Gas Logging

a method of ascertaining the presence of petroleum and gas deposits by means of the systematic analysis of the gaseous and light liquid hydrocarbons in drilling fluid and, less frequently, in core samples.

During the drilling of wells through an oil- and gas-bearing layer, hydrocarbons enter the drilling fluid and are carried to the surface. Occasional or continuous chemical stripping of the drilling fluid takes place, and the gas obtained is analyzed. The results of the analyses are plotted on charts (logs) that record the changes in the hydrocarbon composition and content for a cross section of the well. The depth of the location of the oil-bearing or gas-bearing bed is determined from these charts.

Gas-logging units (automatons) are used to conduct this work. These units contain various instruments that make it possible to monitor the well bottom depth, well sinking rate, and drilling fluid circulation, to analyze the gas entering from the gas stripper, to determine the presence of petroleum in the drilling fluid, and so on. The results of the gas analyses are automatically recorded. After taking into consideration the well depth and sinking rate, adjustments may be made permitting a more precise determination of the location of the petroleum and gas deposits according to the well profile.

Gas logging is also performed during well drilling shutdown. The drilling fluid stays in a well for some time and is enriched with hydrocarbons in those portions of the fluid in contact with oil- and gas-bearing layers. Later, the regular drilling fluid circulation is begun (as during well drilling), and the gas logging is performed, permitting the analysis of the fluid intervals that are rich in hydrocarbons. By introducing corrections for well depth and the drilling fluid circulation rate, the locations of the oil and gas deposits are determined according to the well profile.

Gas logging is also performed on core samples that have been subjected to chemical stripping and to analysis of the extracted gas. The results of the analyses allow inferences concerning the location of oil- and gas-bearing strata. The gas logging method is also used to investigate the gas-bearing capacity of coal seams. The joint use of gas and electrical logging of wells is envisaged for the future.

Gas logging in the USSR was first developed in 1933.