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
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
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
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
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
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
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.