Effective porosity
Encyclopedia
The term effective porosity lacks a single or straightforward definition. Even some of the terms used in its mathematical description ("
” and “
”) have multiple definitions. However, it is most commonly considered to represent the porosity
of a rock or sediment available to contribute to fluid flow
through the rock or sediment, often flow to a borehole. Porosity that is not considered effective porosity includes water bound to clay particles and isolated "vuggy" porosity (vugs not connected to other pores). The effective porosity is of great importance in considering the suitability of rocks or sediments as oil or gas reservoir
s, or as aquifer
s.
Figure 1 illustrates core and log derivations of porosity for a sandstone and infers (some) different definitions of effective porosity.
The figure is discussed below going from left to right along the strip.
, or in core analysis terms, part of the grain volume.
is dried in a normal dry oven (non-humidified atmosphere) the clay layers and quartz together form the grain volume, with all other components constituting core analysis “total porosity” (notwithstanding comments in (6) below). This core total porosity will generally be equivalent to the total porosity derived from the density log when representative values for matrix and fluid density are used.
The clay layers contain groups (often termed “structural water”). This structural water is never part of the pore volume. However, since neutron
logs sense H (hydrogen) and all hydrogen
so-sensed is allocated as pore space, then neutron logs will overestimate porosity in argillaceous rocks by sensing as part of the pore space.
bound water (clay-bound water or CBW) which varies in volume according to the clay-type, and the salinity of the formation water (see the Attachments section). The most common definition of effective porosity for sandstones excludes CBW as part of the porosity, whereas CBW is included as part of the total porosity. That is:
To assess the effective porosity, samples are dried at 40-45% relative humidity
and 60 °C. This means that one to two molecular layers of CBW can be retained, and a form of “effective porosity” can be measured on the samples. However, the CBW retained by the humidity-dried core plugs is not necessarily representative of CBW in the formation at reservoir conditions. This lack of reservoir representation occurs not only because CBW tends to a minimum value in cores humidity-dried at the specified conditions but also because the amount of CBW at reservoir conditions varies with the salinity of formation water in the “effective” pore space.
Humidity-dried cores have no water in the “effective” pore space, and therefore can never truly represent the reservoir CBW condition. A further complication can arise in that humidity drying of cores may sometimes leave water of condensation in clay-free micropores (ref. 7).
Log derivation of effective porosity includes CBW as part of the volume of shale (Vsh). Vsh is greater than the volume of Vcl not only because it incorporates CBW, but also because Vsh includes clay size (and silt-size) quartz (and other mineral) grains, not just pure clay.
water which is different from CBW in that it is physically (not electrochemically) bound to the rock (by capillary forces). Capillary water generally forms part of the effective pore space for both log and core analysis.
However, microporous pore space associated with shales (where water is held by capillary forces and hence is not true CBW) is usually estimated as part of the Vsh by logs and therefore not included as part of the effective porosity. The total water associated with shales is more properly termed “shale water” which is larger in value than CBW (ref. 8). If we humidity dried core samples, (some of) the electrochemically bound CBW would be retained, but none of the capillary-bound microporous water (notwithstanding comments in ref. 7). Therefore, although the figure infers that a humidity-dried core could produce an effective porosity similar to a log analysis effective porosity, the effective porosity from the core will usually be higher (see “Examples” section)—notwithstanding comments in (6) below. Traditionally, true CBW has been directly measured neither on cores nor by logs, although NMR measurement holds promise (ref. 9).
At a given height above the free-water level, the capillary water becomes “irreducible”. This capillary water forms the irreducible water saturation (“Swi”) with respect to effective porosity (notwithstanding the inclusion of microporous water as Vsh during the log analysis) whereas for total porosity, the CBW and capillary water combined form the “Swi”.
s (in a hydrocarbon bearing formation). Above the transition zone, only hydrocarbons will flow. Effective porosity (with reference to the figure below) can be classified as only the hydrocarbon-filled large pore spaces above the transition zone (ref. 10).
Anecdotally, effective pore space has been equated to displaceable hydrocarbon pore volume. In this context, if residual hydrocarbon
saturation were calculated at 20%, then only 80% of the hydrocarbon-filled pores in the figure would constitute effective pore space.
s, make a negligible contribution to porosity. There are exceptions. In some carbonates, for example, the tests of microscopic organisms can become calcified to create significant isolated intra-particular pore space which is not connected to the inter-particular pore space available for hydrocarbon storage and flow. In such cases, core analysis will only record the inter-particular pore space, or “effective porosity”, whereas the density and neutron logs will record the total pore space. Only by crushing the rock can the core analysis yield the total porosity seen by the logs. The traditional Petroleum Engineering
and core analysis definition of effective porosity is the sum of the interconnected pore space—that is, excluding isolated pores (ref. 11). Therefore in practice, for the vast majority of sedimentary rocks, this definition of effective porosity equates to total porosity.
