随钻多模式电阻率成像测井仪响应的三维有限元数值模拟（英文）

3D FEM simulation of responses of LWD multi-mode resistivity imaging sonde

设计并成功地研制了一台以螺绕环为激励源的新型随钻多模式电阻率成像测井仪样机,该仪器可在方位电阻率、侧向电阻率和钻头电阻率三种模式下进行测量。相比现有仪器,该仪器节省了时间成本和经济成本。文中将螺绕环激励源等效为延伸的电偶极子,推导了在均匀地层中的测井响应方程。利用3D有限元法模拟了设计的新仪器在三种不同测量模式下的探测深度、纵向分辨率、周向方位探测特性以及井眼尺寸、泥浆电阻率、目的层厚度、围岩层电阻率和泥浆侵入的影响。数值模拟结果表明,新仪器的三种测量模式之间具有不同的探测深度和纵向分辨率,可为井场实时决策提供更多的参考。方位纽扣周向探测能力取决于异常体与周围地层电阻率对比度和异常体周向张开角度。井眼在钻头处截断与否对模拟结果有较大影响。井眼、泥浆侵入对不同模式的影响都存在,而围岩层厚的校正只需要在侧向测量模式下进行。本文结果可为仪器的研发及随钻测井解释提供理论依据。

A new multi-mode resistivity imaging sonde, with toroidal coils as source, can conduct three resistivity measurements: azimuthal resistivity, lateral resistivity, and bit resistivity measurements. Thus, the logging time and cost are greatly saved. The toroidal coils are simplified as an extended voltage dipole and the response equations are derived for a homogenous formation. Based on 3D FEM, the depth of investigation(DOI), vertical resolution, circumferential azimuthal capacity, borehole diameter, mud resistivity, thickness of target formation, and the resistivity of the surrounding formation and mud invasion are simulated. The results suggest that the three measurement modes of the new sonde are different in vertical resolutions and DOIs. The circumferential detection ability of the azimuth button depends on the contrast between the anomaly and formation resistivity and the open angle of the anomaly. Whether the borehole is truncated at the bit or not has a great influence on the simulation results. The borehole and mud invasion affect the apparent resistivity in all modes, but the effects of resistivity of surrounding formation and thickness of the target formation are only corrected for lateral resistivity measurement.