Chapter 4- Formation Pressure

CHAPTER 4:

FORMATION PRESSURES

DRILLING ENGINEERING I(CGE577)

Contents2

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 Pressure so 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 fluidsinflux from shallow formation blow out

Blowout of Deepwater Horizon semi-submersible Mobile Offshore Drilling Unit (MODU), Off Louisiana, 20 th April 2010 .

Happened on 20 April, 2010 while drilling Macondo Oil Prospect.

Killed 11 Workers, injured 16 others

Deepwater Horizon Rig (Transocean)

13000 ft seabed

Transocean executive-pressure had accumulated inside the marine riser and as it came up ,it expanded rapidly and ignited

BOP failure was blamed by BP.

BOP Manufacturer: Cameron International Corp.

12,000 19,000 bopd leaks

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Formation Pressure5

Formation pressure/Pore pressure :

Pressure of fluid contained in pore spaces of the rock (spaces between grains)

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The pressure in the formations to be drilled is often measured in pressure gradient.

In vertical column of fluid, gravitycauses 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.

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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.

0.465 psi/ft

0.35 psi/ft

0.1 psi/ft

Formula for Pressure

Pressure at the bottom of a column of fluid = Density x Height x Constant**The constant depends on the units you chose

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

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

Density Height

(depth)

*Constant Answer is in

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/in2 )

* Pressure gradient (psi/ft ) = ppg x 0.052

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* Hydrostatic pressure (psi) = 0.052 x f (ppg) x D (ft)

Subsurface Pressures9

Subsurface Pressures

Normal

Abnormal (Subnormal)

Overburden

Abnormal (Overpressure)

Fracture

1. Normal Pressure (Hydrostatic Pore Pressure)

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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 ppmsalt content) normal pressure gradient/hydrostatic pressure.

Example-1

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 = ?????

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Example-213

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 Pressures14

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 ).

(a) Thermal Expansion

- As sediments and pore fluids are buried the temperature rises. If the fluid is expand, the density will decrease thus reducing the pressure

(b) Formation Foreshortening

- 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 Pressure15

(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 Subnormal Formation Pressure

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(a) Incomplete sediment compaction or undercompaction

In the rapid burial of low permeability clays or shalesthere 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.

Origin of OverpressuredFormation

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Pressure

Water escapes upward to make room for the sand

Slow deposit of sand into water

Pore pressure is hydrostatic

Lithostatic pressure

How does overpressure occur?

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Pressure

Water cannot escape quickly enough. Gets trapped and pressurized by overlying deposits

Rapid deposit of clay into water

Pore pressure is hydrostatic down to A then increases abnormally

Lithostatic pressure

A

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(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

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(d) 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 Overpressured21

(c) 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.

(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


(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

- Shaleswhich 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 Pressure24

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:

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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/ftand rounded up to 1 psi/ft

This is the maximum possible overburden pressure gradient.

4. Fracture Pressure27

Fracture Pressure is the pressure inside the well/borehole that would fracture the formation

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

Sandstone

Formed by cemented sand grains/ quartzOpen pore space networkOil and gas reservoirsAllows flow (dissipates pressure quickly)

Shale

Formed by claysTight pore space networkOil and gas sealsRetards flow, but allows flow in long term

1 mm

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Pressure (psi)

De

pth

(m

)

00

0 1000010,000 ft

Fracture gradient: pressure at which the formation will

fracture

Lithostatic(overburden)

Normal Hydrostatic pressure 0.445 psi/ft

Pore Pressure (depends on location)

Subsurface Pressures

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Example 3:30

a) At depth of 1300 ft, the formation pressure is 650 psi. This formation pressure is:

Normal presssureOR 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

gallbmh

p ft/11

4000052.0

2283

052.0

4000

Determination of Fracture Gradients34

1. Field Determination

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

1. Drill 5 to 10 ft below casing shoe2.

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.

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Typical Rig for LOT37

Why do we need to measure fracture gradient??

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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.

Determining Fracture pressure using LOT40

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 = Fracture Pressure, psi.

OMW = Original Mud weight, ppg.

D = Casing Shoe Depth, ft TVD - RKB.

LOP = Leak-off Pressure, psi.

P Hyd = Mud Hydrostatic Pressure, psi.

EMW = Equivalent Mud Weight,

Determining Maximum Allowable Mudweightusing LOT

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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)

A leakoff test was carried out just below a 13-3/ 8" casing shoe at 7000 ft . TVD using10.0 ppg mud. The results of the tests are shown below. Determine the fracturepressure at the casing shoe and the maximum allowable mudweight for the 12-1/ 4"hole section ?

Volume pumped, bbl Pressure, psi0 01 4

1.5 1002.0 1902.5 2803.0 3703.5 4604.0 5504.5 6405.0 7305.5 8206.0 8506.5 880

Using a graph paper, plot Pressure vs Volume graph

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Leak -off test (LOT) - Solution43

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. Mathews & Kelly

Consider the matrix stress and varies only with the degree of compaction

3. Eaton

Extended concept from Mathews and Kelly by introducing Poisson ratio

4. Christman

Accounted for the effect of water depth

Theoretical determination of Fracture Pressure

Eaton proposed the following equation for estimating fracture gradient

Gf = fracture gradient (psi/ft)

Go = overburden gradient (psi/ft)

Gp = pore pressure gradient (observed or predicted) (psi/ft)

in one direction (least principle stress) when pressure is applied in another direction.

ratio varies with depth and degree of compaction

In Malaysia: Poison ratio is between 0.4 to 0.5

Eaton Method (Commonly used)

ppof GGGG1

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.

59849000

780010000

68109500

1017111000

45048500

40678300

24505000

13203000

Pore Pressure (psi)TVD (ft)

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

ECD = effective circulating density (ppg)

MW = mud weight ( ppg)

Pd = annulus frictional pressure drop at given circulation rate

D = depth (ft)

The Equivalent Circulating Density (ECD)

D

PMWECD d

052.0

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.

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.

Maximum Allowable Annulus Surface Pressure (MAASP)

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Chapter 4- Formation Pressure