Design Of Oil-based Mud Electrical Parameter Measurement Circuit

Oil-based mud(OBM) is widely used in a variety of drilling platform because of its many advantages. OBM is mainly composed by oil, water and clay. The proportion of these components has a great influence on electrical parameters. The resistivity and permitivity are two important parameters of OBM. The value of these parameters impacts amplitude and phase of the logging signal, which can lead to inaccurate logging interpretation. Therefore, to determine the electrical parameters of OBM is an essential work. The resistivity and permitivity can be acqured by measuring the complex impedance parameter of the OBM when the metal electrode structure is fixed. This paper introduces the electrical parameter measurement method and illustrates the equivalent by analyzing the circuit model.According to the capacitive and high impedance characteristics of OBM, this paper focuses on research of broadband and wide range measurement technology and circuit for complex impedance. Vector voltammetry is used as the basic measurement principle and amplitude-phase demodulation technology is adoped as measurement scheme. The design makes full use of the demodulator’s characteristic curves and DDS’ phase shift function to complete the phase polarity discrimination. The impedance modulus calibration is finished by two-point linear method. The circuit has three main functional modules: the excitation signal source, amplitude and phase detection circuit and data acquisition module. The measuring circuit contains three main parts: the excitation signal source, amplitude-phase detection circuit and data acquisition module. The stable voltage source with low spurious takes dual channel device AD9958 as frequency generator followed by low pass filter and amplifier. The amplitude, frequency and phase parameter of the device can be flexibly adjusted. AD8302 detects amplitude and phase of the broadband signal. The signal conditioning circuit and adjustable amplitude of the signal soruce improve the detectable signal range. FPGA controls two pieces of AD7694 to realize synchronous acquisition of phase and amplitude parameters, and then it finishs preliminary data processing to improve stability. The master chip of the system is STM32. it undertakes many tasks—management of each fuctional modules,data processing and achieving automatic measurement. This design use MFC based on VC6.0 platform to develope PC application software which completes the tasks of curcuit debugging, result display, and data storage.The signal source frequency reaches 5MHz with resolution better than 1Hz. The measuring circuit can work well in wide frequency range of 500Hz~5MHz. The test results show that, the measuring circuit has good linearity, modulus error is less than 1.6% in the range 100Ω~5MΩ, phase error is less than 1.5° in capacitance-resistance testing.Compared with common mud resistivity measuring instrument, this circuit, working stablely, has wider measurement range and better accuracy. Its performance index meets the demand for practical engineering.