Res Eng PP Ch2

8/13/2019 Res Eng PP Ch2

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Heriot-Watt University

DEPARTMENT OF PETROLEUM ENGINEERING

HeriotHeriot--Watt UniversityWatt University

DEPARTMENT OF PETROLEUM ENGINEERING

Reservoir PressuresReservoir Pressures

Adrian C Todd


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Magnitude and variation of pressures in a

reservoir are an important aspect of reservoir

understanding during exploration and

production phase


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Oil and gas occur at a range of sub-surfacedepths.

At these depths pressure exists as a result of:

the depositional process

the fluids contained.


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Lithostatic Pressures & Fluid Pressures

Lithostatic pressure

grain to grain transmission of weight of rock

sometimes termed geostaticor overburden

pressure.

Function of depth, density

1 psi./ ft

Pov at depth D = 1.0 x D psi.


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Lithostatic pressure is balanced in part by the

pressure of fluids within pores,pore pressure

and by grains of rock under compaction.

Unconsolidated sands, overburden totally

supported by fluid pressure. In deposited rocks, like reservoirs, fluid

pressure is not supporting the rocks but arises

from the continuity of the aqueous phase

from surface to the depth.

Termed hydrostatic pressure.


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Hydrostatic Pressure

Imposed by a column of fluid at rest.

Value depends on the density of fluid.

Water - salinity

0.433 psi/ft - fresh water 0.45 psi/ft for saline water 55,000ppm.

0.465 psi for 88,000ppm

Pfluid = fluidDg g=acceleration due to gravity


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Hydrostatic pressure

Lithostatic pressure


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Hydrodynamic Pressure

Arises as a result of fluid movement. This is the fluid potential pressure gradient

which is caused by fluid flow


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Fluid Pressure

14.7Wwater

dPP D psiadD

= +

Dictated by prevailing water pressure in vicinity of reservoir.

Normal situation

dP/dD is the hydrostatic gradient

Assumes continuity of water pressure from surface

and constant salinity

If pressure extrapoloted to zero depth is atmospheric

pressure

- normal pressured reservoir


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Fluid Pressure-Normal Pressure

Atmos. Pressure

0 psig.

14.7psia.

Normal pressured

reservoir


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Fluid Pressure-Abnormal Pressure

Under certain conditions fluid pressures are not

normal.

Overpressured reservoirs.

Hydrostatic pressure greater than normal

pressure

Underpressured reservoirs

Hydrostatic pressure below normal pressure


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Abnormal Pressure

Overpressured

reservoir

Underpressured

reservoir


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Abnormal Pressure

Pressure

D

ep

th Water-normal

0.45psift.

Overpressured

0.45psi/ft.

1000-2000psiN. Viking Graben-N.Sea


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Abnormal Pressure

14.7W

water

dPP D Cpsia

dD

= + +

C - constant positive - overpressured

C - constant negative - underpressured


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Causes of Abnormal Pressure

Thermal effects-expansion or contraction of

water

Rapid burial of sediments

Geological changes.

Osmotic effects via salinity differences


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Causes of Abnormal Pressure

Geological changes


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Abnormal Pressure Regional Trends


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North SeaExamples


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Fluid Pressures-Hydrocarbon Systems

Hydrocarbon pressure regimes different sincedensities of oil and gas are less than water.

0.45 /water

dPpsi ftdD

=

0.35 /oil

dPpsi ftdD

=

0.08 /dP gas psi ftdD

=

Pressure

Depth

0

P di t ib ti f il i ith


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Pressure distribution for an oil reservoir with a

gas-cap and oil water contact.

Impermeablebed

Path of well

Pressure

Gradient in

aquifer

Gradient in oil

column

Gradient in gas

column

Over pressured

reservoir



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gas-cap and oil water contact.

H d b P R i


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Hydrocarbon Pressure Regimes

Nature and magnitude of pressures and the positionof fluid contacts important to the reservoir engineer.

Data for fluid contacts from:

Pressure surveys

Equilibrium pressures from well tests

Fluid flow from minimum and maximum depth

Fluid densities from samples

Saturation data from logs

Capillary pressure from cores

Fluid saturation from cores.

T h i f P M t


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Techniques for Pressure Measurement Earlier tests for pressure logging have been replaced by open-hole

testing devices which measure vertical pressure distribution in a well.

E l f P M t


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Examples of Pressure Measurement

Pressuredistributions before

and after production

provide importantreservoir description

information.

Production from here

Original pressure

profile

Pressure survey afterproduction

E l f P M t


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Examples of Pressure Measurement

After subsequent

production

Evidence of layering

Examples of


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Examples of

Pressure

Measurement

Can also be

used to indicate

lack of

hydrodynamiccontinuity.

Examples of


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Examples of

Pressure

Measurement

As an interference

test can indicate

flow behaviour

between wells.

Reservoir Temperature


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Reservoir Temperature Earth temperature increases from surface to centre

Heatflow outwards generates a geothermal gradient.

Conforms to local and regional gradients as influenced by

lithology, and more massive phenomena.

Obtained from wellbore temperature surveys.

Reservoir geothermal gradients around 1.6o

F/100ft (0.029K/m).

Because of large thermal capacity and surface area of

porous reservoir, flow processes in a reservoir occur atconstant temperature.

Local conditions , eg around the well can be influenced by

transient cooling or heating effects of injected fluids.


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Documents

Res Eng PP Ch2