Hello everybody and thanks for tuning in! You are on the online course EASY Geosteering.

In this course, we explore the key geosteering aspects in simple words, following a clear and structured approach.

This article contains the full transcript of a video lesson also available on YouTube:

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Course structure

We are currently in the second part of the course — BHA components and in the previous video, we covered MWD surveys.

And today, we begin with the logging while drilling.

Let’s get started!

BHA components — characteristics of LWD

So main LWD methods used for geosteering are GR, RES and neutron-density.

We will discuss each type separately, but for now, let’s review some common characteristics shared by all logging methods.

The first characteristic is the depth of investigation. It is the distance from the tool’s axis at which the tool receives half of the signal.

The key point here is when we talk about logging, we cannot say we are measuring an exact point or a specific layer in the rock. Instead, each measurement represents a response from some volume of investigation around the tool, which contributes to the final value.

Many factors affect depth of investigation, including tool design, sensor characteristics and rock properties, and in our lesson we will provide approximate values to give a general understanding.

The second point is vertical resolution — the thickness of the thinnest layer that the tool can detect and measure. This parameter is generally inversely proportional to the depth of investigation.

For example, the density log has high resolution, allowing it to detect thin sublayers, but its measurement is relatively shallow. In contrast, resistivity has lower resolution and struggles to differentiate thin sublayers, but it can measure deeper into the formation.

The third point is environmental corrections — external factors that affect logging readings. Many factors come into play, such as mud parameters or bit size, and each LWD method has its own set of corrections.

For example, potassium content in the mud can significantly increase GR readings due to its natural radioactivity. If MWD engineer does not apply necessary correction, the GR value on the log will appear higher than the actual formation value.

Ok, we’ve covered common characteristics of LWD methods, and now let’s start with GR logging.

Natural Gamma Ray

Gamma Ray measures the natural radioactivity of rocks. The word ‘natural’ is critical because some LWD methods also measure GR but use artificial sources.

Three common elements — potassium, uranium, and thorium — are naturally radioactive and present in rocks. Due to mineral composition and sedimentation processes, these elements are mostly found in shale and clay formations rather than in sandstone.

This means GR can be used for lithology identification and reservoir characterization.

If we speak about the depth of investigation, we can say it’s approximately 40 cm.

Now let’s take a look at the behavior of the GR in a typical terrigenous formation, which here is represented by shale, sandstone with gas, oil and water and also carbonate and coal sublayers.

First, pay attention to the blue horizontal line — this is a reference value of 60 API and it serves as a conditional threshold: values above 60 indicate shale, while values below 60 suggest a potential reservoir.

Reference values for each logging method are determined by petrophysicists and should be provided in the geological project for a specific well.

Of course, it’s a pretty common situation when reference values are unavailable. In such cases, interpretation in RT becomes more challenging for us because a specific GR level — for example, exactly 60 API — could indicate a pay zone in one oilfield but represent a non-reservoir in another.

Ok, in the shale formation, there is a high level of GR due to high content of potassium, uranium, and thorium.

Then, as we enter the sandstone, the GR value drops. As we can see, there are no significant differences between gas, oil, and water, meaning we cannot detect fluid contacts and saturation using the GR curve alone.

Then, we encounter hard sublayers that contain even fewer radioactive elements than sandstone, leading to a lower GR level.

And the same pattern appears in coal sublayers. Coal often has a lower GR than carbonate sandstone, but without additional LWD methods, we cannot definitively distinguish between them.

Alright, we outlined shared characteristics of LWD logging and covered GR. Hope you liked this video! Please feel free to leave your thoughts in the comments and don’t forget to follow my profile for upcoming lessons!