Research On The Induction Logging Method In Cased Hole

Petroleum is a crucial strategic resource of a country.The stability and increases in oil production are the top issues to reduce the dependency on imported crude oil.The reexamination,the excavating potential and the discovery of new production layers in old production wells(metal casing wells)are significant ways to produce steadily and increasingly.However,on one hand,the long-term mining of production wells leads to changes in crude oil reservoir,which cannot be accurately explained and evaluated by the original logging data of open wells.On the other hand,the formation resistivity,the key parameter for oil and gas evaluated by conventional lateral instruments and induction logging tools,is severely hindered by metal casings in old production wells.Therefore,it is urgently indispensable to study new methods of detecting the resistivity of outter formation of metal casing,reinterpreting and reevaluating reservoirs,and discovering new oil and gas reservoirs.This paper studies distribution characteristics of electromagnetic fields of cased wells based on current coils by using numerical methods,puts forward a new method,the induction logging method of measuring resistivity of outter formation of casing in cased hole,and studies systematically choices of key parameters of cased well,responses of non-uniform formation and influences of non-uniform metal casing.Meanwhile,it also proposes the method of casing parameter inversion based on the forward response database,and finally studies technical issues related to the new method application into practical projects.The major work and contributions of this paper are as follows:(1)Based on electromagnetic induction principles,the electromagnetic field calculation method of a metal casing well is put forward when medium parameters of both permeability and conductivity are simultaneously considered,the numerical calculation of electromagnetic field in cased hole is obtained.The key to measuring formation resistivity in metal cased well is to solve the electromagnetic field calculation of cased holes based on coil sources.First,the calculation methods of electromagnetic field,voltage,and formation resistivity in uniform medium are offered,and rules of electromagnetic field distribution in uniform medium are obtained.Based on the above,the electromagnetic field calculation method is proposed in nonuniform medium of vertical and radial directions respectively so that characteristics of influence of wellbore,casing and formation are accurately analyzed and a theoretical basis for research method of formation measurement in cased hole is established.(2)An induction logging method of measuring formation resistivity in cased hole is put forward based on phase differences so that the formation resistivity measurement in cased hole is solved.First,aiming at the calculation inaccuracy of oscillatory integral functions of the electromagnetic field,characteristics of oscillatory functions are analyzed in depth and segmented integral strategies are proposed.Second,through numerical calculations,the relations are analyzed between the electromagnetic field distribution,the formation conductivity and the transmitter-receiver distance in cased hole and open hole,results of which show that: when the transmitter-receiver distance is more than 3m,the imaginary part of electric field(the real part of magnetic field)changes with formation conductivity in cased hole,while the real part of electric field(the imaginary part of magnetic field)changes with formation conductivity in open well.Second,the calculation method of phase difference of electric field is offered,rules of influences of different transmitter-receiver distances and various formation conductivities on phase differences are analyed,and a new induction logging method of measuring formation resistivity in cased hole is proposed based on phase differences.Finally,characteristics of influences of excitation frequencies and transmitter-receiver distances upon phase differences and receiving voltages are calculated and analyzed;and,optimal selection regions for key parameters of excitation frequency and transmitter-receiver distance are established according to the measurable signal magnitude.(3)The determination methods of vertical resolution and radial probing depth of induction logging in cased hole are proposed,and the calculation of performance parameters of logging instruments in cased hole is obtained.The vertical three-layer and the radial two-layer models are establised,characteristics of influences of formation conductivity,formation thickness and invasion radius upon various phase differences of transmitter-receiver distances are calculated and analyzed;the method of determining the vertical resolution is put forward by using the ratio between transmitter-receiver distance and targeted layer thickness and finally the method of determining the radial penetration depth of induction logging is proposed by using the normalized phase difference.When the normalized phase difference is 0.4,the radial probing depth is greater than the transmitter-receiver distance.(4)The logging module of cased hole with nonuniform casing is established,characteristics of influences of changes in casing parameters upon logging are revealed.Influences of nonuniform casing,such as corrosion,collar,crack and partial changes in conductivity etc.must be solved during the application of the new induction logging method.So four simplified modules of corrosion,collar,crack and partial changes in conductivity are designed respectively,rules of influences of changes in nonuniform parameters on phase difference and receiving voltage are calculated and analyed,results of which offers a theoretical basis for eliminating influences of nonuniform casing in practical logging.(5)The inversion method of casing parameters is put forward based on forward response database,and parameters of casing in hole are measured.The parameters that influence the logging response are inner diameter,thickness,conductivity and permeability of the casing,among which the inner diameter can be measured by using caliper tool,and three other parameters can only be obtained through inversion.So,first,a coil array of casing parameter measurement with one transmitter and three receivers is designed,a forward response database is estsblished by using numerical calculations,and then the rapid calculation of forward response is realized.Second,the searching method of fast inversion of 3-D parameters is designed by using dimension reducing idea based on forward response database.Finally,the inversion algorithm is designed,the inversion program is written,and the validity of this algorithm is tested by using inversions of various combined parameters.(6)The project implementation of induction logging instruments in cased holes is studied.The system of induction logging instruments in cased hole is designed,the signal source in the project implementation and receiving coil are studied in depth.The method of realizing the separation of response signals in the real part and signals in the imaginary part and detection of weak signals are put forward.And finally the method of signal noise elimination and the performance of detection algorithm through simulation experiments are studied.The research results establish a theoretical foundation for applications of the induction logging method of formation resistivity measurement in cased hole,and propose the method and basis for the design and research of induction logging instruments in cased hole.