JNTU B.Tech III WELL LOGGING Previous Question Paper (October - 2020 - R15)

The Natural Spectral Gamma Ray Log is an advanced logging technique that measures the total natural radioactivity of formations and distinguishes between the three primary radioactive elements: potassium (K), uranium (U), and thorium (Th). Unlike a standard gamma-ray log that measures total radioactivity, this tool utilizes a scintillation detector (typically sodium iodide or bismuth germanate) to perform spectral analysis of the energy levels of incoming gamma rays.

The Principle is based on the fact that each radioactive isotope emits gamma rays at characteristic energy levels, measured in million electron volts (MeV):

• Potassium-40 (K-40) emits gamma rays at 1.46 MeV.
• Uranium series (Bi-214) emits gamma rays at 1.76 MeV.
• Thorium series (Tl-208) emits gamma rays at 2.62 MeV.

The tool counts the number of gamma rays detected within specific energy windows corresponding to these peaks. By applying a stripping matrix to account for the Compton scattering effect—where higher-energy gamma rays lose energy and contribute to lower-energy windows—the tool calculates the individual concentrations of K (in weight percent), U (in ppm), and Th (in ppm).

Applications include:

• Lithology Identification: Distinguishing between different clay types (e.g., illite, montmorillonite, kaolinite) and identifying non-clay radioactive minerals.
• Clay Mineralogy: Potassium and thorium concentrations help differentiate between mica-rich clays and other clay minerals, which is critical for evaluating reservoir quality and cation exchange capacity.
• Sedimentology and Provenance: The Th/K ratio is a powerful indicator of depositional environments and the maturity of sediments.
• Fracture Detection: Uranium enrichment is often associated with fluid movement or fracture zones, as uranium can precipitate from groundwater.
• Correlation: Providing a more detailed stratigraphic correlation than a total gamma-ray log, especially in complex lithological sequences.

A well logging unit is a specialized mobile laboratory, typically mounted on a truck, designed to acquire, process, and record formation data. The system consists of three primary subsystems: the surface equipment, the downhole tool string, and the transmission link.

The major components of the logging unit include:

• Logging Cable and Winch: A multi-conductor armored cable provides mechanical support for the tool string and acts as the electrical conduit for power and data transmission. The winch system ensures precise depth control and constant cable tension.
• Surface Acquisition System: This includes the surface panel, which provides power to the tools, and a computer-based data acquisition system that digitizes, processes, and displays the incoming signals in real-time.
• Depth Measuring Device: Often called a depth encoder or measuring head, this component tracks the cable movement to correlate tool position with borehole depth accurately.
• Downhole Tool String: A modular assembly of sensors (e.g., resistivity, nuclear, sonic, or imaging tools) that measure physical properties of the formation as they are pulled up the borehole.
• Power Supply and Signal Conditioning: These units regulate the voltage sent downhole and filter the returning signals to remove noise before processing.

The logging setup involves the following operational steps:

1. The tool string is assembled and calibrated at the surface.
2. The tools are lowered to the bottom of the well using the winch system.
3. Data acquisition typically occurs during the upward movement (up-log) to ensure the cable remains under constant tension, preventing tool sticking and ensuring consistent depth tracking.
4. The surface computer synchronizes the sensor measurements with the depth data to generate a continuous log, which is then stored digitally and plotted as a visual representation of the formation characteristics.