Capillary pressure (Pc): buoyancy, height, and pore throat radius
Exploring for Oil and Gas Traps | |
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Series | Treatise in Petroleum Geology |
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Part | Predicting the occurrence of oil and gas traps |
Chapter | Predicting reservoir system quality and performance |
Author | Dan J. Hartmann, Edward A. Beaumont |
Link | Web page |
Store | AAPG Store |
A capillary pressure (Pc) curve is generated in the lab using a mercury pressure cell, a porous plate, or a centrifuge. Fluid systems used in these techniques include air–water (brine), oil–water, air–mercury, or air–oil. Data generated by these techniques cannot be compared directly to each other or to reservoir conditions. Below, we demonstrate how to convert pressures measured in the lab to
- Standard pressure scale (mercury Pc)
- Reservoir pressure
- Height above free water
- Pore throat size
If true reservoir conditions are at least partially oil wet, then the water saturation–height plot shifts away from the pore volume–height plot (Pc curve).
Converting lab capillary pressure data
Follow the steps in the table below to convert Pc to buoyancy pressure, pore throat size (r), or hydrocarbon column height (h′). Assume water-wet conditions in the reservoir (γ = interfacial tension, Θ = contact angle).
Step | Action | Equation |
---|---|---|
1 | Rescale Pc from one lab system to a common technique (i.e., air–brine to air–mercury). | |
2 | Convert lab Pc to reservoir Pc (i.e., air– mercury to water–oil). | |
3 | Convert reservoir Pc to height ( h ′). | Typical gradients in psi/ft:
|
4 | Convert lab Pc to pore throat radius ( r ) in microns. | where C is the constant 0.29 |
Example
The following is an example of applying the conversion of lab Pc data to reservoir conditions.
Use these assumptions:
- Maximum air–brine Pc value for a sample is pressure::40 psi.
- Air–mercury is the base data set.
- The reservoir is oil filled.
Determine these parameters:
- Equivalent air–mercury Pc (Pc Hg)
- Equivalent oil Pc (buoyancy pressure in the reservoir)
- Height above the free water (h′)
- Pore throat radius for Pc Hg as determined at A
Answers (refer to the table below for values)
Conversion variables
The conversion from one lab system to another requires values for the contact angle of the fluids to the grain surfaces (Θ) and interfacial tension (γ) between the two fluids. Theta is a reflection of wettability. This information is also required to determine an equivalent buoyancy pressure in the reservoir. Some typical lab and reservoir values are shown in the table below.
Laboratory Measurements | ||||
---|---|---|---|---|
System | Θ | cosΘ | γ | γcosΘ |
Air–water | 0 | 1.0 | 72 | 72 |
Oil–water | 30 | 0.866 | 48 | 42 |
Air–mercury | 40 | 0.766 | 480 | 367 |
Air–oil | 0 | 1.0 | 24 | 24 |
Reservoir Measurements | ||||
System | Θ | cosΘ | γ | γcosΘ |
Water–oil | 30 | 0.866 | 30 | 26 |
Water–gas | 0 | 1.0 | 50 | 50 |
See also
- Pore-fluid interaction
- Hydrocarbon expulsion, migration, and accumulation
- Characterizing rock quality
- Pc curves and saturation profiles
- What is permeability?
- Relative permeability and pore type