Porosity
- Also known as
- effective porosity
Porosity is the fraction of a rock's volume that is empty space, which can hold fluids such as oil, gas and water. Effective porosity counts only the interconnected pores through which fluids can move. Porosity determines how much fluid a reservoir can store, not how easily it flows.
What is porosity?
Sedimentary rocks such as sandstone and limestone are made of grains or crystals with spaces between them. Porosity is the volume of those spaces divided by the total volume of the rock, expressed as a fraction or percentage. A rock with 20 percent porosity has one-fifth of its volume available for fluids.
Total and effective porosity
- Total porosity includes all void space, including isolated pores and water bound to clays.
- Effective porosity includes only connected pores that can contribute to flow, which matters most for production.
What controls porosity
- Grain sorting. Well-sorted grains of similar size leave more space; poorly sorted grains fill gaps.
- Grain packing and shape.
- Cementation and compaction, which reduce porosity as rocks are buried.
- Dissolution and fracturing, which can create additional porosity, especially in carbonates.
Many productive conventional sandstone and carbonate reservoirs have porosities in the range of roughly 10 to 30 percent. Shales and tight rocks typically have lower porosity, and their pores are much smaller.
Porosity versus permeability
Porosity measures storage. Permeability measures flow. A rock can be highly porous but have low permeability if its pores are poorly connected or its pore throats are very narrow, as in many shales. A conventional reservoir needs both.
How porosity is measured
- Well logs, such as density, neutron and sonic logs.
- Core analysis in the laboratory.
Frequently asked questions
It means more space for fluids. The amount of oil also depends on how much of that space oil occupies, its saturation, and on the reservoir’s size.
Slightly. Declining pressure can compact some rocks, and precipitation or formation damage can reduce effective porosity near the well.
Sources
- porosity; effective porosity, SLB Energy Glossary
- Porosity and permeability of rocks, U.S. Geological Survey