Reservoir pressures are an important aspect of understanding oil and gas reservoirs. Pressures in reservoirs are primarily determined by lithostatic pressure from the weight of overlying rock and hydrostatic pressure from fluid columns. Abnormal pressures can occur when fluid pressures are higher or lower than hydrostatic gradients due to factors like rapid burial or temperature changes. Proper characterization of reservoir pressures is important for reservoir engineering and requires measurement techniques like pressure surveys and well tests.

1. Reservoir Pressures
Reservoir Pressures
Adrian C Todd
Heriot
Heriot-
-Watt University
Watt University
INSTITUTE OF PETROLEUM ENGINEERING
Heriot
Heriot-
-Watt University
Watt University
INSTITUTE OF PETROLEUM ENGINEERING

2. Reservoir Pressures
Reservoir Pressures
 Magnitude and variation of pressures in a
reservoir are an important aspect of reservoir
understanding during exploration and
production phase

3. Reservoir Pressures
Reservoir Pressures
 Oil and gas occur at a range of sub-surface
depths.
 At these depths pressure exists as a result of:
– the depositional process
– the fluids contained.

4. Lithostatic Pressures & Fluid Pressures
 Lithostatic pressure
 grain to grain transmission of weight of rock
 sometimes termed geostatic or overburden
pressure.
 Function of depth, density
 1 psi./ ft
 Pov at depth D = 1.0 x D psi.

5. 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.

6. 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 = rfluidDg g=acceleration due to gravity

7. Lithostatic Pressures & Fluid Pressures
Hydrostatic pressure
Lithostatic pressure

8. Hydrodynamic Pressure
 Arises as a result of fluid movement.
 This is the fluid potential pressure gradient
which is caused by fluid flow

9. Fluid Pressure
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 extrapolated to zero depth is atmospheric
pressure
- normal pressured reservoir
14.7
W
water
dP
P D psia
dD
 
 
 
 
 
 
 

10. Fluid Pressure-Normal Pressure
Atmos. Pressure
0 psig.
14.7psia.
Normal pressured
reservoir

11. 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

12. Abnormal Pressure
Overpressured
reservoir
Underpressured
reservoir

13. Abnormal Pressure
Pressure
Water-normal
0.45psi’ft.
Overpressured
0.45psi/ft.
1000-2000psi
N. Viking Graben-N.Sea

14. Abnormal Pressure
14.7
W
water
dP
P D Cpsia
dD
 
 
  
 
 
 
 
C - constant positive - overpressured
C - constant negative - underpressured

15. Causes of Abnormal Pressure
 Thermal effects-expansion or contraction of
water
 Rapid burial of sediments
 Geological changes.
 Osmotic effects via salinity differences

16. Causes of Abnormal Pressure
Geological changes

17. Abnormal Pressure Regional Trends

18. North Sea
Examples

19. Fluid Pressures-Hydrocarbon Systems
 Hydrocarbon pressure regimes different since
densities of oil and gas are less than water.
0.45 /
water
dP
psi ft
dD
 

 
 
0.35 /
oil
dP
psi ft
dD
 

 
 
0.08 /
dP
gas psi ft
dD
 

 
 
Pressure
Depth
0

20. Pressure distribution for an oil reservoir with a
gas-cap and oil water contact.
Impermeable
bed
Path of well
Pressure
Gradient in
aquifer
Gradient in oil
column
Gradient in gas
column
Over pressured
reservoir

21. Pressure distribution for an oil reservoir with a
gas-cap and oil water contact.

22. Hydrocarbon Pressure Regimes
 Nature and magnitude of pressures and the position
of 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.

23. 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.

24. Examples of Pressure Measurement
 Pressure
distributions before
and after production
provide important
reservoir description
information.
Production from here
Original pressure
profile
Pressure survey after
production

25. Examples of Pressure Measurement
After subsequent
production
Evidence of layering

26. Examples of
Pressure
Measurement
 Can also be
used to indicate
lack of
hydrodynamic
continuity.

27. Examples of
Pressure
Measurement
 As an interference
test can indicate
flow behaviour
between wells.

28. 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 1.6oF/100ft (
0.029K/m).
 Because of large thermal capacity and surface area of
porous reservoir, flow processes in a reservoir occur at
constant temperature.
 Local conditions , eg around the well can be influenced by
transient cooling or heating effects of injected fluids.