JNTU B.Tech III WELL LOGGING AND FORMATION EVALUATION Previous Question Paper (September - 2021 - R18)

A well logging unit is a mobile laboratory mounted on a truck, designed to acquire, process, and record subsurface data. The major components include:

• Logging Truck/Unit: A heavy-duty vehicle equipped with a power generator, climate-controlled cabin, and a hydraulic winch system.
• Winch and Cable System: A high-strength armored multi-conductor cable (logging cable) used to lower and raise tools into the borehole, providing both mechanical support and electrical communication.
• Logging Tools (Sondes): Specialized sensors (e.g., resistivity, nuclear, sonic) that measure physical properties of the formation and borehole fluids.
• Surface Acquisition System: A computer-based control system that manages tool power, processes incoming raw signals, and digitizes data in real-time.
• Depth Measuring Device (Sheave): A calibrated wheel system that tracks the cable movement to correlate measurements precisely with borehole depth.

The relationship between water resistivity, porosity, and water saturation is governed by Archie's Law, which relates the formation resistivity to the fluid content and the rock matrix. The fundamental empirical relationship is expressed as:

Where the components are defined as follows:

• Rt: True formation resistivity measured by the logging tool.
• Rw: Resistivity of the formation water at formation temperature.
• φ: Porosity of the formation.
• Sw: Water saturation (fraction of pore volume occupied by water).
• a: Tortuosity factor (typically 1).
• m: Cementation exponent (reflecting pore geometry, typically 2).
• n: Saturation exponent (typically 2).

This equation is derived from the combination of the Formation Factor (F = a/φ^m) and the Resistivity Index (I = Rt/Ro = 1/Sw^n). By rearranging these, one can solve for water saturation (Sw) to evaluate hydrocarbon potential, as Sw is inversely proportional to the resistivity of the formation.