Geological classification of porosity
As sediments are deposited in geologically ancient seas, the first fluid that filled pore paces in sand beds was seawater, generally referred to as connate water. A common method of classifying porosity of petroleum reservoirs is based o whether pore spaces in which oil and gas are found originated when the sand beds were laid down ( primary \ matrix porosity), or if they were formed through subsequent diagenesis (dolomitization in carbonate rocks), catagenesis, earth stresses and solution by water flowing through the rock (secondary or induced porosity). The following general classification of porosity, adapted from Ellison, is based on time origin, mode of origin and distribution relationships of pore spaces.
Types of geologic porosities
- Primary porosity is the main or original porosity system in a rock or unconfined alluvial deposit.
- Secondary porosity is a subsequent or separate porosity system in a rock, often enhancing overall porosity of a rock. This can be a result of chemical leeching of minerals or the generation of a fracture system. This can replace the primary porosity or coexist with it (see dual porosity below).
- Fracture porosity is porosity associated with a fracture system or faulting. This can create secondary porosity in rocks that otherwise would not be reservoirs for hydrocarbons due to their primary porosity being destroyed (for example due to depth of burial) or of a rock type not normally considered a reservoir (for example igneous intrusions or metasediments).
- Vuggy porosity is secondary porosity generated by dissolution of large features (such as macrofossils) in carbonate rocks leaving large holes, vugs, or even caves.
- Effective porosity (also called open porosity) refers to the fraction of the total volume in which fluid flow is effectively taking place (this excludes dead-end pores or non-connected cavities). This is very important for groundwater and petroleum flow, as well as for solute transport.
- Dual porosity refers to the conceptual idea that there are two overlapping reservoirs which interact. In fractured rock aquifers, the rock mass and fractures are often simulated as being two overlapping but distinct bodies. Delayed yield, and leaky aquifer flow solutions are both mathematically similar solutions to that obtained for dual porosity; in all three cases water comes from two mathematically different reservoirs (whether or not they are physically different).
- Macro porosity refers to pores greater than 50 nm in diameter. Flow through macropores is described by bulk diffusion.
- Meso porosity refers to pores greater than 2 nm and less than 50 nm in diameter. Flow through mesopores is described by knudsen diffusion.
- Micro porosity refers to pores smaller than 2 nm in diameter. Movement in micropores is by activiated diffusion.
Characteristic features of the two basic porosity types:
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Primary porosity
- intercrystalline - voids between cleavage planes of crystals, voids between individual crystals and void in crystal lattices. Many of these voids are subcapillary, i.e. pores less than 0.002mm in diameter. The porosity found in crystal lattices and between mud-sized particles has been called “microporosity”. Usually high recovery of water in some productive carbonate reservoirs may be due to the presence of large quantities of microporosity.
- intergranular (interparticle) – voids between grains, i.e. interstitial voids of all kinds in all types of rocks. These openings range from subcapillary through super-capillary (voids greater than 0.5mm in diameter).
- bedding planes – voids of many varieties are concentrated parallel to the bedding planes. The larger geometry of many petroleum reservoirs is controlled by such bedding planes. Differences of sediments deposited, of particle sizes and arrangements and of the environments of deposition are causes of bedding plane voids.
- miscellaneous sedimentary voids – (1) voids resulting from the accumulation of detrital fragments of fossils; (2) voids resulting from the packing of oolites; (3) vuggy and caverneous voids of irregular and variable sizes formed at the time of deposition; (4) voids created by living organisms at the time of deposition.
Secondary porosity
Secondary porosity is the result of geological processes (diagenesis and catagenesis) after the deposition of sediment. The magnitude, shape, size and interconnection of the pores may have no direct relation to the form of original sedimentary particles. Induced porosity can be subdivided into three groups based on the most dominant geologival process:
- solution porosity – channels due to the solution of rocks by circulating warm or hot solutions; openings caused by weathering (enlarged joints or solution caverns); and voids caused by organisms and later enlarged by solution.
- dolomitization – a process by which limestone is transformed into dolomite. Some carbonate rocks are almost pure limestones and if the circulating ore water contains significant amounts of magnesium cation, the calcium in the rock can be exchanged for magnesium in the solution. Because the ionic volume of magnesium is considerably smaller that that of the calcium which it replaces, the resulting dolomite will have greater porosity. Complete replacement of calcium by magnesium can result in a 12-13% increase in porosity.
- fracture porosity – openings created by structural failure of the reservoir rocks under tension caused by tectonic activities such as folding and faulting. These openings include joints, fissures and fractures. Porosity due to fractures alone in carbonates usually does not exceed 1%.
- miscellaneous secondary voids – (1) saddle reefs which openings at the rest of closely folded narrow anticlines; (2) pitches and flats which are openings formed by the parting of beds under gentle slumping; (3) voids caused by submarine slide breccias and conglomerates resulting from gravity movement of seafloor material after partial lithification.
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In carbonate reservoirs secondary porosity is much more important than primary porosity. Primary porosity is dominant in clastic (detrital \ fragmental) sedimentary rocks (sandstones, conglomerates and certain oolite limestones). It is important to emphasis that both types of porosity often occur in the same reservoir rock.
There are several ways to estimate the porosity of a given material or mixture of materials, which is called your material matrix.
- The volume/density method is fast and surprisingly accurate (normally within 2 % of the actual porosity). To do this method you pour your material into a beaker, cylinder or some other container of a known volume. Weigh your container so you know its empty weight, then pour your material into the container. Tap the side of the container until it has finished settling and measure the volume in the container. Then weigh your container full of this material, so you can subtract the weight of the container to know just the weight of just your material. So now you have both the volume and the weight of the material. The weight of your material divided by the density of your material gives you the volume that your material takes up, minus the pore volume. (The assumed density of most rocks, sand, glass, etc. is assumed to be 2.65 g/cm3. If you have a different material, you may look up its density) So, the pore volume is simply equal to the total volume minus the material volume, or more directly (pore volume) = (total volume) - (material volume).
