Introduction
Well logging is an overarching term that encompasses the use of various instruments to measure physical characteristics of a borehole and its surrounding geologic formation. In oilfield, well logging is a critical tool used to evaluate the presence and production capabilities of hydrocarbon reservoirs. In civil engineering and geotechnical engineering fields well logging is commonly used to create a profile of the borehole lithology, groundwater levels, casing condition, porosity of the borehole, permeability and more. Well logging provides data to engineers and geologists that aid in understanding field conditions, so that well construction and engineering decisions can be made. With the emergence of digital well logging technologies, better data quality at faster speeds can be generated.
Geophysical Well Logging
Geophysical well logging is subsurface exploration technique that utilizes the measurement of lithological and physical properties of geological formations to determine the conditions within a borehole. Generally, geophysical well logging is divided into two categories: open-hole logging and cased-hole logging. Open-hole logging is conducted after the borehole has been drilled and cased, and before any casing has been installed. Cased-hole logging is conducted when the borehole has been cased and filled with a drilling fluid.
Open-Hole Logging
Open-hole logging is typically used to evaluate hydrocarbon reservoir rock properties and its capability of producing hydrocarbons. As the borehole is already cased, the open-hole logging tools can easily measure subsurface properties of the formation. Open-hole logging is completed in two ways: conventional and digital logging. Conventional well logging is completed in the form of wireline logging. A logging tool is lowered into the borehole and measurements are taken as the tool is pulled back. Common wireline logging tools include Gamma Ray and Caliper Logging. Gamma Ray Logging provides values for gamma radiation emitted from the formation in the borehole, which can be used to identify rock types and lithology. Caliper Logging provides information about the borehole diameter
Digital Logging
Digital well logging, also known as modern well logging, has become increasingly popular over the past few years, as it allows for faster and more accurate measurements. Digital logging is completed using modern Logging-While-Drilling (LWD) and Measurement While Drilling (MWD) tools. These tools measure the same physical properties as the wireline logging tools, but can do so in real-time while drilling and convey the data to the surface. This type of technology produces high quality data at faster speeds and is more reliable than traditional wireline logging. Common digital logging tools include gamma ray cameras, acoustic televiewers, Pressure while Drilling (PWD) and directional sensors.
Conclusion
Well logging is an invaluable tool used to evaluate the presence and production capabilities of hydrocarbon reservoirs in oilfield, as well as geotechnical properties of the subsurface in civil and geotechnical engineering. With the emergence of digital well logging, better data quality at faster speeds can be generated. Geophysical well logging is divided into two categories: open-hole and cased-hole, with wireline logging and digital logging completing the processes. By using well logging measurements from either open-hole or cased-hole logging, engineers and geologists are able to make informed decisions regarding well construction and engineering.
