formation pressure evaluation · 2018. 10. 15. · pore pressure terminology #2 0 1000 2000 3000...

© 1996 Baker Hughes IncorporatedAll rights reserved.

Formation Pressure Evaluation

Basic Terminology


Pascal's Law

n The pressure at any point in a static fluid is the same in all directions.n Any pressure applied to a fluid is transmitted undiminished through the

fluid .

10 lb.

10 lb.


Hydrostatic Pressure

5000

12 ppg 12 ppg12 ppg 12 ppg 12 ppg12 ppg

Hydrostatic Pressure = 0.0519 x 12 ppg x 5000 ft = 3114 psi


What is normal pore pressure ?


Pore Pressure Terminology #1

Sea Level

DV

DV

DV

ABNORMALPF > 0.0519 x WF x DV

SUBNORMALPF < 0.0519 x WF x DV

NORMALPF = 0.0519 x WF x DV

Water Table



0 1000 2000 3000 4000 5000 Pressure

Depth

5000

2500

Gas Gradient 0.07 psi/ftOil Gradient 0.30 psi/tfWater Gradient 0.433 psi/ftNormal Formation Gradient 0.465 psi/ft10ppg Mud Gradient 0.519 psi/ft15ppg Mud Gradient 0.779 psi/ft21ppg Mud Gradient 1.090 psi/ft


Where is abnormal pressure located?


Abnormal Pore Pressure Locations #1


Abnormal Pressure Locations #2


Abnormal Pressure Locations #3


Mechanisms for generating pore pressure

Formation Pressure Evaluation


How is abnormal pore pressure created ?


MECHANISMS FOR GENERATION OF OVERPRESSURE

Ø Compaction Disequilibrium Ø( Rapid Loading / Overburden Effect )

Ø Aquathermal PressuringØ Clay Diagenesis

Ø( Montmorillonite Dehydration )Ø Sulphate Diagenesis Ø Salt DiapirismØ Tectonic ActivityØ Hydrocarbon Maturation and PlacementØ Piezometric ChangesØ OsmosisØ Pingos


Compaction Disequilibrium

Ø Under “normal” conditions sediments will dewater with burial, with the overburden acting as the main cause for fluid expulsion.

Ø As sediments compact normally their porosity will become reduced at the same time as dewatering occurs.

Ø If the rate of sedimentation is slow then normal compaction will occur and the sediments are considered to be in equilibrium, as the rate of burial is equal to the rate of dewatering.

Ø Normal clay compaction will therefore depend on an overall balance between :-

Øclay permeabilityØsedimentation and burial rateØdrainage efficiency

Ø If any of the above factors are not satisfied and dewatering proceeds more slowly then an abnormal pressure will be created due to thecompaction disequilibrium method.



Interstitial Water(% initial vol.)

Surface 300m 1000m 3000m

75.9

4.1

20

7366.6

80 13.3 7

202020

Expelled Water(% initial vol.)

Solid(% initial vol.)

Porosity can vary from 80% to less than 10% over a 5,000m interval. It can be demonstrated using Dickinson’s data for the Gulf Coast (1953), that at a depth of 3,000mthe total volume of water expelled is more than 75% of the original volume of the argillaceous sediment.



Shale

Shale

Shale

Shale

Sand

Sand

Sand

Shale porosityFluid pressure

in shale

Dep

th

Dep

th

Direction ofcompactionfluid flow

Hydrostatic Pressure

• As dewatering continues, the pore fluids will attempt to escape via the path of least resistance to flow and so channelling will occur to adjacent porous bodies.

•The dewatering close to the drains will then lead to compaction in the immediately adjacent clays.•The resulting compaction will reduce the clays porosity and permeability, which will further retardfluid flow.•This retardation to fluid flow will then create an abnormal pressure build up, typically with the higher pressures reaching a peak within the clay bodies.



Ø The overburden effect is defined as the result of the action of subsidence on the interstitial fluid pressure of a formation. If fluids can only be expelled with difficulty relative to burial conditions, they must support all or part of the weight of overlying sediments.

Ø Porosity decreases less rapidly that it should with depth, and clays are then said to be undercompacted.

Ø Formation pressure intensity is controlled as much by the rate of subsidence as by the dewatering efficiency. Imbalance between these two factors is the most frequent cause of abnormal pressure.


Aquathermal Pressuring

If a body of water has a temperature applied to it then it’s volume will increase with expansion. If that body of water is also in a perfectly sealed environment then thatresultant expansion will exert a pressure equal in intensity in all directions.

