Patent GB-715870-A (GB)
Title: Improvements in or relating to differential pressure well logging
IPC: E21B47/06
Filed: 1952-08-19
Abstract: 715,870. Electric measuring systems. SCHLUMBERGER WELL SURVEYING CORPORATION. Aug. 19, 1952 [Sept. 27, 1951], No. 20815/52. Class 40 (1). Permeable earth formations traversed by a borehole are located and logged by measuring the pressure difference between that of the drilling liquid and that of the fluid within such formations. The drilling liquid 11, Fig. 1, comprises small particles suspended in a liquid base of such weight that the pressure of the liquid column in the borehole 10 is always greater than the fluid pressure within a permeable formation such as 14 at the same level. The suspended particles then flow into the permeable formation, as at 15, and form a "mud-cake" 16 on the wall of the borehole opposite the permeable formation such that substantially the entire pressure difference between the liquid column and that of the fluid in the formation occurs across the mud-cake. This pressure difference is then measured by sliding down the bore-hole a pad 21 which removes the mud-cake so that the fluid pressure within the formation is applied through a duct 23 to one side 17 of a manometer 19 the drilling fluid pressure being applied to the other side 18. Any manometer translating the pressure difference into a corresponding electrical variation may be used, e.g. an elastic member and strain gauge combination, piezo-electric quartz device, Bourdon or mercury tube, so that the signal after amplication may be transmitted over cable 27, 28 to an indicator 31 or a recorder which correlates pressure and depth. Manometer construction. Figs. 1, 2, 3, 5.- The pad 21 and a back-up pad 38 are carried by leaf-springs 32 secured to collars 34, 35, slidably supported on a central housing 26. The pad 21 is preferably made of hard alloy steel and formed with edges as shown in Fig. 2 for cutting through the mud-cake. The manometer acts on the electro-filtration potential principle, and comprises a permeable disc 66 of glass, porcelain or clay secured in a rubber disc 67, mounted in insulation 65. Electrolyte of low ionic concentration, e.g. distilled water, contained in a bladder 72, subjected to the drilling fluid pressure, flows through a duct 71 in insulating members 76, 77 to the side 18<SP>1</SP> of the disc 66, and passes through a duct 68 to a plug 75 which prevents clogging of the port 23 and has far greater permeability than disc 66. Owing to the pressure differential, the flow of electrolyte is always in the same direction, i.e. from the face 18<SP>1</SP> to face 17<SP>1</SP> of the disc 66, and the potential difference of the two faces arising from the flow is detected by metallic screens 69, 70, secured to the faces, and fed over leads 24, 25 to an amplifier located in the housing 26. Modifications.-A direct indication of formation pressure may be logged by combining the manometer indication with one of the drilling fluid pressure at the same level, or a manometer may be used which does not depend on a fluid flow, and have one side subjected to atmospheric pressure. Other logging measurements may be made simultaneously by means of electrodes housed in the back-up pad 38.