双向电磁随钻测量信号下传特性与双向通讯实验研究

Bidirectional electromagnetic measurement while drilling is not affected by drilling fluid, and it has high transmission rate and less vulnerable parts. It can improve the adaptability to the complex resistivity strata and extend downhole continuous working time by adjusting the working parameters of downhole instruments while drilling on ground. It can also cooperate with downhole tools and engineering control technology to develop the steering drilling system based on electromagnetic signal communication. So far, the uplink communication technology of bidirectional electromagnetic measurement while drilling has been relatively mature, and the breakthrough of downlink communication technology is the key to the development of bidirectional electromagnetic measurement while drilling. Therefore, the characteristics of downlink signal transmission and interaction mechanism of bidirectional communication will be sutdied in this project. By use of ANSYS three dimensions simulation and experiments, this project will research the effect of ground emission electrode, downhole pole pitch, frequency, formation resistivity and depth on downlink signal transmission. And the multiple formulae of downlink signal attenuation related to frequency, formation resistivity, depth will be established. The basis for setting of ground emission electrode length, diameter, emission pole pitch and downhole pole pitch will be presented. The interaction scheme of bidirectional communication will be put forward. The research achievements can provide theoretical guidance and technical support for the development of bidirectional electromagnetic measurement while drilling.

双向电磁随钻测量不仅具有不受钻井液影响、传输速率高和易损件少等优点，而且可以通过地面调节井下仪器工作参数，提高仪器对复杂电阻率地层适应性，增大仪器井下连续工作时间，亦可配合井下工具和工程控制技术，形成基于电磁信号通讯的导向钻井系统。目前双向电磁随钻测量信号上传技术已经较为成熟，在信号下传方面取得突破是实现双向电磁随钻测量的关键。因此，本项目对信号下传特性及双向通讯信号交互机理展开研究，综合应用ANSYS三维有限元仿真及室内外实验，研究地面发射电极、井下极距、发射频率、地层电阻率、井深对信号下传的影响，建立关于发射频率、地层电阻率和井深的下传信号衰减的多元关系式；提出地面发射电极长度、直径、发射极距和井下极距的设置依据；并提出双向通讯信号交互方案。研究成果可为双向电磁随钻测量发展提供理论指导和技术支持。

电磁随钻测量采用低频电磁信号将井底测量的工程参数实时传输至地面，可以适用于气体钻井和漏失地层钻井，其是完钻一口高质量定向井的关键技术之一。双向电磁随钻测量可以通过信号下传通讯技术随钻调节井下仪器工作状态，解决单向EM-MWD对复杂地层适应差、井下连续工作时间短的问题，且可以形成基于电磁信号双向通讯的旋转导向钻井系统，但国内目前研制的双向电磁随钻均为井底向地面传输数据的单向系统，双向电磁随钻测量系统还未实现，主要难在信号下传方面还未取得突破。基于此，首先应用ANSYS建立了“电流场”型的信号下传三维非轴对称有限元模型，并考虑到有限元三维模型计算效率较低的问题，采用二级离散复镜像法、线天线理论、矩量法和体等效原理建立了信号下传数值模型。其次，应用建立的有限元三维模型和矩量法模型研究了下传信号衰减与信号频率、地层电阻率、井深、电极长度及直径、电极极距等因素的关系，并给出了电极长度及直径、电极极距、井下极距的设置依据。再次，考虑到井下信号发射对信号接收的强干扰，设计了间歇式双向信号交互方法。最后，在此基础上设计加工了一套电磁随钻测量型号信号收-发系统，并开展了地面浅井和井内双向通讯实验。结果表明：所建数值模型在分析井内信号下传时准确性良好，本研究关于双向电磁随钻测量信号下传特性认识具有可行性；提出的双向信号交互方法具有实用性。此研究为进一步研制双向电磁随钻测量系统提供理论依据。