JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III WELL LOGGING AND FORMATION EVALUATION Previous Question Paper (February - 2022 - R18)

Conventional coring involves retrieving a cylindrical core sample from the formation using a core barrel attached to the drill string. The data collected includes lithology, sedimentary structures, porosity, permeability, fluid saturation, and grain size distribution. This method provides continuous, high-quality samples that allow for detailed sedimentological and petrophysical analysis.

Sidewall coring is performed after drilling, using either mechanical or percussion tools to extract small plugs from the borehole wall. The data collected is primarily restricted to lithological identification, mineralogical composition, and localized porosity estimates. It is useful for verifying formation tops and calibrating wireline logs.

Limitations of coring analysis include:

• High operational costs and significant rig time requirements.
• Risk of core loss or damage during retrieval, especially in unconsolidated formations.
• Fluid invasion and flushing by drilling mud, which alters original formation fluid saturations.
• Limited recovery depth and diameter, which may not be representative of the entire reservoir volume.
• Potential for core expansion or fracturing due to pressure release during ascent to the surface.

Temperature measurements in boreholes are essential for geothermal gradient studies, fluid flow identification, and log correction. The primary methods include:

• Maximum Reading Thermometer (MRT): A simple, rugged mercury-in-glass thermometer lowered with wireline tools to record the maximum temperature at a specific depth.
• Resistance Temperature Detectors (RTD): Highly accurate sensors that measure changes in electrical resistance of a metal (usually platinum) as a function of temperature.
• Thermistor Logs: Semiconductor devices that exhibit a large change in resistance with small temperature variations, providing high-resolution, continuous temperature profiles.
• Fiber Optic Distributed Temperature Sensing (DTS): Uses laser pulses sent through fiber optic cables to measure temperature along the entire length of the wellbore in real-time.
• Downhole Pressure-Temperature Gauges: Electronic gauges integrated into drill stem testing (DST) or production logging tools to monitor thermal stability during transient flow tests.