Utilising the Eslinger & Pevear concept, the following terms are illustrated in Figure 2.
Effective porosity
The sum of all the interconnected pore space. In the vast majority of cases, this core analysis and Petroleum Engineering definition of effective porosity equates to total porosity.
Effective porosity
Effective porosity measured on core samples which are dried in a humidity oven so that clays retain one or two molecular layers of bound water—however, this CBW tends to a minimum and is likely not reservoir representative.
Effective porosity
Log effective porosity. In essence, total porosity minus shale water, where solid minerals and the volume of shale (Vsh) constitute the matrix (non-effective porosity) and the remaining volume constitutes the effective porosity. For practical purposes, Vsh includes solid clays and the clay-sized and silt-sized fraction of non-clay minerals plus CBW and capillary bound water associated with shale micropores.
Effective porosity
In a hydrocarbon-bearing reservoir above the transition zone, only that pore space which is filled with hydrocarbons. From the NMR log, this equates to the Free Fluid Index (FFI), in other words, all pore space above the T2 cut-off.
Effective porosity
The volume of pore space which contains only producible hydrocarbons.
where
is total porosity, 
is salinity
factor (
) where 
the salinity in g
/l
, and
is the Cation Exchange Capacity
, meq/ml pore space
reservoirs in Western Australia
. Greensands are green because of iron-bearing glauconite
which is usually recognized as illite
/mica
or mixed layer illite-smectite clay by x-ray diffraction. The glauconite per se will incorporate electrochemically bound water (CBW) because of the clay types. More importantly for the consideration of effective porosity, though, glauconite grains (part of the Vsh) have intra-particular microporous pore space which retains capillary-bound water. Glauconite can constitute a large percentage of the reservoir rock, and therefore the associated intra-particular pore space can be significant. Log effective porosities calculated at 25% in some Greensand reservoirs have yielded core analysis effective porosities of 35% at equivalent depths. The difference is the glauconitic microporosity which contains water at reservoir conditions and is included as part of the Vsh (non-effective porosity) by log analysis. However, glauconitic microporosity is measured as part of the effective porosity in core plugs, even if they are humidity dried.
Greensands may cause varying degrees of difficulty for porosity log analysis. radicals affect neutron logs; the iron component is troublesome, and varying clay hydration needs to be considered for density log interpretation. The iron component affects the NMR logs and clay affects the sonic log. Therefore it is essential to have a core - or at least a good understanding of the geology - before invoking total vs effective porosity relationships.
” and “”) have multiple definitions. However, it is most commonly considered to represent the porosityPorosity
Porosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%...
of a rock or sediment available to contribute to fluid flow
Fluid dynamics
In physics, fluid dynamics is a sub-discipline of fluid mechanics that deals with fluid flow—the natural science of fluids in motion. It has several subdisciplines itself, including aerodynamics and hydrodynamics...
through the rock or sediment, often flow to a borehole. Porosity that is not considered effective porosity includes water bound to clay particles and isolated "vuggy" porosity (vugs not connected to other pores). The effective porosity is of great importance in considering the suitability of rocks or sediments as oil or gas reservoir
Reservoir
A reservoir , artificial lake or dam is used to store water.Reservoirs may be created in river valleys by the construction of a dam or may be built by excavation in the ground or by conventional construction techniques such as brickwork or cast concrete.The term reservoir may also be used to...
s, or as aquifer
Aquifer
An aquifer is a wet underground layer of water-bearing permeable rock or unconsolidated materials from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology...
s.
Background for multiple definitions
The figure is discussed below going from left to right along the strip.
Quartz
"Quartz" (more aptly termed “non-clay minerals”) forms part of the matrixMatrix (geology)
The matrix or groundmass of rock is the finer grained mass of material in which larger grains, crystals or clasts are embedded.The matrix of an igneous rock consists of finer grained, often microscopic, crystals in which larger crystals are embedded. This porphyritic texture is indicative of...
, or in core analysis terms, part of the grain volume.
Clay layers
"Clay layers" are dry clay (Vcl) which also form part of the grain volume. If a core sampleCore sample
A core sample is a cylindrical section of a naturally occurring substance. Most core samples are obtained by drilling with special drills into the substance, for example sediment or rock, with a hollow steel tube called a core drill. The hole made for the core sample is called the "core hole". A...
is dried in a normal dry oven (non-humidified atmosphere) the clay layers and quartz together form the grain volume, with all other components constituting core analysis “total porosity” (notwithstanding comments in (6) below). This core total porosity will generally be equivalent to the total porosity derived from the density log when representative values for matrix and fluid density are used.
