What process is used to determine the pore size distribution in a formation sample?

What process is used to determine the pore size distribution in a formation sample?

• A. Inject the sample with mercury then plot capillary pressure vs water saturation
• B. Dissolve the sample in acid and measure porosity
• C. CT scanning of the sample
• D. Measuring electrical conductivity of the sample

Answer: (C)

Pore size distribution is determined by a method that links how a liquid invades the pore space to the size of the pore throats. Mercury intrusion porosimetry does exactly this: mercury, a non-wetting liquid, is forced into the rock sample while pressure is gradually increased. Smaller pores resist intrusion until higher pressures are applied. By recording how much mercury enters at each pressure and applying the Washburn relationship between capillary pressure and pore radius, you get a quantitative distribution of pore throat sizes. This approach provides a direct, quantitative view of how many pores exist at each size across a wide range, which is essential for understanding fluid flow in formation rocks. Dissolving the sample in acid and measuring porosity would tell you how much pore space exists overall, but not how that space is distributed by size. CT scanning can image the pore structure in three dimensions and give qualitative or indirect size information, but its ability to precisely define a pore size distribution depends on resolution and image analysis, so it’s not the standard direct measurement. Measuring electrical conductivity reflects porosity and connectivity to some extent but does not yield a pore size distribution.

Pore size distribution is determined by a method that links how a liquid invades the pore space to the size of the pore throats. Mercury intrusion porosimetry does exactly this: mercury, a non-wetting liquid, is forced into the rock sample while pressure is gradually increased. Smaller pores resist intrusion until higher pressures are applied. By recording how much mercury enters at each pressure and applying the Washburn relationship between capillary pressure and pore radius, you get a quantitative distribution of pore throat sizes. This approach provides a direct, quantitative view of how many pores exist at each size across a wide range, which is essential for understanding fluid flow in formation rocks.

Dissolving the sample in acid and measuring porosity would tell you how much pore space exists overall, but not how that space is distributed by size. CT scanning can image the pore structure in three dimensions and give qualitative or indirect size information, but its ability to precisely define a pore size distribution depends on resolution and image analysis, so it’s not the standard direct measurement. Measuring electrical conductivity reflects porosity and connectivity to some extent but does not yield a pore size distribution.