DRILLING ENGINEERING I (CGE577) CHAPTER 4:
FORMATION PRESSURES
Contents 2
Definition of normal pressure, abnormal pressure, overburden pressure and fracture pressure Origin of abnormal pressure Leak Off Test Procedure Pore Pressure Profile Prediction and Detection of Abnormal Pressure
Contents 2
Definition of normal pressure, abnormal pressure, overburden pressure and fracture pressure Origin of abnormal pressure Leak Off Test Procedure Pore Pressure Profile Prediction and Detection of Abnormal Pressure
Why is Formation Formation Pressure so Important Important?? ?? 3
It is an important consideration in many aspects of well planning and operations. To be able to design casing and mud weight selection. To be able to predict and detect high pressure zones where there is the risk of blow-out. To be able to predict pressure at which the rocks will fracture losses of large volumes of drilling fluids influx from shallow formation blow out
Blowout of Deepwater Horizon semi-submersible Mobile Offshore Drilling Unit (MODU), Off Louisiana, 20th April 2010. 4
Happened on 20 April, 2010 while drilling Macondo Oil Prospect. Killed 11 Workers, injured 16 others Deepwater Horizon Rig (Transocean) Drilling at 5000‟ water depth and under 13000 ft seabed Transocean executive- “Abnormal pressure had accumulated inside the marine riser and as it came up ,it expanded rapidly and ignited (blowout)”. BOP failure was blamed by BP. BOP Manufacturer: Cameron International Corp. 12,000 – 19,000 bopd leaks
Formation Pressure 5
• Formation pressure/Pore pressure:
Pressure of fluid contained in pore spaces of the rock (spaces between grains)
Formation Pressure (Cont‟) 6 •
•
•
The pressure in the formations to be drilled is often measured in pressure gradient. In vertical column of fluid, gravity causes the pressure inside the fluid to change with depth. The pressure in the fluid at a particular depth has to support the weight of the fluid above that depth.
7
Figure 3-3: P-Z Diagram illustrates how measured pressure at any point increases steadily as you move down a column, in direct proportion to the depth and density of fluid. If the density of the fluid changes, so does the slope of the curve.
0.1 psi/ft
0.465 psi/ft
0.35 psi/ft
Formula for Pressure 8 •
Pressure at the bottom of a column of fluid = Density x Height x Constant*
*The constant depends on the units you chose
Density
Height *Constant Answer is in (depth)
g/cc, kg/l, SG, RD
metres
9.81
Kpa kilopascals
g/cc, kg/l, SG, RD
metres
1.42
psi (pounds per square inch)
ppg, lbs/gal
feet
0.052
psi (lbs/in 2 )
• Example; the pressure exerted by a column of seawater 1000 m deep is: Pressure = 1.03 g/cc X 1000 m X 1.42 = 1,463 psi Pressure = 1.03 g/cc x 1000 m x 9.81 = 10,104 Kpa Pressure = 8.58 ppg x 3281 ft x 0.052 = 1463 psi
* Hydrostatic pressure (psi) = 0.052 x
f (ppg)
* Pressure gradient (psi/ft) = ppg x 0.052
x D (ft)
Subsurface Pressures 9
Normal Abnormal (Subnormal)
Abnormal (Overpressure) Subsurface Pressures
Overburden
Fracture
1. Normal Pressure (Hydrostatic Pore Pressure) 10
is a pore pressure created by salt water column Most of the fluids found in the pore space of sedimentary formations contains a proportion of salts (brines). Dissolved salts may vary between 0 to over 200,000ppm. Pore Pressure gradient:
Pure water = 0.433psi/ft Salt water = 0.442-0.478psi/ft Most geographical area = 0.465 psi/ft (assumes 80,000 ppm salt content) normal pressure gradient/hydrostatic pressure.
Example-1 12
Compute the normal formation pressure expected at a depth 8,500 ft in the Malaysian basin area. The normal gradient in Malaysian Basin is 0.442 psi/ft.
