Exquisite Simulation And Borehole Correction Method Research Of Electromagnetic LWD Response

With the development of oil and gas exploration,complicate reservoirs(e.g.thin interbeds)have become the main target and the reservoirs logging evaluation has entered into the era of evaluation of high deviated and horizontal wells.In recent years,because of the rapid development of LWD technology and the widely use of LWD tools,people place greater demands on numerical simulation of electromagnetic wave while drilling.Numerical simulation should depend on the electromagnetic instrumental structures and analyze the impact of different factors,such as instrumental parameters and environment,on log response.By doing that,people can optimize parameters and correct the measurement environment.In this paper,finite difference time domain method is used to study the fine simulation of electromagnetic tool.By using the sub-grid and conformal grid techs,the computational speed and accuracy have been improved significantly.Firstly,sub-grid is used to improve the speed of computation.More specifically,in fine places such as coils and tool grooves,local adaptive fine meshing is used.While in other place such as formation,coarse mesh generation is used.Secondly,conformal grid is used to improve the accuracy of the computation,which is to assign the grid parameter in boundary of complicated beds and off-centre of tool.After that,simulation of the V-groove electromagnetic tool is used to study the impact on the logging response of the tool parameters,such as groove angel,transmit frequency,spacing and so on.Meanwhile,the impact of measurement environment,such as mud filtrate resistivity,shoulders,the thickness of beds,is analyzed.And the correction chart of well is established.Finally,the logging response of tool with uniform grave is discussed and the impact of duty ratio,fiberglass resistivity,the number of grave and other tool parameter on the strength of signal is studied.Comparing to the nonuniform grid,the sub-grid tech improves 5.3 times computational speed with the relative error in 0.8%.The accuracy of simulation is 3.42 times to the ladder approximation by using the conformal grid tech in situation of off-centre.As to the V-groove instrument,the smaller the groove angle is,the stronger the signal of receiving coil will be.And the bigger the groove angle is,the bigger logging response difference between the formation and the simple tool will be.When the thickness of the bed is thin enough,the log data of resistivity will be different from the real resistivity of the formation.The results conform that the off-center making a great impact to the response.To the uniform groove tool,it is most appropriate to value the duty ratio in 23%.And the more the number of groove is,the stronger the formation signal will be.Generally,the resistivity of FRP makes a little contribution to the formation signal.