Effective porosity

Quartz "Quartz" (more aptly termed “non-clay minerals”) forms part of the matrix, 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 sample 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 That is: :\text{Effective porosity} = \text{Total porosity} - \text{CBW} 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. 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 capillary 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. 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 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 hydrocarbons (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. 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. Isolated pores “Isolated pores” in clastics, and most carbonates, 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. Therefore, in practice, for the vast majority of sedimentary rocks, this definition of effective porosity equates to total porosity. ==Summary of terms==

curve. The cumulative distribution for the fully saturated sample is compared to the cumulative distribution after centrifuging at 100 psi. The cutoff time which separates the T2 distribution into macro-porosity and micro-porosity is defined as the relaxation time at the point where the cumulative porosity of the fully saturated sample equals the irreducible water saturation. ;Effective porosity \phi_{e6}: The volume of pore space which contains only producible hydrocarbons. ;Clay-bound water (CBW): The amount of Clay-bound water is determined by the following equation ::\text{CBW} = \phi_t \cdot \text{SF} \cdot \text{Qv} :where \phi_t is total porosity, \text{SF} is salinity factor :and \text{Qv} is the Cation Exchange Capacity, meq/ml pore space ;Salinity factor (SF): 0.6425 \cdot S^{-0.5} + 0.22 :where S is the salinity in g/L, ==Examples==

A dramatic example of a core effective porosity vs log effective porosity discrepancy comes from some Greensand 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. ==See also==