微电极测井的三维有限元数值模拟

Simulation of Micro-electric Log Based on 3D FEM Method

利用三维有限元法计算了微电极测井仪的电场、电流分布。为模拟微电极测井仪的电场分布,建立了低电阻率泥岩中存在高电阻率渗透性砂岩层时的三维有限元模型。为定量了解微电极系测井曲线与泥饼厚度的关系,建立了泥饼厚度在0.001～5cm变化的模型,并计算了相应的微电极测井响应。计算结果表明,当泥饼厚度小于1.08cm时,微电极测井曲线的正幅度差随泥饼厚度迅速增大,在泥饼厚度为1.08cm处达到峰值,之后正幅度差随泥饼厚度的增大而逐渐减小。为定量了解微电极测井曲线与泥饼电阻率的关系建立了泥饼电阻率在0.01～10Ω.m变化时的模型。在已知钻头程序和井径曲线的情况下可以精确确定泥饼的电阻率值,这对测井曲线的环境校正具有重要意义。

The electric field and its distribution of micro-electric log is accurately calculated with 3-dimentional finite element method （FEM） and the relationship between log response and mud cake thickness and mud resistivity is calculated quantitatively. Established is a 3-dimentional fi- nite element model for low resistivity mudstone where there are high resistivity permeable sand- stones to simulate electric field distribution of micro-electric log. In order to quantitatively study the relationship between micro-electric log curves and mud cake thickness, established is a mud cake thickness model when the thickness changes from 0. 001 em to 5 cm, and the correspondent micro-electric log response is computed. The calculated result shows that when the thickness is less than 1.08 cm, positive separation of micro-electric log curves increases rapidly with the rise of the thickness. And the positive separation reaches peak value when the thickness is 1.08 cm, and then the positive separation decreases gradually with the increase of the thickness. Estab-lished is a mud cake resistivity model with the resistivity changing from 0.01 Ω·m to 10 Ω· m so as to quantitatively study the relationship between micro-electric log curve and the thickness. The study shows that the mud cake resistivity can be calculated accurately when the drill bit pro-gram and caliper log curve are given. And this has important significance in log curve environ-ment correction.