Numerical Simulation Of Transient Electromagnetic Logging Response

With the development of complex oil and gas exploration and efficient development technologies,electromagnetic logging tools need to realize the identification of geological structures within the range of a few meters to tens of meters around the well.Time domain electromagnetic methods,also known as Transient Electromagnetic Methods(TEM),is a method that utilizes the intermittent of primary field produced by the step-function or pulse signal,and then observe the secondary field through a coil or an electrode,thereby extracting the formation information contained in it.It has the advantages of wide frequency band,rich information,extraction of target information in the time domain,wide detection range,etc.Therefore,transient electromagnetic logging has broad prospects for development.But currently there are few domestic studies on it.Therefore,it is necessary to simulate transient electromagnetic wave logging response with numerical simulation methods and systematically study the response characteristics of transient electromagnetic logging.In order to realize the remote detection of transient electromagnetic boundary,the theoretical basis work should be done well.First,the transmitting coil is equivalent to a magnetic dipole source in one-dimensional layered formation model,which is further decomposed into the superposition of vertical and horizontal magnetic dipole sources.By deriving the analytical solutions of the fields generated by the two directional sources respectively,the electromagnetic field distribution in frequencydomain of the magnetic dipole source in any direction in the layered medium is obtained,which can be converted into time-domain response of TEM logging in layered media through the frequency-time conversion algorithm of cosine transform.The diffusion and wave equations of the electromagnetic field in Maxwell’s equations in the rectangular coordinate system in the passive medium are derived.Then the virtual displacement current is added for the explicit difference of the time domain finite difference.A low-frequency approximate processing method is adopted to explicitly include the Divergence Equation of magnetic field.The time-domain finite difference formula under the Yee cell format is derived.Under the premise of ensuring its stability and numerical dispersion,variable space step and time step are introduced to expand the calculation area and speed up the calculation efficiency.In order to reduce the step error,we use conformal grid technology in the simulation of actual well logging and inclined formation.Based on three-dimensional FDTD numerical simulation technology,we study the response characteristics of transient electromagnetic wave logging under different media conditions.For layered formations,the sensitivity of the common influencing factors such as formation resistivity,interface distance,dip angle and source distance are analyzed.Then we study the response characteristics of four typical types of three-layer models(A,H,K,Q)and interlayer models and use the induced electromotive force attenuation curve to calculate the late-time apparent resistivity value.We further analyze the sensitivity for mud influencing factors such as mud resistivity,well diameter and invasion in the formation with borehole.Finally,we establish a corresponding three-dimensional model of abnormal structure around the well and analyze the logging response for the detection capabilities.Finally,we found that TEM logging can achieve forward looking detection with short source spacing and can extract formation resistivity information by analyzing logging response.