Wednesday, August 26, 2015

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
  1. The displacing fluid behavior is Newtonian (Viscosity is independent of applied shear force)   

  2. The porous medium is fully saturated and medium grained

  3. The porous media is inert

  4.  The flow is under isothermal conditions

  5.  The grain geometry is stable

  6.   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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