Using Pressure Tests to Define Fluid Contacts In

8/9/2019 Using Pressure Tests to Define Fluid Contacts In

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Using Pressure Tests to define

Fluid Contacts in the Subsurface


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Reservoir Pressures Fluids confined in pores are under pressure: Reservoir

Pressure, Fluid Pressure or Formation Pressure. We can determine the pressure by measuring the force

per unit area exerted by the fluids on the reservoir rockswhere it is penetrated by the well.

Fluids in communication will transmit pressures freely Pressure normally cited as pounds per square inch (psi)

Pressures normally computed as pressure gradients


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The Hydrostatic Gradient

For a freshwater system = 0.433 psi/ft

If water contains dissolved salts: its

specific gravity will be higher so its

hydrostatic pressure gradient will behigher than freshwater.

Thus on a pressure depth plot, saline fluid

pressure gradients will be shallowerthanhydrostatic.


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Using pressure measurements to

find fluid contacts 2 primary types of fluid contact

Sharp Transitional

Good quality reservoir: good K (2000mD) Sharp contacts

Poorer quality: as K decreases, sharpness of thecontact decreases transitional zone

When we drill the Discovery Well: use thepressure data from this well to locate the fluidcontacts.


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Oil

Discovery Well

Pressure Test Data

used to determine

locations of fluid

contacts:

Appraisal well canbe moved to locate

the OWC.

Pressure gradients

are used to identify

component fluids


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Pressure Gradients determined by

fluids

Dry Gas 0.100 psi/ft

60 API Oil/ Wet Gas 0.321 psi/ft

10 API Oil/ Water 0.433 psi/ft

Heavily saturated brine 0.500 psi/ft

All under Static Conditions We can use this to study reservoir connectivity


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As long as there are no pressure barriers

in the system: Can continue these gradient lines with depth.

Consider a simple hydrocarbon system: We will see different gradients for different

fluids


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oil

gas

H2OH2O

oil

Pressure psi

Dept

hft

gas

oil

water

Hyd

rosta

ticgrad

ientOWC


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Pressure/ depth plots good at predicting

possible contacts but. Not good enough to base expensive decisions

on

RFT tool is used to determine pressure in thewell. MDT tool can make more measurements atlower cost and is better therefore atcompartmentalization definition.

To follow are a few examples of how we can usethese tools.


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psi

depth

A

B

A B

Simple Pressure Gradient


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psi

depth

A

B

A B

Different Pressure Gradients: Flow Barriers


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What pressures do we commonly measure?

Original reservoir pressure can only be

determined from the first well drilled. When a producing well is shut in: pressure startsto rise until an equilibrium highest pressure isreached. Rise is rapid at first, slowing with time .

Maximum pressure = static bottom-holepressure or static formation pressure

Static formation pressure of a producing well isnormally low than virginal reservoir pressure.

Difference between the two = measure ofpressure decline in the reservoir.


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Flowing pressure = measured while well is

producing. Differential Pressure = difference between

flowing pressure and bottomhole static pressure

Casing/ Surface pressure = static pressure atthe top of a shut in well if the pressure has beenallowed to build to a maximum. Can calculate reservoir pressure from this by

incorporating total weight of fluid in the column

And many more.


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Sources of Reservoir Pressure Variety of sources of pressure:

Some continuous add to the present pressure system Some transitory effects

Their importance of various effects is dependant on whether

the reservoir is sealed in or not.

Three primary sources of reservoir pressure are:

Pressure exerted by the water above the point of

pressure measurement

Pressure exerted by the rock overburden

Osmotic phenomena


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Lesser contributors to pressure Temperature changes

Secondary precipitation or cementationphenomena

Earthquakes

Atmospheric and oceanic disturbances

Chemical and biological reactions

Often difficult to tell how much of the pressure is

contributed to by each of these.


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1. Pressure due to the water

column Interconnected pores are generally water filled,

which exerts a pressure. When the water is at rest Exerts hydrostatic pressure, at right angles to the

boundary surface, and in all dirctions at all points in

the fluid at that pressure value. Pressure at any point = pressure gradient x height ofwater column above the point of measurement

Energy stored by the reservoir is a potential

energy as it exists by virtue of its position. The pressure gradient is determined also by the

salinity of the brine.


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2. Compaction and Pressure Load pressure exerted by a column of rock =

100psi per 100ft depth. = 2 x normal hydrostatic pressure gradient

Pressure is transmitted by the particles of the

rock, not the fluid

When grains are allowed to touch (compacted)

they transmit some of the pressure to the fluid.

E.g. rapid basin filling: often causes fluids to be

squeezed out of the rocks.


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3. Osmosis

Clays = semipermeable membranes

Permit osmotic pressures to build upwherever there is a marked contrast in salinity

of the fluids either side of the clays.

Major contributor to subsurface fluidpressures


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a. Temperature Changes Changes in temperature change the fluid

pressure. Increased temperature causes oil, gas and

water to expand, increasing presssure.

Can be caused by an approaching igneousbody.

Fluids would then move towards the coolerregion.

Temperature has a significant effect on fluidviscosity in the subsurface


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b. Cementation & Secondary

Precipitation

Decrease of porosity resulting from

recrystallization/ cementation. In a confined unit this will cause pressure

elevation.

Generally only a temporary effect


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c. Earthquakes

Advance compression waves of

earthquakes cause elastic compression ofshallow reservoirs.

Causes sudden rise and fall of the

groundwater level

Earthquakes can increase production for

short periods in petroleum wells!


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d. Tides, Tsunamis and

Atmospheric pressures

Tidal and other oceanic disturbances cause

minor temporary elastic effects in underlyingrocks.

Ice cap melting and reformation undoubtedlycaused load movement over large surface of theearth.

Small atmospheric changes can have quitesignificant effects in shallow aquifers.

Oil seep speed can respond quite rapidly tochanging atmospheric pressures


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e. Chemical & Biological Processes Decrease in fluid volumes = decrease in fluid pressures.

Volume of a solution is much less than the combinedvolumes of the solvent and solutes.

In fact, adding salt to distilled water decreases volumeas the molecules are forced closer together.

Breakdown of hydrocarbons tends to increase volume. Catalytic reactions, biological activity, temperature

changes can all cause this volume change.

Using Pressure Tests to Define Fluid Contacts In

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