The calculation of porosity and water saturation plays an important role by the time of estimating oil and gas reserves, because a considerable mistake calculating these two values can bring serious economic problems if we are talking about a giant reservoir.
The recovery factor is another important parameter in the reserves estimation calculations, and this value will depend on the natural drainage mechanism(s) of the reservoir (gas cap, acuifer, etc.), which are very different between them. In addition, the recovery factor can increase by the application of secondary and tertiary recovery technics at the reservoir, but this improvement adds more costs for the explotation budget.
In this section, you can find the different equations for oil and gas reserves estimation.
You can calculate the following Reserves
Equations:
Saturation of a fluid is defined as the ratio between the volume that occupies a fluid in a pore space and that the total volume of that porous media.
Saturation = fluid volume / pore volume
So, saturation of water is defined as the ratio between the volume of water accumulated in a rock porous media, and the total pore volume of that rock.
The value of water saturation decreases to minimum values at irreducible water saturation zones, where in oil-wet rocks (read about wettability) lower values are around 0.15 (15%), and in water-wet rocks (water wettable), water saturation values are higher than 0.2 (20%). These values help us as a guide when we calculate water saturation, because if we get a water saturation value lower than 20% in a water-wet rock, we can infer that something is wrong with our calculations. Usually, sedimentary rocks are water-wet rocks, but in the case of carbonates, there is a considerable proportion of oil-wet rocks. However, mixed wettability also exists.
It is important to know how to get a reliable water saturation value, due to that it will impact considerably the hydrocarbon reserves estimation.
Saturation of a porous media is expressed the following way:
Where:
St = total saturation, equals 1 or 100%
So = oil saturation
Sg = gas saturation
Sw = water saturation
The equation to calculate oil saturation could be expressed the following way if the reservoir only accumulates gas and oil:
To calculate water saturation from well logs, we can use resistivity and spontaneous potential (SP) well logs.
In this section, you can find the different equations used to calculate water saturation.
You can calculate the following Saturation
Equations:
Resistivity is defined as the opposition or resistance that a material has to interfere the flow of an electric current.
In the case sedimentary rocks, these contain fluids in their pores that have different kind of resistivities.
A rock pore volume can contain equally oil, gas or water. Oil and gas have a higher resistivity value than water, which allow to detect the presence of hydrocarbons at a well by using resistivity logging tools, for instance.
However, water salinity also affects resistivity in water, and it can a wide range of values. Salty water is less resistivity than fresh water so, it has a lower resistivity value.
In addition to helping detect the presence of hydrocarbons, water resistivity is widely used for the calculation of water saturation, which is a very important value by time of estimating hydrocarbon reserves.
In this resistivity section, you can find thee different equation used to calculate water resistivity:
You can calculate the following Resistivity
Equations:
Permeability is a parameter that is defined on how smooth or easy a fluid flows through a porous media.
This physical property was defined by Darcy, and that is the origin of the name of the units that are used for permeability values.
Where:
Q/A= Darcy velocity = flow rate Q over unit of cross area to flow A
k= absolute permeability
µ= viscosity
p= pressure
Permeability has been used as a cut-off parameter to diferentiate between conventioanl and unconventional reservoirs, but this will depend on the fluid type that is going to be produced (oil or gas).
Generally, permeabilidad is calculated through lab methods, where a rock sample is tested with different techniques that measure how easy a fluid moves through the pores of the rocks. However, these measurements tend to be unprecised due to rocks conditions after they have been extracted from the wellbore, which are in some cases very different that the original conditions.
There are different equations that have been created by diverse authors to calculate permeability, but lab measurements have not been not replaced by them.
Also, there are permeability measurements derived from nuclear magnetic resonance (NMR) well logs, and some oil services companies offer also effective porosity values through these well logs, which are estimated from the hydrogen content that is measured from the rock pores.
It will depen on the petrophycist or reservoir criteria which values are going to be used to make estimatation, if from lab data or from values derived from equations.
Like in other sections of the blog, the permeability equations can be found at the navegation bar above:
You can calculate the following Permeability
Equations:
Porosity is an important petrophysical parameter of the rock, which is defined as the relationship the rock pores volume and the rock total volume.
Porosity = pore volume / total rock volume
This physical property limitates the rock by the time of accumulating hydrocarbons (oil, condensates or gas), however, porosity must be interconnected to add commercial value to a reservoir.
There are total porosity and effective porosity, where the first one is related to all the pores of a rock, and the second one, to the interconnected pores only, which is more important by the time of producing hydrocarbons.
In addition, the shale volume affects reservoirs quality, and that is why corrections have to be made by time of calculating porosity, the ones known as shale corrected values.
At the above window, you can find the ways to calculate porosity values derived from differrent well logs: density, neutron, sonic, and resistivity. There are different authors for porosity calculations from diverse well logs.
You can calculate the following Porosity
Equations:
Shale volumen is a fundamental value to evaluate the quality of a reservoir rock.
According to the shale volume that a reservoir rock has, it will be determined how exploitable a reservoir is or not.
There are different methods to calculate shale volume, where we can name some of these equations: Larionov, Larionov- old rocks, Steiber, Clavier. Some of these equations use the gamma ray well log values, and some of them use values from spontaneous potential (SP) well logging tool.
Also, this value is used to make shale correction for porosity values derived from different well logging tools, so we can have a more reliable porosity value.
From the different links of this section, you can find the different equations used to calculate shale volume:
You can calculate the following Shale Volume
Equations:
