(Total and Effective) Porosity Explained
The porosity is the percentage of the (reservoir) rock occupied by pores (typically 0-35%). Porosity is usually obtained through the neutron and density logs. Neutron logs are effected by all the hydrogen atoms in the rock and thus measure crystalline bound hydrogen (structural water), clay bound hydrogen (clay bound water) and the hydrogen in the connected and isolated pore space as shown in the next figure. The density log measures the electron density of the rock, which in turn is proportional to the actual rock density. If the density of clay and quartz (matrix rock) is known then the remaining porosity can be deducted from this log. This porosity is related to the total porosity, including clay bound water and isolated water. Using other petrophysical techniques the effective porosity can be calculated which is the porosity in which hydrocarbons theoretically could exist.
Some companies model reservoirs using total porosity, while others use effective porosity. When doing so one should keep track on which porosity the NtG cut off was defined and how the water saturation is calculated. A total porosity system usually has a higher porosity cut off and a higher water saturation opposed to an effective porosity system.
When modeling porosity in combination with NtG one should be aware that the PORNET should be modeled. This is only the porosity in the Net reservoir
; the porosity in non-net reservoir is not modeled, but these intervals have a lower NtG therefore already making sure little to no hydrocarbons are calculated in these intervals.Some notes on Porosity from Core Analysis
During core analysis the plugs are dried which is done to remove all clay bound water. After this the plugs are weighed, then soaked (usually with Mercury) and weighed again to get the weight of the fluid in the pores. This is then used to calculate the porosity. This means that the porosity from core is equal to total porosity. However, from comparison to log analysis results, it appears that some clay bound water often remains. Therefore the porosity from core is usually somewhere inbetween total and effective porosity.