JNTU B.Tech III WELL LOGGING Previous Question Paper (September - 2021 - R15)

Well logging methods are categorized based on the physical properties of the formation they measure. The primary types include:

• Electrical Logging: Measures the resistivity or conductivity of the formation. Examples include Spontaneous Potential (SP) logs, which measure natural electrical potentials, and Resistivity logs (Normal, Lateral, Induction, and Laterologs) used to differentiate between water-bearing and hydrocarbon-bearing zones.
• Radioactive (Nuclear) Logging: Utilizes the interaction of radiation with rock formations. Gamma Ray logs measure natural radioactivity to identify lithology (e.g., shale content), while Neutron logs measure hydrogen index to determine porosity. Density logs use a gamma-ray source to determine bulk density.
• Acoustic (Sonic) Logging: Measures the travel time of sound waves through the formation. It is primarily used to determine porosity and for seismic correlation.
• Imaging Logging: Includes Borehole Televiewers and Micro-resistivity imaging (FMI), which provide high-resolution visual representations of the borehole wall to identify fractures, bedding planes, and sedimentary structures.

Drilling mud filter cake forms on the permeable walls of a borehole when mud filtrate invades the formation. Its presence significantly impacts log interpretation in the following ways:

• Resistivity Measurements: The filter cake is often thin and conductive. If a logging tool is not properly pad-mounted or focused, the resistivity measurement may be biased toward the mud cake properties rather than the formation, leading to erroneous water saturation calculations.
• Borehole Geometry: A thick filter cake reduces the effective diameter of the borehole. This can cause tool standoff issues, where the logging tool is pushed away from the formation wall, necessitating borehole corrections for density and neutron logs.
• Invasion Profile: The thickness and permeability of the filter cake dictate the rate of mud filtrate invasion. This invasion alters the fluid saturation near the wellbore, creating a transition zone that complicates the determination of true formation resistivity (Rt) from shallow-reading tools.
• Interpretation Accuracy: Failure to account for the filter cake effect in quantitative analysis leads to inaccurate porosity and permeability estimates. Modern interpretation software uses specific environmental correction charts to mitigate these effects based on mud weight, temperature, and cake thickness.