Porosity Measurement: Methods, Formulas, and Ranges
Technical note
Porosity Measurement: Methods, Formulas, and Ranges
How the pore fraction of a core is measured, the equations behind each method, and the porosity ranges you should expect by rock type.
Porosity sets the storage capacity of a reservoir rock. It is the first property measured in routine core analysis and it feeds almost everything downstream, from hydrocarbon in place to the pore volumes used in permeability, resistivity, and capillary pressure work. This note covers what porosity is, the laboratory methods used to measure it, the equations each method relies on, and the ranges that different rocks return.
What porosity is
Porosity is the fraction of a rock occupied by pore space rather than solid grains. It is reported as a fraction or a percentage of the bulk volume.
Two definitions matter in practice. Total porosity counts every void, including pores that are sealed off from the rest. Effective porosity counts only the connected pore space that fluid can enter and leave. Reservoir engineering uses effective porosity, because isolated pores hold no producible fluid. Helium and fluid-saturation methods measure effective porosity. Some log and imaging methods report values closer to total.
The three volumes
Every porosity method resolves two of three volumes and solves for the third.
Bulk volume is the outer volume of the plug. Grain volume is the volume of the solid framework. Pore volume is the difference. A method that measures bulk and grain volume gives pore volume by subtraction. A method that measures pore volume and bulk volume gives grain volume. The choice of which two to measure defines the technique.
Helium expansion porosimetry
Gas expansion is the reference method for grain volume in routine core analysis. A dry plug is sealed in a calibrated sample chamber. Helium is charged to a reference cell at a known pressure, then a valve opens and the gas expands into the sample chamber. Because helium is inert and its small molecule reaches connected pores quickly, the equilibrium pressure fixes the volume the gas could occupy, and grain volume follows from Boyle's law.
Bulk volume is measured separately, by caliper on a regular plug or by mercury or buoyancy for irregular pieces. Pore volume then comes from equation (2) and porosity from equation (1). Helium porosimetry gives effective porosity, is nondestructive, and takes a few minutes per plug. It is the method built into combined permeameter-porosimeter systems such as the Coretest AP-608 Neo, which reads grain volume and permeability on the same core.
Liquid saturation method
A cleaned, dried plug is weighed, saturated under vacuum with a fluid of known density, then weighed again. The mass gained divided by the fluid density is the pore volume.
Weighing the saturated plug suspended in the same fluid also gives bulk volume by buoyancy, so a single saturation run can return both volumes. The method is simple and accurate for clean, competent rock, but it needs full saturation and careful drying, and it is slower than gas expansion.
Bulk volume methods
Bulk volume is measured by caliper for right cylinders, by mercury displacement for irregular samples, or by Archimedes buoyancy in a wetting fluid. Mercury displacement suits pieces that cannot be shaped into a clean plug. Buoyancy pairs naturally with the saturation method above. Caliper is fastest but assumes a true cylinder, so it carries more error on chipped or tapered plugs.
Mercury injection
Mercury injection capillary pressure forces mercury into the pore system at rising pressure. The total mercury taken up at high pressure approximates the connected pore volume, so the test returns porosity alongside its main output, the pore-throat size distribution. The rock is not recoverable afterward, so mercury injection is used where pore structure is the goal rather than a routine porosity number.
NMR porosity
Nuclear magnetic resonance measures the hydrogen in pore fluids. The total signal scales with fluid volume and gives porosity without a grain-density assumption, while the relaxation-time distribution separates clay-bound, capillary-bound, and free fluid. NMR is valuable in shaly and low-permeability rock where gas expansion and saturation are slow or ambiguous, and it links the laboratory core to the downhole NMR log.
Digital rock and CT
Micro-CT and thin-section imaging resolve the pore space directly and compute porosity by counting void voxels. Digital methods add pore geometry and connectivity that bulk measurements cannot show, but resolution limits them below the micron scale, so they undercount microporosity in tight and carbonate rock. They complement, rather than replace, gas expansion for a reservoir-average number.
Ranges by rock type
Expected porosity varies widely with lithology, depth, and diagenesis. The table lists typical total porosity for common reservoir rocks.
| Rock | Typical porosity | Notes |
|---|---|---|
| Unconsolidated sand | 35 to 45 percent | shallow, poorly compacted |
| Reservoir sandstone | 10 to 35 percent | falls with depth and cementation |
| Carbonate | 1 to 35 percent | strongly controlled by diagenesis |
| Chalk | up to 40 percent | high porosity, low permeability |
| Shale and mudrock | 1 to 10 percent | mostly nanoporosity |
| Tight and unconventional | under 10 percent | needs NMR or crushed-rock methods |
Which method, and when
For routine reservoir sandstone and most carbonate, helium expansion with a measured bulk volume is the standard, and combined systems report porosity and permeability together. For friable or vuggy rock, or where a wettability-preserved plug is needed, liquid saturation with buoyancy is preferred. For pore structure, mercury injection adds throat-size data at the cost of the sample. For shale and tight rock, NMR and crushed-rock techniques handle pore space that gas expansion reaches slowly. Reporting effective porosity, and stating the method used, keeps the number comparable across a study.
Porosity and permeability are read together in most core programs. For the companion methods on the flow side, see Gas Permeability Measurement: Methods, Formulas, and Ranges.
Related equipment: the Coretest porosity and grain-volume systems measure grain volume by helium expansion, several on the same core as gas permeability.