The amount of resultant pressure rise will depend upon:-

1) the density of the fluid.2) the increase in temperature.3) that the pore volume is constant.4) the body of fluid is completely isolated in a sealed

environment.5) that the temperature has been applied after isolation.


Aquathermal Pressuring

n Aquathermal expansion has been proposed as an effect producing increased pressure in sedimentary sequences due to a temperature raise in a closed system.

n The effect is governed not only by thermal conditions and water density, but more particularly by the permeability of the environment and the time factor.


Clay Diagenesis

Clay Mineralogy

Ø Argillaceous minerals form part of the phyllosilicates group or the lattice layer group, which are characterised by alternately arranged sheets of T2O5 tetrahedra and octrahedrawhere T = Si, Al or Fe3+

Ø The simplest clay minerals are the pyrophyllite, which are formed from the superposition of two tetrahedral sheets bonded by Al3+ ions.

Al2 [Si4 O10] (OH)2Ø These tetrahedras are connected by a weak electromagnetic link called a Van der Waals

bond and are electrically neutral.Ø Substitution of Si4+ cations in the tetrahedral layers by Al3+ creates a negative charge, which

is compensated by the adsorption of cations and interlayer water and produces a “swelling clay”, for example a montmorillonite from the smectite family.

(Si4x Alx)O 10 (Al(2-x) Rx 2+) (OH)2 Ecx n.H2 0where R2 = Mg, Fe, Mn, Cr etc

EC= exchangeable cationsØ Further substitution of Si4+ cations by Al3+ increases the electrical imbalance and allows for

potassium and calcium ions to be fixed in the interlayer position. The clay loses its capacity to adsorb water and will gradually change to an illite.

Ky Al4 (Si 8-y Al y) O20 (OH)4with 1 < y < 1.5


Clay Diagenesis


Clay Diagenesis

n Powers (1959) suggested a two stage model for the expulsion of water from smectites.

– free pore water expelled near the surface under the influence ofpressure (Compaction Disequilibrium).

– interlayer water released gradually, first under the effects of pressure, then increasingly due to the influence of temperature (Aquathermal Pressuring).

n Burst (1969) improved upon this model and proposed three stages of dehydration.

– expulsion of free pore water and part of the interlayer water, as far as the last two molecular layers, under the influence of pressure, taking place increasingly slowly as permeability declines relative to depth.

– expulsion of the last but one molecular layer of water under theinfluence of temperature.

– Gradual expulsion of the last molecular layer.n The depth at which the three stages of clay dehydration take place are purely

dependant upon the geothermal gradient.


Clay Diagenesis


Sulphate Diagenesis

Ø Gypsum is the only precipitated form of CaSO4 in areas of sedimentation.

Ø At temperatures of less than 40 degrees C gypsum is stable in the presence of pure pore fluids.

Ø In the presence of a salt gypsum will be stable at lower temperatures.Ø Pressure also affects the stability of gypsum, with higher pressure

gypsum becomes unstable.Ø When gypsum becomes unstable it dehydrates to form anhydrite.

CaSO4.2H2O CaSO4 + 2H2OGypsum Anhydrite + Water

Ø When gypsum is transformed to andydrite upto 38% of the originalwater volume is released and therefore abnormal pressure can develop if this fluid cannot escape.

Ø Anhydrite can rehydrate to gypsum and this is accompanied by an increase in volume and therefore can also generate abnormal pressures.


Salt Diapirism

Ø Salt diapirism causes the formation of abnormal pressure in a variety of ways:

Ø Salt is a mobile formation and when it moves it can flow as a wall. As the salt moves it develops internal vortices, which actlike billowing smoke spiralling upwards. These billows can trap and encapsulate the surrounding country rock, typically dolomiteor anhydrite and lift these fragments to shallower depths. Thesefragments would be normally pressured at depth but being taken up the formation means that they are abnormally pressured when compared to the surrounding formation at this shallow depth.

Ø These fragments of country rock will be perfectly sealed in the salt, as salt acts as a perfect plastic seal this will inhibit pressure bleeding off.

Ø These fragments of country rock may also contain gases such as hydrogen sulphide, oil or water.


Salt Diapirism


Salt Diapirism

Banff Diapir and Field


Tectonic Activity

Where deformation occurs due to tectonic stress, modifications in fluid pressures and distribution of the masses will occur. This may create abnormal pressure or restore abnormal pressure back to normal.

The process of overthrusting in the earth’s crust is dependant upon abnormal pressures at the base of the thrust for lubrication otherwise the rock masses could not move in the way that they do.

A thrust may overload the underlying sediments and if seals are present then this can impose an extra pressure on the contained pore fluids.

