The Multi-channel Acquisition Module Design Of Oil-based Mud Logging Tool

In recent years, the oil-based mud logging tool with independent intellectual property rights is scrambling to research in the domestic and overseas. Due to its advantages of tolerating high temperature and corrosion, enhancing the wellbore stability and improving drilling efficiency compared to conventional water-based mud. Besides, it?s suited for the logging environment of water-based mud. Multi-channel acquisition module is one of the most important functional modules of the oil-based mud logging tool. Its main function is to detect the current signal which flow through the button electrode, complete small signal extraction, amplification, filtering, data collection, data operations and receive the command that is distributed by processing module and transmit the date. The performance of the acquisition module determines the accuracy of measurement of formation resistivity of the logging tool directly.Because of the reason that the signal which has been collected by the multi-channel acquisition module is extremely weak current signal, a comprehensive analysis of the noise of oil-based mud logging acquisition module has been made in the thesis and the mathematical model has been established. According to the mathematical models, we adopt the method of algorithm design and circuit design to reduce noise. In the design of the analog circuit and the digital logic of this module, the principle of restraining noise and improving signal-to-noise ratio is considered and followed to help oil-based mud logging tool measure the formation resistivity more accurately. Besides, the current input model of acquisition module has been established through mathematical derivation and analysis and the signal extraction circuit internal resistance value of acquisition module as well as its interference degree of stratum impedance measurement has also been analyzed.The thesis mainly deals with that the resistor thermal noise analysis, the op-amp inherent noise analysis, the ADC inherent noise analysis, the analog switch crosstalk noise analysis, the analog switch switching noise analysis and the online crosstalk noise analysis, the design of the ADC peripheral circuit with stray capacitance, the ground segmentation and the processing of the signal across the system, the circuit design of the weak current signal?s extraction and regulating, the design of the phase lock loop synchronization scheme, etc.. Besides, the DPSD algorithm has been used to improve the measurement accuracy.According to the technic index requirements of the multi-channel acquisition module, a series of experiments have been made to verify the rationality and accuracy of the module. Among them the logging cycle time experiment is used to verify if the logging time meet the requirement of indicators, the channel calibration experiment is used to verify the rationality of the design of the calibration channel, the current and voltage acquisition synchronous experiment is presented to verify the correctness of the phase-locked loop synchronization scheme, the high temperature experiment verify the working performance of the module under the environment of high temperature, the current sampling circuit noise suppression experiment is used to verify the ability of suppressing the noise of acquisition module; the ability of the small current detection is verified by the small current collection experiments, and downhole field logging experiment is illustrated to verify that the acquisition module can meet the logging demand of the whole instrument.