density log

Unveiling the Secrets of Formation Density: A Look at Density Logs in Drilling and Well Completion

Understanding the properties of the rock formations encountered during drilling is crucial for successful well completion. One of the key parameters assessed is formation density, a measure of the mass per unit volume of the rock. This is where density logs, a specialized type of radioactivity log, come into play.

Density logs are invaluable tools in open-hole surveying, providing detailed information about the formation's density variations. They operate on the principle of gamma-gamma scattering. The logging tool, held against the borehole wall, emits neutrons. These neutrons collide with the atoms in the formation, causing them to emit secondary gamma radiation. This scattered gamma radiation is then detected by the tool. The intensity of this secondary radiation is directly related to the density of the formation.

Why is density information so important?

• Porosity Determination: Density logs are particularly useful for calculating porosity, which is the proportion of void space in the rock. Knowing the formation density and its mineral composition allows for a precise estimation of porosity. This information is critical for determining the potential for oil or gas production.
• Shaley Sand Analysis: Density logs excel at characterizing shaley sands, formations that contain a mixture of sand and clay. These formations pose a challenge for other logging methods, as they can exhibit variable density. Density logs provide a robust solution for assessing porosity and fluid content in these complex formations.
• Lithology Identification: Density logs can assist in identifying different rock types, providing valuable insights into the geological structure of the well. This information is essential for well planning and completion strategies.

Popular Density Log Variations:

• Formation Density Log (FDC): This is a standard density log, widely used for porosity evaluation.
• Gamma-Gamma Density Log (GGDL): This variation emphasizes the use of gamma radiation in the measurement process.
• Densilog: This is a trade name for a specific type of density logging tool.

Advantages of Density Logging:

• High Accuracy: Density logs provide precise measurements of formation density, leading to reliable porosity calculations.
• Wide Applicability: They are effective across various formation types, including shaley sands, which are difficult for other logging methods.
• Comprehensive Data: Density logs yield valuable data on porosity, lithology, and fluid content, contributing to a comprehensive understanding of the well's geological environment.

Limitations of Density Logging:

• Contact Logging: Density logs require the tool to be in contact with the borehole wall, limiting their effectiveness in highly deviated wells.
• Influence of Mudcake: The presence of mudcake, a layer of drilling mud on the borehole wall, can affect the accuracy of the measurements.

Overall, density logs are an essential tool in the arsenal of drilling and well completion engineers. They provide valuable information about formation density, porosity, and lithology, contributing to the success of exploration and production activities.