Important for drillers and operators, well logs are measurements acquired at specific depths of the well that identify subsurface formations. Based on a ship's log, the well log started as a journal of depth versus the type of formation, effective drilling muds and different drill bits required to move through the formation.
This early journaling system has evolved to encompass lowering logging tools into the wellbore via wireline in order to measure porosity, resistivity, permeability, temperature, water saturation and more. While logging-while-drilling techniques have transformed drilling operations to include real-time data for drilling engineers, some logging methods occur after drilling has ended.
Open-hole logging refers to logging operations that are performed on a well before the wellbore has been cased and cemented. In other words, the logging is done through the bare rock sides of the formation. This is the most common type of logging method because the measurements are not obstructed and it's done during or after the well has been drilled.
On the other hand, cased-hole logging involves retrieving logging measurements through the well casing, or the metal piping that is inserted into the well during completion operations. Cased-hole logging is performed more rarely but still provides valuable information about the well.
Cased-hole logging is used to help operators obtain additional information from a well or reservoir that has already been completed. For example, the well may have already started production and a cased-hole log could help determine what has hampered flow. In some cases, the decision must be made to plug and abandon the well or recomplete it, and the cased-hole log will help identify what lies beyond the casing of the well.
Cased-hole logging can be used to evaluate the formation and completion of the well, as well as determine the state of the cement, corrosion and perforation. Both gamma ray and neutron porosity logs can be run through the casing of a well, and better ideas of thermal decay and interval transit time can be achieved through porosity, hydrocarbon saturation and producibility measurements.