Introduction to Formation Evaluation

The purpose of formation evaluation is to quantify the production potential of a well. By using remote sensing tools and retrieving samples of the reservoir rocks (core), various properties and behaviors of the rocks can be determined. Operators want to know the size of the reservoir, the quantity of hydrocarbons in place, and the ability of the reservoir to produce hydrocarbons. The ultimate goal of formation evaluation is to decide whether it is worthwhile to complete a well, and, if so, how the well should be completed. In the end, these are technical results that lead to business decisions which come down to economic considerations.

Formation evaluation is a continuous process that begins during the exploration phase. Once a well is drilled, however, much more information becomes available to operators.

During formation evaluation, operators can collect physical rock samples and/or examine the rocks within the hole using downhole sensors. In a productive field, data collected during the drilling of other wells provides additional information about the area. The final stages of formation evaluation involve testing the well to estimate how much oil and gas it will produce.

Another goal of operators during the formation evaluation process is to evaluate the potential for geologic hazards that might occur during the completion and production phases of operations. These hazards include well bore failure, sand production, and subsidence. We’ll get into detailed descriptions of those problems a little bit later. For now, keep in mind that many of the preemptive steps taken to identify hazards before they happen during the formation evaluation phase of wellsite development.

In this module, we’ll take a look at the tools and methods available to operators during the formation evaluation process. It is important to remember that not all of these techniques will be used on every well. Selective of the tools and methods employed depends on the data desired to make appropriate decisions.

Why It Matters

What story does the formation have to tell? Are there unique geologic or engineering considerations? The tools of formation evaluation are useful for exploration and production companies to determine the size of a reservoir and the potential for hydrocarbon recovery. The same tools used for economic decisions can also prove useful in determining proper well design and construction standards. Since reservoirs cannot physically be observed from the surface, formation evaluation tools and techniques are the “eyes and ears” of the engineers and geoscientists to help determine what the reservoir looks like.

Learning Objectives

  • Interpret a mud log correctly.
  • Describe how a core sample is retrieved.
  • Describe ways that downhole sensors do their jobs.
  • Describe how various logging tools function.
  • Describe a drill stem test.
  • Recognize the importance of well log interpretation.
  • Understand how cores and logs are correlated.

Images: “Core Lab” by Penn State University for Top Energy Training