Well logging, or borehole logging, creates an in-detail record of the geologic configuration that has been penetrated by a borehole. This record is called a well log. There are two types of well logs. Geological logs are compiled by visually examining samples obtained from within the borehole. Geophysical logs are based on the measurements gathered from instruments that were lowered into the borehole. Both types are referred to as either measuring while drilling (MWD) or logging while drilling (LWD). The effective interpretation of well log results requires MWD training and MWD technical support.
There are types of geophysical well logs that can be performed at any time during a well bore, including the drilling, completing, producing or abandoning phases. MWD is common in directional drilling boreholes that have been drilled for oil, gas or groundwater. They are also valuable when conducting mineral and geothermal explorations or geotechnical and environmental studies.
The History of Measuring While Drilling
Marcel and Conrad Schlumberger, the 1926 founders of Schlumberger Limited, are recognized as the inventors of electric well logging. Conrad devised a method of prospecting for metal ore deposits called the Schlumberger array, which was a surface technique. The two brothers then modified that technique for subsurface use. In late 1927, one of the Schlumberger crews put an electric tool down a well in Pechelbronn, Alsace, France, which resulted in the first well log. Nowadays, that initial log would be referred to as a resistivity log or a 3.5-meter upside-down lateral log.
Two Schlumberger employees in 1931, Henri Doll and G Dechatre, noticed that a galvanometer reacted even though there was no current passing through the logging cables within the well. Their discovery led to the development of a process that could differentiate between impermeable nonproducing beds and permeable oil-bearing beds by measuring the spontaneous potential (SP). The SP was a natural reaction at the edges of permeable beds by the borehole mud. When the SP and resistivity were recorded concurrently, loggers were able to see if their well bores would be successful.
The Schlumbergers devised the spontaneous potential dipmeter in 1940. It permitted the calculation of the dip and the direction of a layer’s dip. This led to the resistivity dipmeter in 1947 and the continuous resistivity dipmeter in 1952.
During 1948, oil-based mud (OBM) was initially used at Rangely Field in Colorado. Electric logs need water-based mud. However, OBM is not conductive. This problem was solved by the induction log in 1949. In the 1960s, new transistors and integrated circuits improved the reliability of electric logs.
The computerization seen in the 1970s enabled speedier log processing, which expanded the ability to gather log data. Resistivity logs and porosity logs, called combo logs, could be recorded simultaneously in the borehole.
Today, MWD rental kits enabled companies to make use of this technology without investing in the hardware singulair medication. These kits included accelerometers, magnetometers and gyroscopes. More advanced MWD kits use temperature, pressure and strain gauges along with gamma ray sensors to measure mudflow volume, downhole temperature, bit torque, bit weight, drillstring rotation and the severity of downhole vibration.
An onboard logic unit processes the collected data and converts it into binary code. That code is then transmitted to the surface by Mud Pulse Telemetry (MPT). Depending on the binary code, a Pulser Unit varies the drilling fluid pressure accordingly. Fluctuations in pressure are converted into usable data on the surface.
Improvements to existing MWD kits and modern technology are continuously advancing the ability to measure while drilling. Real-time data enables faster decisions during the well bore. In some cases, video cameras are also being effectively employed in the process.