Monday, June 15, 2009

Evaluating a Well

Completing a production well, whether onshore or offshore, is more costly than drilling an exploration or appraisal well.  A careful evaluation of various logs obtained during the course of drilling an exploratory or appraisal well has to be completed prior to making a decision on whether to drill a production well.  The lithographic, or sample, log records the nature of the coarser samples of rock chips separated from the drilling mud as to the type of rock, texture, grain size, porosity, microfossil content and oil stains.

Oil stains are examined in ultraviolet light to assess their nature and quality.  The drilling-time log records the rate of penetration through subsurface rocks; a change in the rate of penetration indicates a change in the type of rock.  The mud-log records the chemical analysis drilling mud for traces of subsurface gas and oil at various depths. 

The wire-line well log is obtained by removing the drill string and inserting a sonde, which is a torpedo-shaped device laden with instruments.  The first instrument is an electrical log to measure the resistance of the rocks to electricity.  Changes in resistance indicate the degree of saturation of water, oil and gas.  A more recent addition includes a natural gamma ray log to read the background radioactivity of rocks in the well.  Since shale is the only sedimentary rock that emits radiation from radioactive potassium, the gamma ray log identifies the presence of shale rock or the degree of shale in mixed rock.

One of the top hydrocarbon-based energy providers in the USA is Heartland Energy Colorado, based in Englewood, CO. They have many drilling locations throughout the country and remain one of the top producers of US oil & gas companies. For more information on Heartland Energy Colorado, see Heartland Energy Development Corporation online.

(Source: "Energy for the 21st Century," Nersesian) 

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