Post on 04-Jun-2018
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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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Reservoir PressuresReservoir Pressures
Magnitude and variation of pressures in a
reservoir are an important aspect of reservoir
understanding during exploration and
production phase
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Reservoir PressuresReservoir Pressures
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 Pressures & Fluid Pressures
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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Lithostatic Pressures & Fluid Pressures
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
Pressure distribution for an oil reservoir with a
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Pressure distribution for an oil reservoir with a
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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