- Water saturation method is slightly harder to do, but is more accurate and more direct. Again, take a known volume of your material and also a known volume of water. (Make sure the beaker or container is large enough to hold your material as well.) Slowly dump your material into the water and let it saturate as you pour it in. Then seal the beaker (with a piece of parafilm tape or if you don't have parafilm tape a plastic bag tied around the beaker will do.) and let it sit for a few hours to insure the material is fully saturated. Then remove the unsaturated water from the top of the beaker and measure its volume. The total volume of the water originally in the beaker minus the amount of water not saturated is the volume of the pore space, or again more directly (pore volume) = (total volume of water) - (unsaturated water).
- Water evaporation method is the hardest to do, but is also the most accurate. Take a fully saturated, known volume of your material with no excess water on top. Weigh your container with the material and water and then place your container into a heater to dry it out. Drying out your sample may take several days depending on the heat applied and the volume of your sample. Then weigh your dried sample. Since the density of water is 1 g/cm3, the difference of the weights of the saturated versus the dried sample is equal to the volume of the water removed from the sample (assuming you are measuring in grams), which is exactly the pore volume. So once again, (pore volume in cubic centimeters) = (weight of saturated sample in grams) - (weight of dried sample in grams).
- Mercury intrusion porosimetry requires the sample to be placed special filling device that allows the sample to be evacuated followed by the introduction of liquid mercury. The size of the mercury envelope is then measured as a function of increased applied pressure. The greater the applied pressure, the smaller the pore entered by mercury. Typically this method is used over the range of pores from 300 µm to 0.0035 µm. This method is used to characterize a variety of porous material from coal to fabrics. Because of increased concern over use of mercury, several non-mercury intrusion techniques have been developed.
- Nitrogen gas adsorption is used to determine fine porosity in materials such as charcoal. In very small pores, nitrogen gas condenses on the pore walls less than 0.090 µm. This condensation is measured either by volume or weight.
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| void space | пустое пространство |
| fraction | часть \ доля |
| grain volume | объем зерен |
| pore volume | поровое пространство |
| porosity value | значение пористости |
| packing arrangement | выкладка породных полос \ закладка кусковым материалом |
| wide-packed system | |
| close-packed system | плотно уложенная система |
| uniformity (sorting) | однородность |
| grain size | размер зерен |
| gradation | постепенный переход из одного состояния в другое |
| effective (intercommunicating) | действующая (эффективная) поростость (сообщающаяся \взаимосвязанная) |
| squeeze out | выжимать |
| expulsion | выделение \ вытеснение |
| overburden pressure | давление покрывающих пород |
| random (packing) | хаотическая \ неупорядоченная |
| closer (packing) | плотная |
| consolidated – unconsolidated | затвердевший |
| interconnected (interconnection) | связанное |
| conductivity | удельная проводимость |
| vesicular (porosity) | везикулярный \ пузырчатая \ вспучения |
| hydration | гидратация |
| heterogeneity | неоднородность |
| leaching | выщелачивание \ вымывание |
| dead-ends | пустой |
| irreducible (fluids) | остаточные флюидов |
| connate water | погребенная \ реликтовая вода |
| primary (matrix) porosity | первичная пористость |
| intercrystalline | интеркристаллический |
| cleavage | кливаж \ спайность |
| plane | плоскость \ горизонт |
| lattice (crystal) | решетка |
| subcapillary | субкапиллярный |
| intergranular (interparticle) | междузерный |
| opening (s) | пора (мн. пустоты- в породе) |
| bedding pane | плоскость напластования |
| miscellaneous (sedimentary voids) | смешанный |
| detrital (fragments) | обломочный |
| vuggy | пористый |
| cavernous | пещеристый \ кавернозный |
| secondary (induced) porosity | вторичная ( наведенная) пористость |
| diagenesis | диагенез |
| catagenesis | катагенез |
| dolomitization | доломитизация |
| solution porosity | |
| cavern | каверна (карстовая пустота) |
| fracture porosity | |
| structural failure | структурное оседание |
| saddle reef | пластовая жила, имеющая форма антиклинали |
| crest | гребень \ сводная часть складки |
| flat | горизонтально залегающий пласт |
| pitch | угол падения \ погружение антиклинали |
| slumping | оползание |
| fissure | разрыв \ трещина в породе |
PERMEABILITY
A reservoir rock must have the ability to allow petroleum fluids to flow through its interconnected pores. This rock property is termed permeability. The permeability of a rock depends on the effective porosity. Therefore, permeability is affected by the rock grain size, grain shape, grain size distribution (sorting), grain packing and the degree of consolidation and cementation. Permeability is affected by the type of clay present, especially where fresh water is present.
French engineer Henry Darcy developed a fluid flow equation that since has become one of the standard mathematical tools of the petroleum engineer. One Darcy is a relatively high permeability and the permeability of most reservoir rock is less than one Darcy. The common measure of rock permeability is in millidarcies (mD) or μm2 in SI units.
The term absolute permeability is used if the porous rock is 100% saturated with a single fluid (phase), such as water, oil or gas. When two or more fluids are present in the rock, the permeability of the rock to the flowing fluid is called effective permeability. Because fluids interfere with each other during their movement through the pore channels in the rock, the sum of effective permeability will always be less than the absolute permeability. The ratio of effective permeability of one phase during multiphase flow to the absolute permeability is the relative permeability to that phase.
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