Patent GB-1298355-A (GB)
Title: Improvements in and relating to data processing
IPC: G01V11/00
Filed: 1969-11-04
Abstract: 1298355 Borehole well-logging SCHLUMBERGER INLAND SERVICES Inc 4 Nov 1969 [5 Nov 1968] 53915/69 Heading G1A [Also in Division G4] A well-logging system combines two or more of three porosity parameters # N , # D  and # S  to obtain values of formation clay content (q), formation effective (i.e. non-clay-filled) porosity (# e ) or formation hydrocarbon (gas or oil) saturation (S ny ) in an analogue or digital computation involving an iterative process in which certain assumed constants are successively adjusted until a feasible result is obtained. Parameter # N  is related to formation hydrogen content and is measured (preferably by a side wall epithermal neutron log 14a sesitive to scattered neutrons; or by a conventional neutron log sensitive to capture gamma-rays. Parameter # D  is related to bulk density and is measured by a gamma source 15a with two detectors spaced at different distances from it. Parameter # S  is related to sonic travel time and is measured by  a sonic device 13. Formulae are given for all these relations. The logging tool is provided with skids 14b, 15b for keeping the logs in contact with the sidewall, spacers 16, a caliper recording borehole diameter, and a depth recorder 20. In certain modes, use may also be made, in the calculation of water saluration, of measurements of formation resistivity (R t ) by an induction device; and a spontaneous potential (SP) log may be used to indicate excessive shaliness. Analogue computing. Signals G 1 , G 2  from the bulk density log are combined in circuits 26-29 to produce # D . Signal C N  from the neutron log passes through circuits 23-25 to produce # N . Signal # t  from the sonic log is converted by circuits 17, 18, 21 into # S . All three are passed through inhibit gates 32 to matrix computer 33. Gate 32 is inhibited, when certain unfavourable conditions exist (e.g. excussive shale), through OR gate 37. The computer 33 generates, inter alia, the quantities q, S gxo  (invaded zone gas saturation) and S wxo  (invaded zone water saturation). These are applied to a limit testing circuit 43 which tests the feasibility of these quantities and, if there is non-feasibility, provides fresh values of compaction factor C p  or sonic matrix travel time # tm  to circuit 21 to give a recalculation of # S ; the iteration proceeds until feasibility is established. All the calculated quantities are then passed through amplifiers 48 to memory bank 50 whence they are transmitted to recorder 51 linked to depth recorder 20. In modifications only the density and neutron log signals may be used to calculate # D  and # N , the total porosity being assumed, Fig. 9 (not shown). Or where only the sonic and density log signals are available, the effect of hydrocarbon content on the parameters # S , and # D  may be ignored. Digital computing The operations performed by the circuit of Fig. 1 may be imitated by a suitably programmed general purpose digital computer and a suitable programme is given in Figs. 7A, 7B (not shown). The modification of Fig. 9 is also programmed, Figs.10A, 10B (not shown).