Solution : HP (psi) = 0.052 x f (ppg) x D (ft) HP = ?????
Example-2 13
Calculate the pressure gradient of 10 ppg of mud Solution: Pressure gradient (psi/ft) = ppg x 0.052 = 10 ppg x 0.052 = 0.52 psi/ft
2. Abnormal Pressures 14
Is a pore pressures which are found to lie above or below the normal pore pressure gradient line. Subnormal/underpressure = pressure gradient < normal pressure gradient Overpressure/undercompaction = pressure gradient > normal pressure gradient The abnormal pressure exist when the pores are isolated (not interconnected) between each other. The permeability barrier that form avoid the pressures from being equalized (undercompaction).
Origin of Subnormal Formation Pressure 15
Thermal Expansion
(a) -
As sediments and pore fluids are buried the temperature rises. If the fluid is expand, the density will decrease thus reducing the pressure
Formation Foreshortening
(b) -
-
During a compression process, there is some bending of strata. The upper beds bend upward and the lower bed bend downwards. The intermediate beds must expand to fill the void and create subnormal pressure zone. This thought apply to some subnormal zones in Indonesia and the U.S. Notice that this may also cause overpressures in the top and bottom beds.
Origin of Subnormal Formation Pressure (Cont‟) 16 (c)
Depletion When hydrocarbon or water are produced from a competent formation in which no subsidence occurs, a subnormal pressure zones may result This will be important when drilling development wells through a reservoir which already been producing for some times. Some pressure gradients in Texas aquifers have been as low as 0.36 psi/ft.
Origin of Overpressured Formation 17
(a) Incomplete sediment compaction or undercompaction In the rapid burial of low permeability clays or shales there is little time for fluid to escape. Under normal conditions the initial high porosity is decreased as the water is expelled through permeable sand structure. If the burial is rapid and the sand is enclosed by impermeable barriers, water in rock is not allowed to escape accordingly, e.g., due to presence of barrier. The trapped fluid will help to support the overburden.
How does overpressure occur?
Pressure
Slow deposit of sand into water
Pore pressure is hydrostatic
Water escapes upward to make room for the sand
Lithostatic pressure
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Pressure
Pore pressure is hydrostatic down to A then increases abnormally
Rapid deposit of clay into water Water cannot escape quickly enough. Gets trapped and pressurized by overlying deposits
A
Lithostatic pressure
Origin of Overpressured Formation (Cont‟) 20
(b) Faulting Faults may redistribute sediments and place permeable zones opposite impermeable zones, thus creating barriers for the fluid to flow This may prevent water from being expelled from a shale, which will cause high porosity and pressure within that shale undercompaction.
Origin of Overpressured Formation (Cont‟) 21
(c)
(d)
Phase changes during compaction Minerals may change phase under increasing pressure. E.g gypsum converts to anhydrate plus free water. The release of water is about 40% the volume of gypsum If the water cannot escape, overpressure will be generated. -
Massive rock salt deposition Since salt is impermeable to fluids the underlying formations become overpressured. (Plastic behavior) Deposition of salt can occur over wide areas (e) Salt diaperism / salt dome The upward movement of a low density salt dome due to buoyancy which disturbs the normal layering of sediments and produces pressures anomalies.
Origin of Overpressured Formation (Cont‟) 22
(f)
Tectonic compression Faulting and uplift have moved a formerly buried formation from an area of high overburden stress to one of lower overburden stress
Origin of Overpressured Formation (Cont‟) 23
(g)
Repressuring from deeper levels This is caused by migration of fluid from a high to low pressure zone at shallower depth High pressure can occur in shallower sand if they are charged by gas from lower formations
(h)
Generation of hydrocarbon Shales which are deposited with a large content of organic material will produce gas. if it is not allowed to escape the gas will cause overpressure.
