GB1405299A - Techniques for evaluating the porosity of earth formations - Google Patents

Abstract

1405299 Borehole logger SCHLUMBERGER INLAND SERVICES Inc 18 Oct 1972 [3 Nov 1971] 47908/72 Headings G1A and G1N [Also in Division H4] In borehole logging, to determine the porosity of formations traversed by a borehole, in which the sonde carries devices to carry out measurements of the speed of sound, density and hydrogen content, and possibly other quantities, in order that the measurements provided by the devices all relate to the same narrow sections of formation, the sonde is structured so that the device sequentially follow the same path along the borehole wall; at any one instant, they are therefore aligned substantially along the same generatrix of the borehole wall. In the particular arrangement described, the sonde comprises five devices which are all directive so as to monitor narrow formation sectors: a sonic logging device 18; a density radiation logging device 19; an electrode-type resistivity logging device 20; neutron logging device 21; natural and radioactivity logging device 22. The sonde is urged against one wall of the borehole by spring-loaded arms 47, standoff wheels 49 preventing direct contact of probes with the wall. Sonic probe, 18. This comprises two receivers 25, 26, mounted on spring urged pad 27, which are used sequentially in association with firstly acoustic pulse source 24 and secondly acoustic source 23, and an add/subtract scaler, to obtain a measure of the speed of sound through the formation as represented by the time of propogation of waves in the region between the two receivers. This measure is indicative of porosity, lithology, vacuoles, clay, gaseous hydrocarbons. The sonic probe may be of the construction described in Specification 1399735. Density probe, 19. This comprises a 7-source 31 with two detectors e.g. a Geiger counter 32 and scintillation detector 33 or two scintillation detectors mounted in spring urged pad 34. The Compton effect is utilized to determine density, which is a function of the porosity, lithology and presence of gas. Resistivity probe, 20. This comprises a central emitting electrode surrounded by four concentric electrodes. The sensed current is indicative of porosity, roughness of the borehole wall and mudcake thickness. Neutron probe, 21. Neutrons emitted by a plutonium-beryllium; americium-beryllium or californium source 43 are detected as epithermals, slowed down by hydrogen nuclei in the formation, detected by 44, 45. Alternatively γ-radiation resulting from neutron capture is detected. The measure of hydrogen content (ratio of detector count rates) is indicative of porosity, lithology, clay, gaseous hydrocarbon. Radioactivity probe, 46, is a scintillation detector and indicates clay content higher natural radioactivity than porous rock).