Invasion profile: Permeability VS Porosity FAQ
This post is an elaboration of the "Mud Invasion Profile" slide which I share in my previous post [CLICK HERE]. This explanation will fit the best if you want to present the "Mud Invasion Profile" slide at page 2
These are very basic questions that many people outside
Petroleum Industry ask. We’ll discuss these terms as simply as possible
What is Porosity?
Porosity is the ratio of the volume of the pores or empty
space/void in the rocks to the total volume of the mass.
What is Permeability?
The ability of the rock to transmit fluids
Are they connected to
each other?
By definition, there is no connection between porosity and
permeability. The porosity might get higher but with the same permeability. The
tendency will show higher permeability, but there is no universal correlation
between permeability and porosity.
But there are people researching the relationship between
porosity and permeability empirically for a certain area. Since the permeability
tests quite take time, people may get porosity from log measurements, which are
faster and easier, then convert it to permeability by the empirical equation to
obtain permeability estimation.
What makes rock get
porosity?
Rock always has original porosity from the void between
every tiny grain building the rocks. Sometime it is too small because the rocks
are well cemented naturally. But then, the void might be formed because of
chemical leaching of minerals or the generation of a fracture system after the
rock had deposited.
What makes rock get
permeability?
Porous rock always has its own permeability. But to test the
permeability, pressure difference should be applied between two ends of the
rock and let the fluid inside flow between the two ends.
Does permeability
depend on the fluid type?
Permeability is a
property of the rock and not the fluid, so it won’t be different. But remember
that permeability measurements are under Darcy’s Law. The conditions for Darcy’s
Law are
- The displacing fluid behavior is Newtonian (Viscosity is independent of applied shear force)
- The porous medium is fully saturated and medium grained
- The porous media is inert
- The flow is under isothermal conditions
- The grain geometry is stable
- Inertial forces are negligible (Flow is laminar)
I can’t see how
permeability works. How could permeability be the same when you see flow of
viscous fluid is not the same at all with the aqueous one!
In simple answer, viscous fluid will flow in different flow
rate than the aqueous one for the same pressure difference applied. Simply when
there are different type of fluid, it will change other flowing properties
which will result in the same permeability in the Darcy’s equation, which is
the properties of rock.
So the permeability
will depend on viscosity of the fluid?
No. Permeability is the property of the rock, which defining
a specific rock being used. In other hand, viscosity is the property of the
fluid, which defining a specific fluid going through the rock. Changing the
fluid, which will change the viscosity, won’t change the rock, so it won’t
change the permeability too. Changing the fluid only changes the flow behavior.
How could I imagine
porosity vs permeability?
Here’s a good illustration for you. Imagine you are in a
classroom filled with big yoga ball. It will take about 6 yoga ball to fill the
classroom. Then there’s another classroom with exact same size, filled with
marbles. It takes thousands of marbles to fill the whole class. Do you know
that both of class will have the same percent of void space between these round
things? Yes they are, but the difference is in yoga ball’s class the voids are more
centered, and in marble’s class the voids are spread between thousands of
marbles. This is the example of two rooms with same porosity.
Then imagine both rooms have door at one side and a window
on the wall across the door. You would like to flow water from the door to
window. Which room will flow water faster from door to window? The answer is
the room of Yoga balls. It takes time to flow water through thousands of
marbles! That’s how permeability works. The yoga ball classroom has bigger
permeability. The water should flow through twisty path to the window. In the
rock, this twisty path is called turtuosity, which is one of parameter which composes
permeability. That is how the porosity could be same between rocks, but
different permeability
If you have any questions or correction, feel free to
comment. Thank you!
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