GB750976A - Improvements in or relating to apparatus for logging formations traversed by a borehole - Google Patents

Abstract

750,976. Geophysical prospecting. SCHLUMBERGER WELL SURVEYING CORPORATION. May 25, 1954 [May 26, 1953], No. 15427/54. Class 118(2) Permeability of earth formations is investigated by measuring the phase relationship between the electro-filtration potential and pressure waves causing it in a fluid permeating the said formations, or, conversely, between electro-osmotic pressure waves and the applied electromotive force causing them. This phase angle is a direct function of the permeability but is also a function of the frequency of the motion-generating waves (periodic pressure waves or periodically varying electric field) and of the viscosity of the fluid in the permeable formations. The viscosity of the fluid (usually water) will generally be known and, for qualitative determination, is not important. At low frequencies the phase angle is governed by the fluid viscosity and will be small, while at high frequencies the inertia of the fluid is dominant and the phase angle will approach 90 degrees, and it will not change much with variation in permeability at either limit; a formula for deciding the optimum frequency to use is given. In the embodiment shown a cable 17 carrying a transmitting transducer 31, a receiving transducer 16 and an electrode 30 is lowered into the somewhat salty water 11 filling a bore hole 10. A suitable generator 32 is connected to the transducer 31, and the output from transducer 16 is fed via conductors 27, 28 to a phase meter 26. The potential picked up by electrode 30 is fed by lead 18 to two parallel circuits. The first circuit comprises a choke 21 whereby only the direct spontaneous potential is fed to a recording galvanometer SP, and the second circuit comprises a blocking condenser 23 and a recording galvanometer VSP for the alternating electro-filtration potential, which is also fed by leads 24, 25 to the phase meter 26. The phase meter 26 thereby indicates the phase angle between the sound waves and filtration potential at substantially the same level in the bore hole and is preferably arranged to record this as a function of the depth of the microphone 16 and electrode 30. Conventional means for raising and lowering the cable are provided but not shown. When the apparatus is opposite substantially impermeable material no alternating electrofiltration potential exists between electrode 30 and ground 20 and any signal present is merely "noise." Preferably, therefore, the phase meter is so biased that no phase angle is measured when the signal input is below a given level. In the alternative embodiment the transducer 31 is replaced by an electrode to receive alternating current from the source 32. The electrode 30 then picks up the alternating potential in the bore hole and the microphone 16 receives the electro-osmotic pressure waves created by oscillatory motion of the fluid in permeable formations. In either embodiment the phase-meter recording locates permeable formations and indicates their relative permeabilities. If the fluid viscosity of a given formation is known, the actual permeability may be determined by previous calibration of the apparatus at a given frequency opposite formations of known characteristics.