Research On Coil Type Complex Resistivity Spectrum Logging Experimental System

Conventional resistivity logging data is not suitable for saturation evaluation of unconventional reservoirs represented by low-contrast oil and gas reservoirs because they have relatively small differences in resistivity with water reservoirs.Petrophysical researches have shown that the complex resistivity spectrum(CRS)of rocks contains a large amount of reservoir parameter information,which is more advantageous in the evaluation of low-contrast oil and gas reservoirs.However,the research of CRS logging tools is relatively lagging behind.There are no mature industrialized CRS logging tools although many institutions have carried out research on multi-frequency resistivity logging tools.The Electrical Logging Laboratory of China University of Petroleum,Beijing has developed an electrode-type CRS logging tool prototype with a measurement frequency range of 1 k Hz to 500 k Hz,but the frequency range is too low to meet the requirements of the complex formation environment.To further promote the industrialization of the CRS logging tool,this paper carried out the design and research of the coil type CRS logging experimental system with a measurement frequency range of 250 k Hz to 8 MHz and the main tasks completed are:The numerical simulation study of the response characteristics of the coil-type CRS logging tool was carried out using the finite element analysis method.The structure of coil system was optimized,and four coil system with different deeps of investigation(DOI)(0.3 m,0.5 m,1 m,and 1.5 m)were obtained,all the four coil system has the same radial resolution of 0.2 m.Also,a simulated coil system with the DOI of 0.3 m and the radial resolution of 0.2 m for measurement in the laboratory was designed.Each coil system can realize the measurement of the complex resistivity at 6frequencies with the same DOI,and the radial resolution of the coil system is determined by the spacing of the receiving coils.A diagram expressing the relationship between the DOI of the coil system,the coil distance and the measurement frequency is designed,which can be more conveniently used for the adjustment of the structure of coil system.The coil system was optimized by adding compensation transmitter coil to overcomes the asymmetry characteristic of the measurement curve of the traditional three-coil system.The hardware structure and embedded unit software design of the coil type CRS logging experiment system were carried out.The hardware system of the coil type CRS logging experiment system is designed with DSP,DDS,mixer,PGA,FIFO and ADC units as the core.The system can transmit 6 frequency(250 k Hz,500 k Hz,1 MHz,2MHz,4 MHz and 8 MHz)sinusoidal signals in turn under the control of the host computer and perform down-frequency acquisition of the two receiving signals.The embedded software system of the DSP unit,in addition to completing the logic control of the experimental system and the signal communication of each unit,can also perform rapid amplitude and phase analysis of the measured signal based on the DFT algorithm and return the results to the host computer.A calibration circuit with known amplitude attenuation and phase shift characteristics and actual physical model were designed to test the experimental system.The results show that the experimental system can complete the design goal and realize the complex resistivity measurement at six frequencies,which confirms the feasibility of coil type CRS logging.The construction of the experimental system can further promote the industrial development of CRS logging tools.