3. Overburden Pressure 24
Also known as geostatic pressure/ lithostatic pressure Overburden pressure originates from the combined weight of the formation matrix (rock) and the fluids (water, oil, and gas) in the pore space overlying the formation of interest. In order to calculate overburden, the average density of the material (rock and fluids) above the point of interest must be determined:
3. Overburden Pressure (Cont‟) 25
3. Overburden Pressure (Cont‟) 26
Overburden pressure varies in different regions and formations. It may vary with depth because of changing in bulk density due to compaction and changing lithology. The specific gravity of rock matrix vary from 2.1 (sandstone) to 2.4 (limestone). Using average of 2.3 and zero porosity the overburden pressure gradient is 2.3 x 0.433 psi/ft = 0.9959 psi/ft and rounded up to 1 psi/ft This is the maximum possible overburden pressure gradient.
4. Fracture Pressure 27
Fracture Pressure is the pressure inside the well/borehole that would fracture the formation It also determines the wellbore‟s strength and ability of the formation to withstand pressure. If borehole pressure exceed formation fracture pressure, the formation would break Exceeding this limit also can cause lost circulation, resulting in formation damage and induced fractures. The pressure in the borehole must always lie between the formation pore pressure and the fracture pressure.
Sandstone and shale 28
1 mm
Shale
Sandstone • Formed by cemented sand grains/ quartz • Open pore space network • Oil and gas reservoirs • Allows flow (dissipates pressure quickly)
• • • •
Formed by clays Tight pore space network Oil and gas seals Retards flow, but allows flow in long term
Subsurface Pressures
Normal Hydrostatic pressure 0.445 psi/ft Pore Pressure (depends on location)
) m ( h t p e D
Fracture gradient: pressure at which the formation will fracture
Lithostatic (overburden)
10,000 ft
0
0 0
Pressure (psi)
10000
Example 3: 30
a) At depth of 1300 ft, the formation pressure is 650
psi. This formation pressure is:
Normal presssure OR Overpressure OR Subnormal pressure
b) Find pressure at 1500 ft if normal pressure
gradient is occurred between 1300ft – 1500ft. c) Find pressure at 1100 ft if normal pressure gradient
is occurred between 1100ft to 1300ft.
Solution: 31
a) Overpressure: 0.5 psi/ft > 0.465 psi/ft b) Pressure @ 1300ft + increase P @ 200ft = 743 psi c) Pressure @ 1300ft - decrease P @ 200ft = 557 psi
Example 4: 32
Consider the gas sand was encountered in the U.S. gulf coast area. If the water-filled portion of the sand is pressured normally and the gas/water contact occurred at a depth of 5000 ft, what mud weight would be required to drill through the top of the sand structure safely at a depth of 4000 ft? Assume the gas has an average density of 0.8 lbm/gal.
Solution: 33
Solution: P5000ft = P4000ft + PGas1000ft P4000ft = P5000ft – PGas1000ft P4000ft = 0.465(psi/ft) x 5000 (ft) – 0.052 x 0.8 (lbm/gal) x 1000 (ft) P4000ft = 2283 psi. The mud density needed to balance this pressure while drilling
p 4 0 0 0 ft 0.052 h
2283
0.052 4000
11
lbm / gal
Determination of Fracture Gradients 34
Field Determination
1.
Leak Off Test (Formation Integrity Test, FIT) – Commonly used
2. Theoretical Determination Hubbert & Willis Mathews & Kelly Eaton (Commonly used) Christman
Leak-off test (LOT) 35
The most common procedure used for the field determination of fracture gradient. It can only be determined after the formations have been penetrated. This test is normally performed at the start of each new hole section just after drilling out of a casing shoe of the previous hole section. In this test, blow-out preventers are closed and then the pressure is applied incrementally to the shut-in system until the formation initially accepts fluid The operation is generally stopped at the first point which deviates from the straight line portion of the plot.
Leak-off test (LOT)- Procedure 36
1. Drill 5 to 10 ft below casing shoe 2. Close the BOP‟s at the surface and
apply pressure down the drill pipe in small increments, using a low volume pump. 3. Record the volume of mud pumped and the pressure in the system at each volume increment. 4. Stop pumping when the pressure in the well does not increase linearly (formation begins to take fluid) for an increase in the volume of fluid pumped into the well 5. Plot pressure versus the pumped volume to determine the initial leak off pressure.
