基于非结构有限元电各向异性介质井-地激发极化法响应特征研究

Study on response characteristics of well ground induced polarization method in electrical anisotropic media based on unstructured finite element method

井中激发极化法因其具备工作效率高、勘探效果好,在深部找矿、油气资源勘查和水资源勘查中得到了广泛应用.目前井-地激发极化法的相关研究均是将地下介质假设为各向同性,但是实际地球介质的电性特征表现为各向异性,此时基于各向同性模型的传统成像及反演成像技术就不再适用于各向异性数据的处理解释.为分析各向异性特征对井-地激发极化法的影响规律,本文基于有限单元法开展不同主轴方向的电导率、极化率各向异性正演算法研究,从点源场满足的偏微分方程和边界条件得出异常电位法下的稳定电流场的边值问题和变分问题,实现各向异性介质的井地激发极化法的三维正演模拟.通过对复杂三维各向同性模型和三维各向异性模型的测试与对比,验证了算法的有效性与正确性.构建典型各向异性地质模型分析了不同主轴方向的电导率、极化率各向异性异常体对井-地激发极化响应的影响.结果表明x主轴电导率各向异性导致视电阻率响应值增大其视极化率响应增强,y轴和z轴电导率各向异性时视极化率均表现出较强敏感度;x轴极化率各向异性导致异常体的视极化率响应减弱而y轴和z轴方向极化率各向异性变化不敏感;y轴方向和z轴方向电导率、极化率各向异性时,视极化率响应变化敏感度低,x轴方向的响应变化比较敏感.该研究不仅有利于理解各向异性对井中激电的影响规律,而且可以为考虑各向异性的井中激电资料的数据解释提供理论支撑.

The method of stimulated polarization in wellshas been widely used in deep-searching,oil and gasresources exploration and water resources explorationbecause it possesses high working efficiency and goodexploration effect.At present,the well-ground excitationpolarization method of the relevant studies are thesubsurface medium is assumed to be isotropic,but theactual electrical characteristics of the earth medium isanisotropic is the existence of anisotropy,this time basedon the isotropic model of the traditional imaging andinverse imaging technology is no longer applicable to theanisotropic data processing and interpretation.In order toanalyze the influence law of anisotropic features on thewell-surface excited polarization method,In this paper,wecarry out the research on the anisotropic orthogonalalgorithm of conductivity and polarizability in differentmain axis directions based on the finite cell method.Marginal and variational problems for the steady currentfield under the anomalous potential method are derivedfrom the partial differential equations and boundaryconditions satisfied by the point-source field to realizethree-dimensional orthogonal simulations of the well groundexcitation polarization method for anisotropic media.Theeffectiveness and correctness of the algorithm is verified bytesting and comparing the complex 3D isotropic model and3D anisotropic model.A typical anisotropic geological model was constructed to analyze the effect of anisotropicanomalies of conductivity and polarizability in differentprincipal axis directions on the polarization response ofwell-surface excitation.The results show thatx-spindleconductivity anisotropy leads to an increase in the apparentresistivity response and an enhancement of its apparentpolarizability response.Bothy-andz-axis conductivityanisotropy show strong sensitivity in visual polarizability,x-axis polarizability anisotropy results in a diminishedapparent polarizability response of the anomaly whilepolarizability anisotropy changes in they-andz-axisdirections are insensitive.When the conductivity andpolarizability are anisotropic in they-axis andz-axisdirections,the sensitivity of the response change in theapparent polarizability is low,and the response change inthex-axis direction is more sensitive.This study not onlyfacilitates the understanding of the law of the influence ofanisotropy on the excitation in wells,but also can providetheoretical support for the data interpretation of theexcitation data in wells considering anisotropy.