Ch. 31] CONTINUOUS INSTRUMENTAL LOGGING 567 



CONTINUOUS INSTRUMENTAL LOGGING 



Continuous instrumental logging combines several logging techniques 

 in one unit. Such data as depth, thickness, and fluid content of porous 

 formations penetrated by the bit are automatically and continuously 

 logged by this method, as shown in Fig. 4. In the process of drilling 

 through a porous formation the drilling fluid tends to become slightly 

 diluted by any formation fluid (whether water, oil, or gas) liberated. 

 These formation fluids are not only entrained by the drilling mud but 

 are also retained on or in the cuttings circulated to the surface. By 

 continuously testing the drilling fluid and cuttings at the surface, the 

 presence of oil or gas can be detected and the depth of origin deter- 

 mined. 



In testing the mud for gas, a motor-driven vacuum pump pulls air 

 through a trap attached to the flow line of the drilling fluid. From the 

 trap it is conveyed to the portable laboratory by means of a flexible 

 hose and passed through a filter, a humidifier, a flow meter, and a "hot- 

 wire" gas-detector instrument where the percentage of combustible gas 

 is determined. This total gas determination gives the percentage of 

 all the combustible gases, including methane. The amount of methane 

 gas present can be determined separately by controlling the tempera- 

 ture of the filament in the gas detector. 



Determination of gas in the cuttings is made by placing a small 

 amount of cuttings with a known quantity of water in the closed con- 

 tainer of a high-speed grinder. The grinding and agitation liberate 

 the natural gases that are mixed with the air in the container. This 

 air-gas mixture is tested for total gas and methane gas in the manner 

 described above. 



All oils, whether crude or refined, fluoresce under ultraviolet light. 

 A sample of drilling mud may be viewed directly under ultraviolet 

 light when it has first been treated to reduce surface tension and gel 

 strength. The relative amounts of observed fluorescence are used as a 



.... . .4 



basis for indicating the magnitude of the oil "shows." Freshly washed 

 cuttings may be examined directly under ultraviolet light for the 

 fluorescent effect of oil. 



Since the cuttings are examined in a routine fashion for oil and 

 gas, the same samples may be examined under the microscope and 

 estimates made of the percentage of sandstone, shale, limestone, and 

 anhydrite. A curve showing a percentage estimate of the amount of 

 sandstone in the cuttings is shown in Fig. 4. 



A drilling-rate curve is obtained from data provided by depth 