Typical Rig for LOT 37
Why do we need to measure fracture gradient?? 38
To investigate the strength of the cement bond around the casing shoe. To determine the fracture gradient around the casing shoe. To validate / invalidate the setting depth of the next casing. To determine the maximum mud weight.
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Determining Fracture pressure using LOT 40
Fracture pressure/ formation strength pressure/ maximum allowable pressure at casing shoe is measured in psi or EQUIVALENT MUD WEIGHT (EMW): FP
= LOP + P Hyd (psi) P Hyd = 0.052*OMW*D (psi)
FP = LOP + 0.052*OMW*D (psi) EMW = LOP + OMW (ppg) 0.052*D Where:
FP OMW D LOP P Hyd EMW
= Fracture Pressure, psi. = Original Mud weight, ppg. = Casing Shoe Depth, ft TVD - RKB. = Leak-off Pressure, psi. = Mud Hydrostatic Pressure, psi. = Equivalent Mud Weight,
Determining Maximum Allowable Mudweight using LOT 41
Maximum Mudweight (ppg) =
LOP + OMW (ppg) 0.052*D
Usually a safety factor of 0.5 ppg (0.026 psi/ft) is subtracted from the maximum mud weight.
If it is anticipated that a mudweight greater than this is required then consideration should be given to setting another string of casing prior to entering the zone that will require this higher mudweight.
Example 5: Leak-off test (LOT) 42
A leakoff test was carried out just below a 13-3/8" casing shoe at 7000 ft. TVD using 10.0 ppg mud. The results of the tests are shown below. Determine the fracture pressure at the casing shoe and the maximum allowable mudweight for the 12-1/4" hole section ? Volume pumped, bbl 0 1 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5
Pressure, psi 0 4 100 190 280 370 460 550 640 730 820 850 880
Using a graph paper, plot Pressure vs Volume graph
Leak-off test (LOT)- Solution 43
Theoretical determination of Fracture Pressure
It is necessary to estimate the fracture pressure of the formation to ensure safe operation and to optimize the design of the well. At the well planning stage, the fracture gradient can be estimated from offset well data. If no offset data is available the fracture gradient can be predicted using any of the published models below: 1. Hubbert & Willis The fracture gradient is a function of overburden stress, formation pressure and a relationship between horizontal and vertical stresses 2.
3.
4.
Mathews & Kelly Consider the matrix stress and varies only with the degree of compaction Eaton Extended concept from Mathews and Kelly by introducing Poisson ratio Christman Accounted for the effect of water depth
Eaton Method (Commonly used)
Eaton proposed the following equation for estimating fracture gradient
Gp 1
G f Go G p
Gf = fracture gradient (psi/ft) Go = overburden gradient (psi/ft) Gp = pore pressure gradient (observed or predicted) (psi/ft) v = Poisson‟s ratio
Poisson‟s ratio is a rock property that describes the behaviour of rock stresses in one direction (least principle stress) when pressure is applied in another direction. Poisson‟s ratio varies with depth and degree of compaction In Malaysia: Poison ratio is between 0.4 to 0.5
Example 6: Estimation of Fracture Gradient
Using the data below, calculate the fracture gradient at the various depths for the following land well. Assume v = 0.4 and overburden gradient = 1.0 psi/ft. Plot a pore pressure profile consist of pore pressure, fracture pressure and overburden pressure lines. TVD (ft)
Pore Pressure (psi)
3000
1320
5000
2450
8300
4067
8500
4504
9000
5984
9500
6810
10000
7800
11000
10171
The Equivalent Circulating Density (ECD)
When mud is circulated through the drillstring, the borehole pressure at the bottom of the annulus will be greater than the hydrostatic pressure of the mud The extra pressure is due to the frictional pressure required to pump the fluid up the annulus. This frictional pressure must be added to the pressure due to the hydrostatic pressure from the colom of mud. An ECD can be calculated from the sum divided by true vertical depth of the well P ECD
MW
d
0.052 D
ECD = effective circulating density (ppg) MW = mud weight (ppg) Pd = annulus frictional pressure drop at given circulation rate D = depth (ft)
Example 7: ECD Calculation
If the circulating pressure losses in the annulus of the above well is 300 psi when drilling at 7500ft with 9.5ppg mud, what would be the ECD of the mud at 7500ft.