Due to overloading then a change in the geothermal gradient can also affect the fluid pressure.

Thrusting can also lift compartments to higher levels without rupturing them, which in turn will create abnormal pressure if seals are present.


Tectonic Activity


Tectonic Activity

Faulting

• Faults can acts as seals or drains.• They can redistribute pressure.

Abnormally pressured sands

Hydrostatically pressuredsands

Pressure dissipated in the hydrostatic series

Hydrostatically pressuredsands


Tectonic Activity

Subduction Zones Accretionary prism

Asthenosphere

Oceanic lithosphere

Magma

Continental lithosphere

• Argillaceous sediments are buried rapidly in geosynclinal zones and in subduction zones, where two tectonic plates converge.

• Undercompacted argillaceous layers are favourable to the development of overlying deformation because they act as lubricants, amplifying movement.

• The Barbados accretionary prism is multiple imbricated overthrust faults, with their development linked to abnormal pressures in undercompacted argillaceous sediments,which is confirmed by the presence of shale diapirism.

• Mud volcanoes are the the ultimate manifestation of shale diapirism and tend to besituated along large active transcurrent faults eg New Zealand, Azerbaijan and Trinidad.


Tectonic Activity

Sediment Influx

Residual shalemass

Growth Faults

• Growth faults are characterised by a curved fault plane, which is invariably concave. In the top section the plane is almost vertical and tends to gradually decrease the curve to conform with the dip of the strata.

•The downstream compartment displays a thickening of the sediments in the form of a “roll-over” near the fault.

• These faults can develop due to compaction of the clay sediments, which during compaction slide under their own weight on a slope of less than 3o . Lowering the dip of the downhill compartment creates a surface depression which traps sediments. The additional weight of these sediments encourages further slipping.

• The base of the undip compartment often contains a compartment of undercompacted shale (residual shale mass) resulting from differential compaction.

• The preferential site for hydrocarbon build up is in the roll-over structure, which when drilled can cause a sudden increase in fluid pressure.


Tectonic Activity

Amount ofShortening Possible Geopressured Zones

Extension Extension

Compression

Compression


Tectonic Activity

Tectonics and fluid pressures interact to give a variety of effects.Extension causes fractures to open and therefore fluid dissipation.Compression has two main effects:

The easy expulsion of fluids, leading to compaction and therefore the formation of normal fluid pressures.

The difficult expulsion of fluids, which causes undercompaction,the formation of abnormal pressures. This can induce hydraulic fracturing and lead onto the expulsion of pore fluids and ultimately compaction and the formation of normal pore pressure.

Fracturing and faulting can lead to fluid expulsion to form normal pore pressures or the dissipation of abnormal pore pressure.

Changes in relief and geometry are a direct cause of pressure redistribution.Deep lying formations can be uplifted to create pressure anomalies if perfectly

sealed.Uplifted sediments can equalise pressure anomalies that are created due to

aquathermal pressuring.


Hydrocarbon Maturation and Placement

Ø At shallow depths organic matter contained in sediments is broken down by bacterial action, generating biogenic methane.

Ø If this methane is contained in a closed environment then the resulting expansion can lead to abnormal pressures. However perfect seals are rare especially at such shallow depths, so the gas will diffuse upwards and may become trapped in pockets.

Ø With increasing depth bacterial activity will increase gradually giving way to thermochemical cracking, which transforms a heavier product into a lighter one under the influence of temperature. The cracking process will increase the number of molecules contained and therefore the volume the hydrocarbon occupies creating abnormal pressures in closed environments.

Ø As compaction proceeds and less dewatering occurs, decomposing organic matter will tend to cause the water to become saturated in gas, which will eventually produce free gas. If this gas is unable to escape it will cause abnormal pressures. This will also magnify the effect of undercompaction due to the overburden effect if hydrocarbons aregenerated at the same time.


Piezometric Changes

Ø A low water table, or an aquifer with an outcrop below the water table, will show a pressure that is subnormal for drillingpurposes. This is not as dramatic as abnormal pressure but the resulting lost circulation will cause a loss of control of the hydrostatic pressure in the well which could result in well control problems.

Ø A water table above the height of the rig will have abnormal pressure on penetrating the aquifer, which will cause the fluid to rise to the piezometric level to equalise the pressure imbalance.


Piezometric Changes

When the seal is punctured fluids in the aquifer will rise to this level to equalise the pressure.


Piezometric Changes

Be aware of possibilities of a Tilted Oil Water Contact zone

This can severely influence the pressure gradients seen in a well


Piezometric Changes


Osmosis

n Osmosis is the spontaneous movement of water though a semi-permeable membrane separating two solutions of different concentration until the concentration of each solution becomes equal or until the development of osmotic pressure prevents further movement from the solution of lower concentration to that of a higher concentration.

