Capillary pressure (Pc) curves: pore throat size determination
Exploring for Oil and Gas Traps  
Series  Treatise in Petroleum Geology 

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 
Capillary pressure (P_{c}) curves are a rock property measurement that relates the volume of pore space controlled by pore throats of a given size (usually given in microns) to a given capillary pressure.
Contents
What is capillary pressure?
P_{c} is the resistant force to hydrocarbon migration. It is a function of the interfacial tension (γ), the wettability (Θ), and pore throat radius (r). P_{c} increases with decreasing pore throat size, increasing interfacial tension, and increasing contact angle (greater oil wetting). It can be expressed as follows:
This expression assumes the capillary phenomenon occurs within a tube with a circular cross section. Real pores only approximate this, and then only if they are intergranular or inter crystalline.^{[1]}
Capillary test procedure
In a mercury capillary pressure test, a rock with a measured porosity is immersed in a mercury pressure cell. The pressure in the cell is raised to a predetermined pressure level (P1, Figure 1). When the cell comes to equilibrium, the volume of injected mercury is measured (V2). Since the porosity of the test sample is known prior to the test, the volume of injected mercury can be converted to the percent of the total pore volume filled with mercury (for example, 10% at 10 psi68.948 kPa
0.0689 MPa
0.68 atm for point M1). All the pores filled with mercury at this point in the test have at least one 10μ pore throat radius or larger and represent 10% of the sample's pore volume. This procedure is repeated several more times at different pressures (for example, points M2 through M5).
Pore throat profiles
A curve is drawn through the measured points at test completion. This capillary pressure curve also represents a pore throat size profile for the tested sample. It relates a given pore throat size to its capillary resistance (P_{c}). Figure 2 shows the curve drawn through the points in Figure 1.
Converting capillary pressure to pore throat size
Capillary pressure curves are converted to profiles of pore throat size by solving the previous equation for r:
Capillary pressure for a given S_{w} can also be converted to an approximation of height above free water (h) within a reservoir system. From a capillary pressure curve at a given S_{w}, we read the capillary pressure and multiply it by a factor that converts P_{c} to buoyancy pressure (P_{b}). If the conversion factor is not known, we use 0.4 for gas and 0.7 for oil.
Using p_{c} to estimate h and r
Use the table below to estimate height above free water (h) and pore throat radius (r) from a mercury capillary pressure curve.
To estimate  Follow this procedure 

Pore throat size ( r ) from S_{w} 

Height above free water level (h) from S_{w} 

Example
Using the curve in Figure 3, if S_{w} = 20% (point 1), then the mercury capillary pressure (P_{c}) that must be overcome to enter pore throats at that point on the curve is 200 psi1,378.951 kPa
1.379 MPa
13.609 atm (point 2). Converting mercury P_{c} to hydrocarbon column height (h):
The minimum pore throat radius entered when S_{w} is 20% and P_{c} is 200 psi1,378.951 kPa
1.379 MPa
13.609 atm is 0.5μ.
See also
 Pore systems
 Pore system fundamentals
 Pore system shapes
 Pore and pore throat sizes
 Pore throat size and connectivity
References
 ↑ Coalson, personal communication, 1997