Beneath the Surface

Date: 03/11/2013

Understanding logging tools and measurements

Oil and gas reservoirs lie deep beneath the Earth’s surface. Geologists and engineers cannot examine the rock formations in situ, so tools called “sondes” go there instead. Specialists lower these tools into a wellbore and obtain measurements of subsurface properties.

Evolving measurements

The data are displayed as a series of measurements covering a depth range in a display called a “well log.” Often, several tools are run simultaneously as a logging string, and the combination of results is more informative than each individual measurement.

The first well log was obtained in 1927 in Pechelbronn field in Alsace, France. The tool, invented by Conrad and Marcel Schlumberger, measured electrical resistance of the Earth. Engineers recorded a data point each meter as they retrieved the sonde, suspended from a cable, from the borehole. Their data log of resistivity changes identified the location of oil.

Today, geologists depend on sets of well logs to map properties of subsurface formations. By comparing logs from many wells in a field, geologists and engineers can develop effective and efficient hydrocarbon development plans.

Logging defined

“Defining Logging,” the first in a series of introductory articles by the Oilfield Review that describes basic concepts of the E&P industry, examines the history of logging and explains the different types, such as openhole, acoustic, and electric logging. 

The article also explains concepts and approaches, such as logging while drilling, and the multitude of measurements involved in the logging process. A common combination of logging measurements includes gamma ray, resistivity, and neutron and density porosity combined on one toolstring.

Whether new to the industry or just looking to review the basics, this short article is a resource for understanding the fundamental concepts of logging in the E&P industry.

Download the “Defining Logging” article here.

Discover other introductory articles about concepts and technologies in the E&P industry on the Oilfield Review Web site.

Defining Logging

Assembling a logging tool on a rig floor. One logging operator holds a logging tool in place (left) while another assembles a connection (right).