The clay layers contain groups (often termed “structural water”). This structural water is never part of the pore volume. However, since neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
logs sense H (hydrogen) and all hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
so-sensed is allocated as pore space, then neutron logs will overestimate porosity in argillaceous rocks by sensing as part of the pore space.
Clay surfaces and interlayers
“Clay surfaces and interlayers” comprise electrochemicallyElectrochemistry
Electrochemistry is a branch of chemistry that studies chemical reactions which take place in a solution at the interface of an electron conductor and an ionic conductor , and which involve electron transfer between the electrode and the electrolyte or species in solution.If a chemical reaction is...
bound water (clay-bound water or CBW) which varies in volume according to the clay-type, and the salinity of the formation water (see the Attachments section). The most common definition of effective porosity for sandstones excludes CBW as part of the porosity, whereas CBW is included as part of the total porosity. That is:
To assess the effective porosity, samples are dried at 40-45% relative humidity
Relative humidity
Relative humidity is a term used to describe the amount of water vapor in a mixture of air and water vapor. It is defined as the partial pressure of water vapor in the air-water mixture, given as a percentage of the saturated vapor pressure under those conditions...
and 60 °C. This means that one to two molecular layers of CBW can be retained, and a form of “effective porosity” can be measured on the samples. However, the CBW retained by the humidity-dried core plugs is not necessarily representative of CBW in the formation at reservoir conditions. This lack of reservoir representation occurs not only because CBW tends to a minimum value in cores humidity-dried at the specified conditions but also because the amount of CBW at reservoir conditions varies with the salinity of formation water in the “effective” pore space.
Humidity-dried cores have no water in the “effective” pore space, and therefore can never truly represent the reservoir CBW condition. A further complication can arise in that humidity drying of cores may sometimes leave water of condensation in clay-free micropores (ref. 7).
Log derivation of effective porosity includes CBW as part of the volume of shale (Vsh). Vsh is greater than the volume of Vcl not only because it incorporates CBW, but also because Vsh includes clay size (and silt-size) quartz (and other mineral) grains, not just pure clay.
Small pores
"Small pores” contain capillaryCapillary action
Capillary action, or capilarity, is the ability of a liquid to flow against gravity where liquid spontanously rise in a narrow space such as between the hair of a paint-brush, in a thin tube, or in porous material such as paper or in some non-porous material such as liquified carbon fiber, or in a...
water which is different from CBW in that it is physically (not electrochemically) bound to the rock (by capillary forces). Capillary water generally forms part of the effective pore space for both log and core analysis.
However, microporous pore space associated with shales (where water is held by capillary forces and hence is not true CBW) is usually estimated as part of the Vsh by logs and therefore not included as part of the effective porosity. The total water associated with shales is more properly termed “shale water” which is larger in value than CBW (ref. 8). If we humidity dried core samples, (some of) the electrochemically bound CBW would be retained, but none of the capillary-bound microporous water (notwithstanding comments in ref. 7). Therefore, although the figure infers that a humidity-dried core could produce an effective porosity similar to a log analysis effective porosity, the effective porosity from the core will usually be higher (see “Examples” section)—notwithstanding comments in (6) below. Traditionally, true CBW has been directly measured neither on cores nor by logs, although NMR measurement holds promise (ref. 9).
At a given height above the free-water level, the capillary water becomes “irreducible”. This capillary water forms the irreducible water saturation (“Swi”) with respect to effective porosity (notwithstanding the inclusion of microporous water as Vsh during the log analysis) whereas for total porosity, the CBW and capillary water combined form the “Swi”.
Large pores
”Large pores” contain hydrocarbonHydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
s (in a hydrocarbon bearing formation). Above the transition zone, only hydrocarbons will flow. Effective porosity (with reference to the figure below) can be classified as only the hydrocarbon-filled large pore spaces above the transition zone (ref. 10).
Anecdotally, effective pore space has been equated to displaceable hydrocarbon pore volume. In this context, if residual hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
saturation were calculated at 20%, then only 80% of the hydrocarbon-filled pores in the figure would constitute effective pore space.
Isolated pores
“Isolated pores” in clastics, and most carbonateCarbonate
In chemistry, a carbonate is a salt of carbonic acid, characterized by the presence of the carbonate ion, . The name may also mean an ester of carbonic acid, an organic compound containing the carbonate group C2....
s, make a negligible contribution to porosity. There are exceptions. In some carbonates, for example, the tests of microscopic organisms can become calcified to create significant isolated intra-particular pore space which is not connected to the inter-particular pore space available for hydrocarbon storage and flow. In such cases, core analysis will only record the inter-particular pore space, or “effective porosity”, whereas the density and neutron logs will record the total pore space. Only by crushing the rock can the core analysis yield the total porosity seen by the logs. The traditional Petroleum Engineering
Petroleum engineering
Petroleum engineering is an engineering discipline concerned with the activities related to the production of hydrocarbons, which can be either crude oil or natural gas. Subsurface activities are deemed to fall within the upstream sector of the oil and gas industry, which are the activities of...
and core analysis definition of effective porosity is the sum of the interconnected pore space—that is, excluding isolated pores (ref. 11). Therefore in practice, for the vast majority of sedimentary rocks, this definition of effective porosity equates to total porosity.
