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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.
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