Maximum Allowable Annulus Surface Pressure 49(MAASP)
Is an absolute upper limit for the pressure in the annulus of an oil and gas well as measured at the wellhead. Is the maximum closed in (not circulating) pressure that can be applied to the annulus (drillpipe x BOP) at surface before the formation just below the casing shoe will start to fracture (leak off). MAASP is calculated to provide a surface pressure, which will produce the limiting pressure at the shoe. This is to preserve well integrity to ensure that the annuli remain intact. One major threat to annulus integrity is overpressure within the annulus, which could lead to burst or collapse of a casing or damage to the formation below. This will happen first at the shoe of the annulus because the pressure will naturally be higher with the weight of the column of mud. MAASP = Maximum Allowable pressure at the formation just below the shoe minus Hydrostatic Pressure of mud at the formation just below the shoe.
Example 8: MAASP Calculation 50
If a mudweight of 9.5ppg mud is required to drill the 12-1/4” hole section of a 7000ft well, What would be the MAASP when drilling this hole section? Given the maximum allowable pressure at casing shoe is 4900 psi
Drilling Problem Associated with Abnormal Formation Pressure 51
When drilling through a formation sufficient hydrostatic mud pressure must be maintained to: • prevent the borehole collapsing and • prevent the influx of formation fluids. To meet these 2 requirements the mud pressure is kept slightly higher than formation pressure. This is known as overbalance. If, however, the overbalance is too great this may lead to: • reduced penetration rates • breakdown of formation (exceeding the fracture gradient) and subsequent lost circulation (flow of mud into formation) • excessive differential pressure causing stuck pipe. If the mud pressure is less than the pore pressure then the differential is known as underbalance pressure.
Drilling Problem Associated with Abnormal Formation Pressure 52
The abnormal pressure will influence the design of casing string. If there is a zone with high pressure above a low pressure zone, the same mud weight cannot be used to drill the low pressure zone otherwise the lower zone may be fractured. The upper pressure zone must be cased-off and allowing the mud weight to be reduced for drilling the lower zone. A common problem is where the surface casing is set too high, so that when an overpressured zone is encountered and an influx is experienced. Each casing string should be set to the maximum depth allowed by the fracture gradient of the exposed formations. If this is not done an extra string of protective casing may be required. This will not only prove expensive, but will also reduce the wellbore diameter. This may have implications when the well is to be completed since the production tubing size may have to be restricted. Having considered some of these problems it should be clear that any abnormally pressured zone must be identified and the drilling programme designed to accommodate it.
Pore Pressure Profile 53
54
Basis for all designs related to drilling operations. Mud pressure must be within the window during drilling at any depth. Consequence of violation may be severe.
PREDICTION OF PORE PRESSURE/ ABNORMAL PRESSURE 55
1) Before drilling - for well design and planning 2) While drilling – to adjust the design as necessary
1) Predictive techniques (before drilling) 1. Correlation of available data from nearby wells/offset wells
2. Geophysical measurement at surface: Seismic interpretation data • relies on the fact that abnormally pressured rock is less compacted than normally pressured rock, so has higher porosity. • porosity can be determined from the velocity of sound through rock. Geologists find it easier to use inverse velocity or transit time measured in microseconds per foot, instead of velocity. • The higher the transit time, the more porous the rock.
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2. Detection techniques (whilst drilling) 1.
