduce pH
• H2S is very poisonous and is corrosive
• Raise pH and precipitate out any soluble
  sulfides using scavengers.

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Temperature and abnormal press.
Geothermal temperature vs. depth
Poor conductivity requires higher temperature
gradient to maintain constant heat flux.
Undercompacted rock Lower thermal
conductivity Rock conducts heat better than pore
fluid
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Temperature indicators

• Temperature gradient tends to increase within a
  pore pressure transition
• Rock grains have a much higher thermal
  conductivity than pore fluids
• Well planning predictions may be assisted by
  downhole temperature measurements in offset wells

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Temperature indicators

• Note that wellbore circulation of fluids will
  distort the true temperature profile.
• The drilling fluid temperature increases as the
  fluid moves down the drillpipe.
• As fluid enters the annulus its temperature
  increases for a short while.
• Higher up the annulus temperature decreases

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Flowline temperature from a North Sea well
Predictable increase in temperature of mud
returns as depth increases
Important tool if no shales are present
A deviation from the normal temperature trend may
signal abnormal pore pressure
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Hole Conditions

• Drilling torque when rotating pipe, and drag
  during trips or connections, result from friction
  between the drillstring or bit and the walls of
  the hole.
• Torque and drag (TD) will generally increase
  with depth, gradually.

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Hole Conditions

• A sudden increase in TD may be caused by hole
  instability.
• Circulate bottoms up and observe samples.
• If abnormal pressure caused an increase in TD,
  the rock samples will help to tell the story.

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Sample Shale Cuttings
Abnormally pressured shales
Normally pressured shales