SUMMARY: All About Well Logs


1. What are well logs?

Sensors we send downhole to measure formation properties. They become our eyes, mouth, nose, ears and eyes across our reservoir.

2. When do we acquire well logs?

We send the logging tools to record the formation properties while, after drilling or after we complete the well.

During drilling: Mud logs and Logging-While-Drilling (LWD) logs

After drilling: Openhole logs

After completion: Casedhole logs

3. How do we send the tools inside the borehole?

Using pipes (drill pipes, through tubing, etc), or using wireline.

4. Why do we run well logs?

We want to calculate the hydrocarbon quantity. So, we use logs to:

a. Pick where our reservoir is.

b. Tell what's inside it (oil, gas, or water).

c. Describe how good it is.

d. How much oil or gas we have in the reservoir.


1. Well logs are indirect measurements of the properties we want to measure

- They don't directly measure the properties we want. That's where physics comes in. We measure something else first, engineer the shit of the results, and turn in into the measurement that we want.

- Example: Neutron log doesn't give us porosity directly. We send neutrons into borehole and across the reservoir, let them interact with the formation, and measure the amount, the energy and the characteristic response of the returned neutrons.

- We then use these neutrons to estimate Hydrogen Index which we then use to estimate porosity.

2. Borehole and reservoir conditions affects the logs.

Logging tools are sensitive to borehole conditions.

When we drill or produce or inject the well, we create changes in the borehole and inside the reservoir. Drilling fluids, pressure and temperature changes, and other factors could affect the results of our logs.

That's why people talk about performing environmental corrections to correct for these effects.

3. Logging tools are not perfect.

Logging tools are also affected by other factors other than the properties we are trying to measure. It depends on the tool designs, and what the tool manufactures do to compensate for these external effects.

The logging tools are cannot see beyond what they are supposed to see. It cannot see beyond its depth of investigation, and it cannot see features smaller than their vertical resolution.


1. Lithology logs to separate reservoir and non reservoir rock. Like a Gamma Ray log.

2. Porosity logs to calculate the pore space inside our reservoir. Like Density and Neutron porosity logs.

3. Resistivity logs to calculate water saturation.

4. Other special open hole and cased hole logs like fluid samples, reservoir pressures, magnetic resonance, sonic logs, production logs, and pulsed neutron logs to measure other formation properties.


You can get many properties.

But for this course, we use well logs mainly to determine :

1. Lithology: What kind of rock you have. Carbonate? Limestone? Sandstone? Shale?

2. Volume of Clay: How much clay content is in your rock. The higher clay or shale quantity, the lower rock quality.

3. Porosity: How much pore space you have in your rock.

4. Fluid types: What fluids are in your reservoir (oil, gas, water).

5. Fluid contacts: The depth at which the fluid change into another fluid or gas? Where does the oil changes into water, gas into oil?

6. Net-to-gross: The ratio of the good reservoir thickness versus the total reservoir thickness? Use cutoffs like volume of shale, porosity and/or saturation to determine the ratio.

7. Permeability: The ability of the rock to transmit fluid. How easy fluid flows in the rock? We can't directly measure permeability from logs, but we can indicate how good is the permeability.