Lak wettability index

Key takeaways

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Reproduce the core statement of Lak wettability index from memory before moving on to harder problems.

Reference excerpt

In petroleum engineering, Lak wettability index is a quantitative indicator to measure wettability of rocks from relative permeability data. This index is based on a combination of Craig's first rule. and modified Craig's second rule

I L = A ( 0.3 − k r w , S o r ) 0.3 + B ( 0.5 − k r w , S o r ) 0.5 + C S − R C S 1 − S o r − S w c {\displaystyle I_{\mathit {L}}={\frac {A(0.3-k_{\mathit {rw,Sor}})}{\ 0.3}}+{\frac {B(0.5-k_{\mathit {rw,Sor}})}{\ 0.5}}+{\frac {CS-RCS}{\ 1-Sor-Swc}}}

where

I L {\displaystyle I_{\mathit {L}}} : Lak wettability index (index values near -1 and 1 represent strongly oil-wet and strongly water-wet rocks, respectively)

k r w , S o r {\displaystyle k_{\mathit {rw,Sor}}} : Water relative permeability measured at residual oil saturation

C S {\displaystyle CS} : Water saturation at the intersection point of water and oil relative permeability curves (fraction)

S o r {\displaystyle Sor} : Residual oil saturation (in fraction)

S w c {\displaystyle Swc} : Irreducible water saturation (in fraction)

R C S {\displaystyle RCS} : Reference crossover saturation (in fraction) defined as:

R C S = 0.5 + S w c − S o r 2 {\displaystyle RCS=0.5+{\frac {Swc-Sor}{\ 2}}}

and A {\displaystyle A} and B {\displaystyle B} are two constant coefficients defined as:

A = 0.5 {\displaystyle A=0.5} and B = 0 {\displaystyle B=0} if k r w , S o r < 0.3 {\displaystyle k_{\mathit {rw,Sor}}<0.3}

A = 0 {\displaystyle A=0} and B = 0 {\displaystyle B=0} if 0.3 <= k r w , S o r <= 0.5 {\displaystyle 0.3<=k_{\mathit {rw,Sor}}<=0.5}

A = 0 {\displaystyle A=0} and B = 0.5 {\displaystyle B=0.5} if k r w , S o r > 0.5 {\displaystyle k_{\mathit {rw,Sor}}>0.5}

To use the above formula, relative permeability is defined as the effective permeability divided by the oil permeability measured at irreducible water saturation.

Craig's triple rules of thumb Craig proposed three rules of thumb for interpretation of wettability from relative permeability curves. These rules are based on the value of interstitial water saturation, the water saturation at the crossover point of relative permeability curves (i.e., where relative permeabilities are equal to each other), and the normalized water permeability at residual oil saturation (i.e., normalized by the oil permeability at interstitial water saturation). According to Craig's first rule of thumb, in water-wet rocks the relative permeability to water at residual oil saturation is generally less than 30%, whereas in oil-wet systems this is greater than 50% and approaching 100%. The second rule of thumb considers a system as water-wet, if saturation at the crossover point of relative permeability curves is greater than water saturation of 50%, otherwise oil-wet. The third rule of thumb states that in a water-wet rock the value of interstitial water saturation is usually greater than 20 to 25% pore volume, whereas this is generally less than 15% pore volume (frequently less than 10%) for an oil-wet porous medium.

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Worked examples

Example 1 — a first encounter with Lak wettability index

Start with the simplest possible case. Write down what Lak wettability index claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Lak wettability index before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Lak wettability index ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Lak wettability index

In research: Lak wettability index appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Lak wettability index in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Lak wettability index is common in secondary-school and first-year university syllabi. It links to neighbouring topics Fluid mechanics, Petroleum geology, Surface science, so understanding it makes those chapters shorter.
In everyday life: Look for Lak wettability index outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Lak wettability index in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Lak wettability index means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
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Frequently asked questions

What is Lak wettability index in simple terms?

In petroleum engineering, Lak wettability index is a quantitative indicator to measure wettability of rocks from relative permeability data. This index is based on a combination of Craig's first rule. and modified Craig's second rule I L = A ( 0.3 − k r w , S o r ) 0.3 + B ( 0.5 − k r w , S o r ) 0…

Why does Lak wettability index matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Lak wettability index?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Lak wettability index.