Well logging plays a crucial role in understanding subsurface formations in various industries, from oil and gas to groundwater exploration and environmental studies. The data gathered from these logs helps in making informed decisions about reservoir evaluation, well construction, and resource management. Whether it's geological logs based on visual inspection or geophysical logs from instrument measurements, the information obtained is vital for a wide range of applications and stages in a well's lifecycle.
Categorization of Wireline Logs Based on Physical Properties:
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| Wireline log consisting of caliper, density and resistivity logs IMG Source: Wikipedia |
1.Electrical Logs:
Resistivity logs: Gauge the rock's resistance to electrical current, helping identify formations and fluids.
Induction logs :measure resistivity by inducing a current, providing information on formation properties.
Spontaneous Potential (SP) Logs: Capture natural electrical potential differences, aiding in lithology identification.
2.Acoustic Logs:
Sonic or Acoustic Logs: Assess rock density and porosity by measuring sound wave velocities.
3.Nuclear Logs:
Gamma Ray Logs: Identify the lithology and correlation between wells using natural gamma radiation.
Neutron Logs: Determine formation porosity based on neutron interactions.
Density Logs: Measure bulk density using gamma-gamma or neutron-gamma techniques.
4.Magnetic Logs:
Magnetic Susceptibility Logs: Evaluate rock magnetization properties.
5.Imaging Logs:
Borehole Televiewer Logs: Visualize borehole walls to detect fractures and bedding planes.
Micro Imager Logs: Provide detailed images for identifying small-scale features.
6.Fluid Identification Logs:
Resistivity Logs (differentiation): Differentiate between water, oil, and gas using resistivity measurements.
Pulsed Neutron Logs: Identify fluid types and amounts based on neutron interactions.
7.Pressure Logs:
Formation Pressure Logs: Measure formation pressure to understand reservoir pressures and gradients.
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| Wireline log consisting of a complete set of logs IMG Source: Wikipedia |
Applications Across Industries:
Oil and Gas: Assess reservoir properties, hydrocarbon saturation, and formation pressure to optimize drilling and production strategies.
Mining: Evaluate rock properties to identify mineral deposits and optimize mining operations.
Groundwater Consulting: Determine formation properties to understand aquifer characteristics and groundwater flow.
Environmental: Investigate subsurface properties to assess contamination, groundwater quality, and site suitability.
Data Acquisition and Presentation:
Real-time Mode: Data is recorded against measured cable depth, providing immediate insights during drilling.
Memory Mode: Data recorded against time, later converted to depth, allowing for detailed analysis post-drilling.
Well Log: A presentation of recorded data, provided to clients in either printed or electronic formats, alongside raw data.
Depth Measurement:
Calibrated Wheel Counter: Provides cable length increments; commonly used but less accurate.
Magnetic Marks: Offers calibrated cable length increments, providing more accurate depth measurements.
Correction Factors: Adjustments made for elastic stretch and temperature variations to ensure depth accuracy.
Logging Phases:
Logging While Drilling (LWD): real-time logging during drilling to provide continuous formation evaluation.
Post-Drilling Logging: Conducted once the well reaches total depth, allowing for comprehensive logging of the entire borehole.