Monitoring the drilling parameters Weight on Bit (WOB): The load put on the bit by the drill collars to improve penetration rate RPM: revolutions per minute. Term used to measure the speed at which the drillstring is rotating. Rate of Penetration (ROP): As the compaction is increased with depth, ROP will decrease with depth. Normally measured in feet drilled per hour. Torque: the turning force which is applied to the drillstring causing it to rotate. Torque is usually measured in ft-lbs. Drag: The force required to move the drillstring due to the drillstring being in contact with the wall of the borehole.
•
•
•
•
•
2. Detection techniques (whilst drilling) 2.
Mud Mud and and cutt cuttin ings gs ret retur urni ning ng fro from m the the hole hole (mu (mud d loggi logging ng)) Monitoring the effect effect of overpressured overpressured zone on the mud (influx of oil or gas). The changes in the drilling mud parameter due to overpressure zones includes increase gas cutting of mud, decrease in mud weight, weight, increase in flowline temperature. temperature. Drilled cuttings – try to identify cuttings from sealing zone Overpressu Overpressured red zones are are associated associated with under-com under-compacted pacted shales shales with high fluid content. The degree of overpressure can be inferred from the degree of compaction of the cuttings. The reduction in shale cuttings density could be an indication of transition zone.
•
•
2. 3.
Elec Electr trica icall prope propert rtie iess of the for forma mati tion on (ele (elect ctric ric log loggi ging ng)) Dir Direct ect evid evideence nce (inf (influ lux) x)
58
Typical log responses during drilling in overpressured intervals PRESSURE
SONIC VELOCITY
POROSITY
TEMPERATURE
H T P E D
Top overpressure
Low
High
Low
Hig h
Low
Hig h
Low
High
Monitoring the Drilling Parameters • If WOB and RPM are kept constant, and bit does not wear, then rate of penetration (ROP) will increase in less compacted rock NOTE: it may also increase because the the formation has has changed
ROP (ft/hour)
Depth Something has happened here to cause ROP to increase Needs further investigation
0
20 40 60 80
60
Normal pressure
Transition zone to higher pressure
61
Annular velocity of mud up the hole AV =
Normal pressure
Q x 24.51
d = 8”
D2 – d2
D = 12-1/4” AV (in gauge hole) = 180 ft/min may lift cuttings adequately
Q (flow rate), say, 630 gal / min
Transition zone to higher pressure
D1 = 20” Stuck in the hole (very bad!)
AV (in gauge hole) = 50 ft/min May not adequately carry the sloughings 62
Monitoring the Drilling Parameters •
If WOB and RPM are kept constant, rate of penetration (ROP) will increase in less compacted rock. However, WOB and RPM are rarely kept constant. So we calculate the „drillability ‟ of the formation, called the „d‟ exponent assuming a constant mud weight.
The ‘d’ exponent =
R = Penetration Rate (ROP) N = Rotary Speed (rpm) W = WOB (lb) B = Bit diameter (in.) In normal pressure regions, d exp increases with depth. Any sudden decrease indicates the potential existence of overpressure. 63
Modified d-exponent
It can be seen that the d-exponent equation takes no account of mudweight. Modification is necessary to eliminate the effect of changing mud weight Done by multiplying by a normalized density (mud weight):
d c d MW n - “normal” mud weight MW a – actual mud weight d = d-exponent dc = modified d-exponent
MW n MW a
The d-exponent is generally used to simply identify the top of overpressure zone
How to calculate formation pressure from modified d-exponent??
The value of the formation pressure can be derived from the modified d-exponent, using a method proposed by Eaton (1976):
Exercise 1
Whilst drilling the 12-1/4" hole section of a well the mudloggers were recording the data as shown in the table below.
Exercise 1 (Cont’d)
Assume a normal formation pressure of 0.45 psi/ft, an overburden gradient of 1.0 psi/ft and a normal mud weight for this area of 9.5 ppg. a) Plot the d and dc exponent and determine whether there are any indications of an overpressured zone. b) If an overpressured zone exists, what is the depth of the top of the transition zone. c) Use the Eaton equation to estimate the formation pressure at 8600 ft.
