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| Fig. 1: A Mud Log. (Source: Wikimedia Commons) |
The Oil and Gas industry's supply chain can be broken down into the upstream sector and the downstream sector, although there is sometimes a midstream sector. The upstream sector is sometimes called the Exploration and Production sector (E&P). Exploration involves using surface and subsurface geology methods to carry out oil prospecting. Before the use of subsurface exploration methods, prospecting was based on natural oil seeps. An oil seep is a natural leak of crude oil and gas that migrates up through the seafloor and ocean depths. Since very little was known about petroleum deposits, these were the only indication of oil underground. Electrical resistivity logging was invented by the Conrad Schlumberger and his brother Marcel 1927. They were the first to use electrical measurements to map subsurface formations by measuring how easily electric current passed through a formation.
There are no known direct means for detecting oil. Therefore, geophysicists use surface geological exploration methods to infer what lies below from what is seen on the surface. These methods generally involve collecting and processing indirect images of subsurface earth properties. These images are provided by satellites which measure subsurface variations in rock type and structure via changes in the Earths magnetic fields.
Subsurface geological drilling methods of exploration involve drilling into the subsurface to collect information about the structure and lithology of rocks. Well logging falls under the subsurface geological methods of exploration. Well logging, also known as borehole logging, is the practice of making a detailed record - a well log (Fig. 1) - of the geologic formations penetrated by a well. [1] A hole (or well) is drilled with a depth of several thousand feet. Physical quantities in and around the well are measured and recorded versus depth, or time, or both. These measurements are obtained either by visual inspection of samples brought to the surface (geological logs e.g. cuttings logs, core-logging or petro-physical logging) or on physical measurements made by instruments lowered into the hole (geophysical logs). [1]
Several types of logging instruments are used in the oil industry to measure electrical, acoustic, natural gamma ray, electromagnetic, and other properties of the rocks in the subsurface. All this data is recorded in real-time in an on-site truck equipped with computers that process the information and print out the logs. The different types of logs include Resistivity and Spontaneous (SP) logs, Acoustic logs, Nuclear logs, Gamma Ray logs, Sonic logs, among others. These logs all produce different types of data depending on the physical property of the rock being measured.
The Exploration and Production (E&P) sector of the oil and gas industry generates a significant amount of revenue for the industry. According to the U.S. Energy Information Administration (EIA) upstream cost study in 2016, average horizontal well drilling costs range from $1.8 MM to $2.6 MM and account for 27% to 38% of a well's total cost. [2] The E&P sector is an integral part of the energy industry. With the advanced technological research and development in this field, E&P operations look to become more efficient and less costly.
© Kofi Owusu-Agyeman. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] C. Ofwona, "Introduction to Geophysical Well Logging And Flow Testing," Geothermal Development Company, Ltd., 29 Oct 10.
[2] "Trends in U.S. Oil and Natural Gas Upstream Costs," U.S. Energy Information Administration, March 2016.
