GB1513595A - Radio frequency dielectric induction well logging system - Google Patents

Abstract

1513595 Borehole logging TEXACO DEVELOPMENT CORP 15 June 1976 [21 July 1975 2 Sept 1975] 24702/76 Heading G1N In a method of determining the characteristics of earth formations 15 in the vicinity of a well borehole 13 having a conductive fluid 14 in it, in which an alternating electromagnetic field at a frequency in the range 10-60 MHz is generated from a transmitting coil 17 and detected at at least one receiving coil 19 spaced axially of the borehole from the transmitting coil, the formation of a continuous circumferential path of the conductive fluid about either coil is prevented by the provision of at least one radially-extending, non-conductive fin 101 extending from the logging sonde 11 to the borehole wall. This effectively increases the magnitude of electromagnetic fields from the transmitter coil into the formations and from the formations into the receiver coil(s). Preferably the formation resistivity is determined by measuring the amplitude of the r.f. signal (30 MHz) at one of the receiver coils. A resistivity measurement or the r.f. amplitude measurement is then related to the phase difference between received signals Figs. 4, 5 (not shown) at two axially spaced receiver coils 19, 21 receiving the 30 MHz signal to determine permittivity of the formations. In an alternative embodiment Fig. 8 (not shown) the apparatus combines a (16 MHz) radio-frequency dielectric logging portion with a conventional 20 kHz resistivity logging portion. The former uses a transmitting coil 17 and receiving coil 21 one metre apart. Fins (101) are provided about both the resistivity log receiving coil (111) and the radio-frequency log receiving coil to prevent the formation of circumferential conducting paths in the well fluid about the coils. The measured resistivity is compared with the ratio of the amplitude of the 16 MHz signal received at the coil 21 in the well to that received at the coil 21 in air, Figure 10 (not shown), to derive the formation permittivity.