Research On Tri-axial Array Induction Logging In Complex Formation

The tri-axial array induction logging tool is different from the traditional array induction logging tool.The coil probe of tri-axial array induction logging tool is composed of three mutually orthogonal transmitter and receiver coils,and can measure the tensor conductivity with nine components reflecting the formation of anisotropic information,moreover,the formation dip angle and azimuth angle of the tool are obtained simultaneously,which can be adopted to detect thin reservoirs and low permeability reservoirs that are difficult to be detected by traditional methods.Therefore,reservoir prediction and productivity evaluation can be conducted more effectively by means of the tri-axial array induction logging tool,which has a broad application prospect.However,the existing tri-axial tool has followed the traditional array induction method which is used to offset the coupling signal by the bucking coil.The responses of the existing coplanar coil system have negative responses in radial direction and severe “horns” in the longitudinal direction.It is more difficult to directly evaluate the water(oil)saturation of the reservoir from the measured values.This paper systematically studies the tri-axial array induction logging theory and technology for these problems,and proposes a new coplanar sub-array structure,which consist of two transmitters and one receiver coil.The coplanar sub-array can reduce the negative response area,the amplitude of the horn and the skin effect,moreover,the borehole effect is approximately linear.It is of great theoretical value and practical significance for the design and logging response interpretation and evaluation of tri-axial array induction tool.The main research work and contributions of this paper are listed as follows:1.The complex vector Maxwell equations are constructed.The models of the electromagnetic field generated by the coil current source are established in the infinite media without loss,conductive medium and mixed medium.It describes the relationships between source and fields clearly.The process and rule of excitation of time-varying electromagnetic field are presented which is stimulated by the time-harmonic current source.A new complex vector induction logging model is proposed.The electromagnetic phenomena are analyzed generated by transmitter coils of array induction logging around borehole.It reveals a relationship between the received voltage and the coil current,the formation parameters,the real and the imaginary part of each electric field,magnetic field and induced current.It provides theoretical basis for logging interpretation of the influence of borehole,formation and metal around the array induction logging tool.2.The response characteristics in homogeneous formations,Born geometric factor response characteristics and parameter influence characteristics are calculated between the coplanar coil system and the coaxial coil system using the array induction response calculating method.It is found that the skin effect of the existing coplanar coil system is significantly greater than that of the coaxial coil system,especially for long sub-array.There are drastic negative responses in the radial and longitudinal directions.The response characteristics can't be improved by adjusting transmitting frequency,position and number of turns of the bucking coil.However,negative responses still exist.This is an inherent defect of the coplanar coil system of the tri-axial array induction logging.It reveals the reason that the negative responses is occurred on the coplanar coil systems through investigating the influence rule of space region vortex separated by transmitter and receiver coils on the receiving voltage.3.A forward method is proposed based on the magnetic Hertz potential vector of horizontal magnetic dipole.By introducing the Bessel function and applying the tangential continuity of the layer boundary,the solution methods of the tri-axial array induction logging response are established in the radial columnar and longitudinal stratified inhomogeneous formations.The response characteristics of tri-axial coaxial and coplanar coils in radial and longitudinal inhomogeneous formations are studied in detail.According to the experiment,it is showed that the borehole effects and shoulder effects of coplanar sub-arrays are significantly different from that of coaxial coil systems,and borehole effect is the major reason of negative logging response.Compared the results of simulation in laboratory with field test on the coplanar coil system,the formation boundary can be identified using the transmitter coil or main receiver coil of the tri-axial array induction coplanar coil system.4.A new structure of coplanar system is presented,which consists of two transmitter coils and one receiver coil.The response characteristics of the conventional coplanar coil system and the new coplanar coil system are compared in the homogeneous and stratified heterogeneous strata,the Born geometric factor,the borehole effect in a standard 8in borehole.The measurement results of two different coplanar coils are investigated in complex formations such as the thin interactive bed,Chirp formation and inclined anisotropy formation.Moreover,the new coplanar system mechanisms are revealed for reducing skin effect and negative response through the analysis of the influence of eddy current in different zones separated by the transmitter coil,shielding coil and receiver coil.5.The experimental equipment of tri-axial array induction is designed according to the theoretical research,which includes the transmitting circuit,coil system and receiving circuit.The transmit and receive parameter design principle is determined in the experimental equipment.In order to eliminate noise interference in the transmitting coil,based on the impedance matching method,resonant capacitors are paralleled in the transmitting circuit for the function of frequency filter.The transmitting and receiving signal are measured of the coaxial coil system,coplanar double coil system,conventional coplanar coil system and new coplanar coil system.The experimental results show that the response characteristics of the new coplanar coil system are better than that of the conventional coplanar coil system.The measuring device can also study the influence of parameters such as the number of turns,distance and the transmitting frequency on the receiving signal.It provides experimental basis for the design of tri-axial array induction instrument.Above all,compared with the existing coplanar coil system,the new coplanar coil system improves the negative response and “horns” in the boundary layer.Meanwhile,the borehole effect is approximately linear and the spatial resolution is increased by 2 times.The research achieves a positive effect in the accurate evaluation of formation information and establishes a theoretical foundation for the design and application of tri-axial array induction logging tool.