Well Logging Introduction
Well logging is a method used to determine porosity, lithology, formation depth and thickness, water and/or hydrocarbon saturation, and permeability (REF rigzone) of the subsurface and is important in oil and gas, geothermal, groundwater, and mineral exploration. The process of well logging involves drilling to a desired depth and using various logging tools to measure electrical, acoustic, radioactive, and electromagnetic properties of the formations in the well. These measurements can be made either during or after drilling. This section will cover techniques used in determining lithology since this is important for creating cross sections and structural restorations. All information below references Petrophysics Course Notes from Paul Glover, available online.
Acoustic Log
An acoustic log measures the travel time of an elastic wave (a pulse of sound) through a formation. Specifically, acoustic logs typically measure the P-wave travel time per foot (slowness) from a transmitter to a receiver attached to the same tool. This unit is annotated ∆t and usually in μs/ft. Data is not collected as travel time because different mounts could have various spacing between the transmitter and receiver. The slowness measured is proportional to the density and inversely proportional to the strength of the material. To account for tool misalignment and varying hole diameter, modern tools use two transmitters on either side with four receivers in between (Fig 5). The transmitters alternate pulses, and the ∆t values are averaged. Errors from sonic logging come from noise, ∆t stretch, cycle skipping, mud arrivals and altered zone arrivals.
Acoustic logging is not a good diagnostic tool in determining most sedimentary rock types and should be paired with other log types. However, there are some general patterns: low velocities correlate with shale, middle rang velocities indicate sand, and high velocities are associated with carbonates. Furthermore, this technique can easily detect coal and evaporites, which have low velocities and constant transit times, respectively (Fig. 6).
Neutron Logging
This technique is mainly used in determining porosity of a formation and detects the amount of hydrogen atoms in a formation. In neutron logging, the formation is bombarded with high energy neutrons that scatter due to the presence of hydrogen atoms. This scattering decreases the energy of the neutrons or gamma rays, so gamma ray count back to the tool will decrease. Therefore, formations with high hydrogen concentration will result in low gamma ray detection. In reality, there are other elements that affect attenuation. These errors are minimized by calibrating the tool to pure limestone. The results are not porosities of the formation, but the porosity that would occur if the formation tested was a limestone. The porosity is then corrected for the appropriate rock type.
Various lithologies are identified based on which rock type contains hydrogen atoms (e.g. hydrated minerals are present in metamorphic rocks and bound water is present in shale). Figure 7 shows various neutron porosities for differing rock types. Hydrated evaporites (i.e. gypsum, kainite, carnalite, and polyhalite) are detected using waters of crystallization, which give limestone porosities specific to rock type. Limestone porosities for evaporites that contain no water are low since they lack hydrogen.
Formation Density Log
The main purposes of formation density logging are to determine formation porosity, detect gas-bearing formations, and recognize evaporites. The tool used induces radiation that bombards the formation and a sensor measure how much radiation returns. The gamma rays that enter the formation interact with electrons in the formation and undergo compton scattering. This scattering reduces the energy of the gamma rays, which may undergo photo-electric absorption by interaction with atomic electrons at less than 0.5 MeV. Therefore, the amount of attenuation is dependent on the density of electron in the formation and in turn, the density of the formation itself.
Formation density is not a useful tool for identifying lithology alone since most sedimentary rocks can have a range of densities that commonly overlap (Fig. 8). When used in combination with the neutron log, lithology can be determined. Evaporites are usually very pure and have consistent densities, making them more easily identifiable the other sedimentary rocks.