Pu

re W

ater

P1 P2

Osmotic Flow

H20

H20

H20

H20

H20

H20

Sal

t Wat

er

Cl-

Cl-

Cl-

Na+

Na+

Na+

P2 > P1

Sal

e M

emb

ran

e


Osmosis - Possibility of detection ?

SALTS

Clays

MWD Resistivity

Note the change in trend line of deep reading Resistivity tools in response to changing pore water salinity. Inference of downwards migration to the salt under osmosis


What data will you be given offshore to assist you in pressure evaluation?



Well Data Summary

Well : 15/17-A09Type : Water InjectorBlock : 15/17Licence : P250Licences : Elf Enterprise Caledonia Limited 36.5%

Lasmo 20.0%Texaco 23.5%Union Texas 20.0%

Operator : Elf Enterprise Caledonia LimitedRig : Saltier Platform Rig 51RKB: 168 ft / 51.2 mWater Depth: 474 ft / 144.5 m


Pore Pressure Terminology

Surface Location

Slot : 24 (Side-track below 13 3/8” casing abandoned well 15/17-A03)Geographic : 58o 25’ 00.454” N 00o 20’ 03.834” EUTM : (Zone 31) 6478349 mN 344291 mE

Target Location

3D Seismic Line 1051 CDP 1012 Western Geco 1002/93 SurveyTarget : SCAGeographic : 58o 24’ 30.010” N 00o 19’ 40.813” EUTM : (Zone 31) 6477670.00 mN 343890.00 mE Valhall

(6477665.00 mN 343863.00 mE Piper)AFE Number : 351602AFE Cost : £M 3.087AFE Days : 45



Target Tolerance

The target area is defined as a circle of radius 50m around the target locations at both Top Valhall and Top Piper Formations.

Well Objectives

The objectives of the well are :

– To be a Valhall water injector to support A08 oil production.– To test the Piper Sand distribution for a future Piper West injector and, ideally to provide an

injection location to support A07.

Although the well is planned to be a Valhall injector there may be a short injectivity test into the Piper West Sand (or a production test if Piper Main oil is encountered).



Target / Reservoir Data:

Primary Target: Valhall Formation 10558ft MD (9880ft tvdss)Expected Reservoir Pressure: 4310psia (+/- 100psi) @ 9895ft tvdssExpected Reservoir Fluid: Water (ODT at 9830ft tvdss)

Possible Reservoir : Piper Formation 10902ft MD (10221ft tvdss)

Expected Reservoir Pressure: Piper West 4350psia (+/-100psi) @ 10225ft tvdssPiper Main 4610psia (+/-100psi) @ 10225ft tvdss

Expected Reservoir Fluid: Piper West Water (OWC at 10142ft tvdss)Piper Main 42o API Oil @ GOR 1800scf/bbl

Estimated Total Depth:Rattray Formation 11470ft MD (10787ft tvdss)



Drilling Hazards

Shallow GasSeismic anomalies interpreted to have a low risk of shallow gas were recognisedimmediately below the platform location. Wells 15/17/A01 and A02 confirmed that no gaswas present.

H2S / CO2No H2S has been detected in this area. Small percentages of CO2 have been encounteredin offset wells.

Hole ProblemsNo major hole problems are anticipated, though shale swelling problems in the Sele /Lista Formations, the Lower Cretaceous Sola Formation and the Kimmeridge Clay Formation have been encountered in the previous Saltire Wells. POBM will be used to drill the 12 1/4” and 8 1/2” sections. This may help to alleviate some of the problems previously experienced. Possible differential sticking may also occur in the Palaeocene Sandstones. Care should be taken while drilling to ensure that the open hole condition allows the descent of wireline tools at section TD.



Overpressure

A normal pressure gradient 8.7 ppg EMW is anticipated for this well, with some minor overpressure in the Hordaland Group and possibly in the Sola Formation.

Reservoir Pressure

Due to the ongoing off-take from the Valhall, Piper and Galley Reservoirs, pressures are difficult to predict and the figures quoted above should be taken as most likely. It has been established from RFT/MDT pressure tests on earlier wells that there is some possible communication from the the Piper to the Valhall since the Valhall reservoir pressure is some 100psi above the original pressure.

The figures could be up to 200psi more or less in the Piper and 100psi less in the Valhall depending on the off-take rates.

formation pressure evaluation · 2018. 10. 15. · pore pressure terminology #2 0 1000 2000 3000...

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