Porosity can be measured in the laboratory on core samples, from thin sections under a microscope, or in the well with logs. This article explains each method and what it measures. For the basics, see porosity.
Core Analysis Methods
Bulk volume is determined first, either by displacement of a liquid or by accurately measuring a shaped sample and computing its volume. Then one of the following methods is used to measure either the pore volume or the grain volume:
- Summation of pore fluids: gas, oil and pore water volumes are determined independently from a fresh core sample. The pore volume is the sum of the three.
- Washburn-Bunting method: measures pore volume by vacuum extraction and collection of the gas (usually air) in the pores.
- Liquid resaturation: the pores of a prepared sample are filled with a liquid of known density. The weight increase of the sample divided by the liquid density gives the pore volume.
- Boyle’s law method: a gas is compressed into the pores, or expanded out of them, in a prepared sample. Either pore volume or grain volume can be found, depending on the porosimeter and procedure used.
- Grain density method: measures total porosity. After the dry weight and bulk volume are found, the sample is reduced to grain size and the grain volume is determined and subtracted from the bulk volume.
| Method | Measures | Notes |
|---|---|---|
| Summation of pore fluids | Pore volume | Needs a fresh core sample |
| Washburn-Bunting | Pore volume | Vacuum extraction of pore gas |
| Liquid resaturation | Pore volume | Uses a liquid of known density |
| Boyle’s law | Pore or grain volume | Depends on the porosimeter |
| Grain density | Total porosity | Sample is crushed to grain size |
In all of them, porosity = pore volume ÷ bulk volume. If the grain volume is measured, pore volume = bulk volume − grain volume.
Thin-Section Analysis
Porosity can also be determined by petrographic analysis of thin sections of a rock sample. Pores are counted under a microscope by point counting. Impregnating the sample under vacuum with dyed resin makes the pores easier to identify.
Porosity From Well Logs
A common source of porosity data is well logs. Porosity can be calculated from the sonic, density and neutron logs, which are usually called the porosity logs. Porosity may also be derived from the resistivity logs. Core data are often used to calibrate log-derived porosity.
Porosity Ranges
Porosity values are described according to the classification of porosity. These methods measure pore space, not how easily fluids flow. That is permeability, and the two are compared in relation between porosity and permeability.
Frequently Asked Questions
Which is more accurate, core or log porosity?
Core measurements are direct, but cover only small samples. Logs cover the whole well, so they are calibrated with core data.
Which logs measure porosity?
Sonic, density and neutron logs.
Key Takeaways
- Lab methods find pore volume or grain volume, and bulk volume is measured first.
- Thin sections give porosity by point counting under a microscope.
- Sonic, density and neutron logs give continuous porosity along the well.
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