随钻电磁波短源距前视远探测方法研究

Deep look-ahead logging-while-drilling electromagnetic (LWD EM) technology is critical to enhance the geosteering capability and to help the operators avoid the drilling risk. However, the existing look-ahead technology still has a plethora of disadvantages, e.g. overlong tool spacing, weak azimuthal sensitivity and difficulty in data processing. In this project, we developed a new hybrid dipole antenna which combines the magnetic dipole transmitter and electric dipole receiver. Taking advantage of the large amplitude, slow decay property and strong azimuthal sensitivity of the scattered electric field, a new look-ahead LWD EM method, which is capable of detecting farther boundary with shorter spacing, is established. Firstly, the three-dimensional Born geometrical factor and the analytic solution of the electromagnetic fields in stratified medium are derived. Based on the azimuthal sensitivity and decaying property of the background and scattered electric field, the optimal electric component is selected for detection of boundaries ahead of the bit and the signal definition is determined. Then, the hybrid dipole antennae are implemented by using the closed and open loop antenna. The spacing and array design are also optimized for accurate measurement of the scattered electric field. The responses characteristics of the look-ahead method are investigated and corresponding correction method are developed to eliminate the borehole effects. Finally, an inversion scheme is proposed to accurately extract the boundary and resistivity of the formation ahead of bit.

随钻电磁波前视远探测技术对提升地质导向能力、规避钻井风险具有重要意义，而现有技术仍存在源距过长、方位敏感性弱、资料处理困难等缺点。本项目拟将磁偶极子发射与电偶极子接收相结合，设计新型混合偶极子天线，借助电场散射信号的幅度大、衰减慢及方位敏感性强等独特优势，形成随钻电磁波前探测方法，采用更短的源距实现更远距离的边界探测。首先，推导三维Born几何因子和水平层状介质电磁场的解析解，研究电场的背景场与散射场的方位敏感性和幅度衰减特性，确定前探电场分量组合及信号定义方式；然后，基于闭合和开环天线等方式，设计混合偶极子天线，优化源距及阵列配置，以精确测量电场的散射信号；研究前视测井响应特征及影响因素，形成资料的井眼环境校正方法；最后结合反演算法，准确提供钻头前方地层界面和电阻率信息。

随钻电磁波前视探测技术对拓展地质导向视野、规避钻井风险和最优化油气产能具有重要意义，而现有技术仍面临短源距与远探测难以兼顾的难题。①本项目建立了利用电场散射信息的随钻电磁波前视探测方法：基于各分量空间贡献分布和宏观响应的差异，优选了前视分量，充分利用电场衰减慢、信号量大和方位敏感性强的优势，奠定了短源距前视远探测的理论基础。形成了基于全局幅度传播矩阵算法和多级奇异性消除策略的伪解析快速正演方法，为前视远探测仪器设计、响应研究和资料处理提供了快速模拟手段。②设计了适用于随钻环境的前视远探测天线系统，初步验证了电场信号测量的可行性：提出了开环ME天线+仪器旋转的电场分量测量和提取方案；设计轴向闭合发射+轴向开环天线、横向闭合发射+轴向开环天线的水平井和直井短距远探仪器结构。通过地面实验验证了轴向开环天线和电场分量的可测性、方位敏感性及信号强度，初步验证了新方法随钻前视探测的可行性。③系统考察了前视远探测原始信号与合成响应探边能力，建立了多层探边能力快速定量评估技术：混合偶极子前视天线结构的最大理论探边距离高达30余米，通过多频、多源距的组合，可实现对探边距离的全覆盖。综合趋肤深度衡量波在各地层内的衰减幅度，提出了计算模型自适应边界截断方法，实现了对任意多层边界的前视能力的快速、定量评估。④创建了基于模型降维与并行随机算法的前视远探测资料快速反演技术：综合考虑仪器各个方向贡献分布、钻遇地质结构特征，形成了前视远探测资料降维处理方法，极大降低了反演的难度。引入并行干预马科夫链蒙特卡洛采样算法，建立了混合偶极子电磁波前视远探测资料成像方法，实现了钻遇地层结构快速、精确重构，为实时地质导向和井眼轨迹最优化提供了可靠的界面信息。