Contrast Experiment of Logging Curves Between Formation Electromagnetic Signal and Formation Apparent Resistivity in Casing Well

The transient electromagnetic through-casing resistivity technology is applied to work at contactless and continuous measurement, providing formation resistivity of different exploration depths with time slices. That is more accurate to calculate remaining oil saturation to satisfy the oilfield urgent needs. The electromagnetic probe takes symmetrical, single-transmitter and double-receiver coil system, simultaneously, the receiver coil takes comparative winding mode, and the transmitter coil takes bipolar rectangular wave as exciting waveform. Most part of the casing signal are eliminated by taking symmetrical outputting method and addition circuits, the left relatively is pure, could be regarded as the record of formation information. The formation resistivity is characterized by the voltage value measured in earlier section of the receiving period. We have taken a lots of tests in the indoor casing model well and discovered that the formation signal almost exists in the whole receiving period and overlaps with the casing signal, the contribution ratio of pure formation signal is too small to be extracted if takes single-transmitter and single-receiver coil system. While The logging instrument takes advantage of symmetrical, single-transmitter and double-receiver coil system, the amplitude of pure formation signal record could reach up to the range of mV ~ V at the earlier section of receiving period, very easy to be collected in circuits and making visual display. Then the instrument is carried out in a steel casing well. In data procession, we extract the voltage values in 1 ms time channel and 2 ms time channel respectively to form two resistivity curves. Alternately the two resistivity curves are compared with the counterpart measured in open well, and the results show that both of them are in good agreement. This paper utilizing a symmetrical, single-transmitter and double-receiver coil system and corresponding addition circuits to form a pure record of formation signal, and characterizing the through-casing resistivity with the signal extracted in the earlier section of receiving period, creates a new research direction in the through-casing logging area.