A well log is a foundational tool providing a detailed, continuous record of the rock layers and fluids encountered during the drilling of a well. This process, often called wireline or geophysical logging, uses specialized instruments to gather data deep underground. A well log acts as an objective, measured fingerprint of the geological formations surrounding a borehole, capturing physical properties invisible to the naked eye. The resulting record translates these physical measurements into a standardized format, allowing geoscientists and engineers to understand the subsurface environment. This standardization makes it possible to compare information from wells drilled across different times or locations.
Defining the Physical Record
The physical well log is a graphical display that plots measured data against the depth of the borehole, creating a vertical profile of the Earth’s layers. To create this record, a string of logging tools, known as a sonde, is lowered to the bottom of the wellbore on an electrical cable called a wireline. As the tools are pulled back up the hole, they continuously measure the surrounding rock and fluid properties relative to the measured depth.
The resulting log is typically presented in a multi-column format, commonly divided into three main tracks. The first track often displays the depth and a simple measurement like the Gamma Ray log, used for correlation. The subsequent tracks contain the bulk of the measured data, including curves representing electrical, nuclear, and acoustic properties. These curves are the raw, quantifiable data providing an objective assessment of the subsurface environment.
The final track is sometimes reserved for calculated data or interpretations, synthesizing raw measurements to derive properties like porosity or water saturation. This graphical record archives the history of the well. The log provides a reliable, in-situ snapshot of the formation properties, which is often superior to analyzing small, fragmented rock samples.
Essential Measurement Types
Well logs are categorized by the physical principle they exploit, primarily nuclear, electrical, and acoustic measurements.
Nuclear Logs
Nuclear logs utilize the interaction of the rock with natural or induced radiation to determine rock composition and density. The Gamma Ray log records the natural radioactivity emitted by the formation, primarily from isotopes of potassium, thorium, and uranium. Since these elements concentrate in fine-grained clays and shales, a high Gamma Ray reading indicates a shale layer, while a low reading suggests cleaner rock types like sandstone or limestone.
The Density log is another nuclear tool that emits gamma rays into the formation and measures the returning scattered rays. This measurement is inversely related to the bulk density of the rock, which relates directly to the amount of pore space present. A lower density reading suggests a less compact rock with more void space, indicating porosity.
Electrical Logs
Resistivity logs measure the rock’s ability to impede the flow of an electric current. This measurement is important because hydrocarbons (oil and gas) are electrical insulators and highly resistive, while water containing dissolved salts is highly conductive. By measuring this property, geoscientists differentiate between rock layers containing hydrocarbons and those saturated only with water.
Acoustic Logs
Acoustic logs, frequently called Sonic logs, operate by sending a sound pulse into the rock and measuring the time it takes for the compressional sound wave to travel a specific distance. This travel time is slower in rocks with more void space, making the acoustic measurement a reliable method for determining porosity.
Primary Uses of Log Interpretation
The value of a well log lies in the interpretation, which translates physical measurements into geological and engineering conclusions.
Lithology Identification
Interpretation begins with Lithology Identification, determining the rock type present at each depth. By cross-referencing the Gamma Ray log’s indication of clay content with the Density log’s reading of bulk rock composition, geoscientists distinguish between common subsurface formations like shales, limestones, and sandstones.
Porosity Calculation
Once the rock type is established, the next step is calculating Porosity, the percentage of void space within the rock. This calculation is derived by combining data from the Density log with other porosity-sensitive tools, such as the Neutron log, which measures the amount of hydrogen in the formation. The resulting porosity value relates directly to the rock’s storage capacity.
Fluid Saturation
The most economically significant interpretation is Fluid Saturation, which determines what substance (water, oil, or gas) occupies the calculated pore space. This is primarily accomplished using the Resistivity log, as the high electrical resistance of hydrocarbons contrasts sharply with the low resistance of formation water. These interpretations are also used to assess groundwater resources, characterize reservoirs for geothermal energy, and evaluate subsurface stability.