welllogging

Well logging, also known as borehole logging, is the practice of recording physical, chemical, and structural properties of formations penetrated by a borehole. Tools lowered into the borehole (on wirelines, or integrated into the drill string in logging-while-drilling) measure properties as a function of depth. The collected logs aid formation evaluation, lithology identification, porosity, permeability estimates and fluid saturation, supporting hydrocarbon exploration, groundwater assessment, and geothermal resource development.

Common log types include gamma-ray logs to identify shale content; density and neutron porosity logs to estimate porosity and lithology; sonic (acoustic) logs to measure formation velocity and rock mechanics; resistivity logs (including laterolog and array resistivity) to distinguish hydrocarbons from water; spontaneous potential logs; caliper logs to assess borehole size; and borehole image logs to visualize beds and fractures. Spaced combinations such as neutron-density and density-porosity logs improve porosity estimation. In cased wells, cement and quality logs (Cement Bond Log, CBL; Cement Evaluation Log, CEL) assess cementing integrity.

Interpretation relies on petrophysical models, e.g., Archie’s laws for water saturation, to infer hydrocarbon saturation from porosity and resistivity. Data can be collected on wireline after drilling, with real-time measurements provided by logging-while-drilling (LWD). Logging results guide zone prioritization, well placement, completion design, and reservoir characterization.

Quality control, calibration, and depth correlation with surface measurements are essential. Limitations include borehole effects, mud filtrate invasion, and tool calibration. The field continually evolves with advanced imaging, spectroscopy, and multi-physics logs to improve reservoir understanding.