Summary of terms
Total porosity
The volume of the reservoir rock which is fluid (oil, water, gas) filled, expressed as a percentage or a fraction of the gross (bulk) rock volume.Effective porosity 
The sum of all the interconnected pore space. In the vast majority of cases, this core analysis and Petroleum Engineering definition of effective porosity equates to total porosity.Effective porosity 
Effective porosity measured on core samples which are dried in a humidity oven so that clays retain one or two molecular layers of bound water—however, this CBW tends to a minimum and is likely not reservoir representative.Effective porosity 
Log effective porosity. In essence, total porosity minus shale water, where solid minerals and the volume of shale (Vsh) constitute the matrix (non-effective porosity) and the remaining volume constitutes the effective porosity. For practical purposes, Vsh includes solid clays and the clay-sized and silt-sized fraction of non-clay minerals plus CBW and capillary bound water associated with shale micropores.Effective porosity 
In a hydrocarbon-bearing reservoir above the transition zone, only that pore space which is filled with hydrocarbons. From the NMR log, this equates to the Free Fluid Index (FFI), in other words, all pore space above the T2 cut-off.Effective porosity 
The volume of pore space which contains only producible hydrocarbons.Clay-bound water (CBW)
Clay-bound water iswhere
is total porosity, is salinitySalinity
Salinity is the saltiness or dissolved salt content of a body of water. It is a general term used to describe the levels of different salts such as sodium chloride, magnesium and calcium sulfates, and bicarbonates...
factor (
) where the salinity in gGram
The gram is a metric system unit of mass....
/l
Litre
pic|200px|right|thumb|One litre is equivalent to this cubeEach side is 10 cm1 litre water = 1 kilogram water The litre is a metric system unit of volume equal to 1 cubic decimetre , to 1,000 cubic centimetres , and to 1/1,000 cubic metre...
, and
is the Cation Exchange CapacityCation exchange capacity
In soil science, cation-exchange capacity is the maximum quantity of total cations, of any class, that a soil is capable of holding, at a given pH value, for exchanging with the soil solution. CEC is used as a measure of fertility, nutrient retention capacity, and the capacity to protect...
, meq/ml pore space
Examples
A dramatic example of a core effective porosity vs log effective porosity discrepancy comes from some GreensandGreensand
Greensand or Green sand is either a sand or sandstone, which has a greenish color. This term is specifically applied to shallow marine sediment, that contains noticeable quantities of rounded greenish grains. These grains are called glauconies and consist of a mixture of mixed-layer clay...
reservoirs in Western Australia
Western Australia
Western Australia is a state of Australia, occupying the entire western third of the Australian continent. It is bounded by the Indian Ocean to the north and west, the Great Australian Bight and Indian Ocean to the south, the Northern Territory to the north-east and South Australia to the south-east...
. Greensands are green because of iron-bearing glauconite
Glauconite
Glauconite is an iron potassium phyllosilicate mineral of characteristic green color with very low weathering resistance and very friable.It crystallizes with a monoclinic geometry...
which is usually recognized as illite
Illite
Illite is a non-expanding, clay-sized, micaceous mineral. Illite is a phyllosilicate or layered alumino-silicate. Its structure is constituted by the repetition of tetrahedron – octahedron – tetrahedron layers. The interlayer space is mainly occupied by poorly hydrated potassium cations...
/mica
Mica
The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition...
or mixed layer illite-smectite clay by x-ray diffraction. The glauconite per se will incorporate electrochemically bound water (CBW) because of the clay types. More importantly for the consideration of effective porosity, though, glauconite grains (part of the Vsh) have intra-particular microporous pore space which retains capillary-bound water. Glauconite can constitute a large percentage of the reservoir rock, and therefore the associated intra-particular pore space can be significant. Log effective porosities calculated at 25% in some Greensand reservoirs have yielded core analysis effective porosities of 35% at equivalent depths. The difference is the glauconitic microporosity which contains water at reservoir conditions and is included as part of the Vsh (non-effective porosity) by log analysis. However, glauconitic microporosity is measured as part of the effective porosity in core plugs, even if they are humidity dried.
Greensands may cause varying degrees of difficulty for porosity log analysis. radicals affect neutron logs; the iron component is troublesome, and varying clay hydration needs to be considered for density log interpretation. The iron component affects the NMR logs and clay affects the sonic log. Therefore it is essential to have a core - or at least a good understanding of the geology - before invoking total vs effective porosity relationships.