Research On Key Techniques Of Azimuth Electromagnetic Wave Resistivity LWD

In the exploration and development of oil and gas resources, the resistivity based on conductive characteristics of the reservoir is the basis of the qualitative evaluation whether the formation is oil-bearing strata, oil or water-bearing layer, and it is also one of the important parameters of the reservoir hydrocarbon saturation. LWD has characteristic of real-time and efficient, it can effectively reduce the impact of the drilling fluid invasion to the formation, and it is significance to accurate evaluation of reservoir oil-bearing. In order to enhance oil recovery and reduce costs, horizontal wells, deviate wells and multilateral wells are used, and the number of these wells is increasing. Comparing with vertical wells, the logging response has changed because the electrical parameters surrounding the borehole are the nonaxisymmetrical distribution, research on electric logging of modeling and inversion numerical simulation based on horizontal wells and deviate wells is hot. Meanwhile, in oil drilling, it is urgent need a LWD system that can carry out real-time geosteering and it is suit to the complex oil&gas layer, predict the undrilled formation to maintain the borehole within the desired geological. In recent years, based on the techniques of traditional resistivity LWD, Schlumberger, Halliburton, Baker Hughes have researched on the azimuth resistivity measurement, the corresponding instrument has been developed and been put into commercial use, but this research is still in infancy in China, it is important to solve the key scientific and technological issues about how to develop the azimuth electromagnetic wave resistivity LWD, therefore, it has great significance for us to research it in raising domestic LWD technical level as well as to break the blockade and monopoly of foreign technology.In this context, considering on development of instrument for the future, we focuses on the key technologies of azimuth electromagnetic wave resistivity LWD in this dissertation, study on the forward numerical simulation of logging response and weak signal detection with the theoretical basis of electromagnetic theory, electrical logging method and electronic information technology. To the forward simulation problem, based on the principle analysis of calculation method and electromagnetic wave LWD, we employ ADI-FDTD directly with Yee’s non-uniform staggered grid in three-dimensional cylindrical coordinates, transfer coordinate system between strata space and instrument space, which the conductivity tensor of the anisotropic and dipping formation can be expressed in coordinates of instrument, use area weighted average to compute the effective conductivity of partially-filled grid cells at interfaces and use UPML absorbing boundary conditions to truncate the computational domain. To the weak signal detection of logging problem, based on the analysis of noise from LWD, we use nonlinear theory and electronic information technology to research weak signal detection method of electromagnetic LWD, make a Duffing model to detect weak signal based on one-dimensional nonlinear oscillator, reveal the mechanism of Duffing oscillator which is used in weak signal detection based on the analysis of the dynamics characteristics, design circuit and use Duffing circuit to measure the amplitude attenuation ratio and phase difference of electromagnetic LWD with method of time-scale transformation. We master the logging response characteristics of the instrument by forward modeling, it is not only provides a theoretical basis for the logging data processing and interpretation but also provides a guidance of design methodology for the development of instrument. By researching of logging weak signal detection, we can down the detected SNR, improve the radial depth of instrument and enhance the predicted distance of the undrilled formation, it provides the technical support of signal acquisition for the development of the instrument.In this dissertation, those work was researched such as the calculation method of electromagnetic field with FDTD in three-dimensional, numerical simulation algorithm, forward simulation of electromagnetic wave resistivity LWD, dynamic behavior of Duffing oscillator and effect of noise, discriminate algorithm on phase state of system, electromagnetic wave logging weak signal detection and realization based on Duffing oscillator, its main research work and achievements are as follows:1. FDTD calculation method of electromagnetic field in three-dimensionalResearch on FDTD calculation method of electromagnetic field is based on the theory of time-domain Maxwell equations and Yee’s grid. From the calculation of differential, iteration of differential equations, numerical stability and numerical dispersion, absorbing boundary conditions, spatial discretization, effective parameters of conformal grid, the excitation of source, and the extraction of time-harmonic field for amplitude and phase is discoursed, respectively. On this basis, for the problem of Courant criterion leads to oversampling in time, the alternating direction implicit (ADI)-FDTD method and UPML boundary conditions of ADI-FDTD are studied, for the problem of staircasing error from rectangular grid and based on the actual shape of borehole, the non-uniform cylindrical Yee grid is studied, for the singular electric field problem of iterative equation in the axial direction, special processing method of the integral form of the Maxwell equations is used near the axial. Based on the research of FDTD calculation method, the time-domain iterative equation is derived in Cartesian and cylindrical coordinate system, combining characteristics of implicit differential, UPML absorbing boundary conditions is extended to the ADI-FDTD method, and the formula of UPML absorbing boundary conditions in3D ADI-FDTD method is derived. 2. Numerical simulation algorithm of azimuth electromagnetic wave resistivity LWDWe use Yee’s staggered grid to differential discretize the time domain Maxwell’s curl equations of the anisotropic formation in cylindrical coordinates, thus time domain recursive equation of each staggered node is obtained. To the deviate wells, we use rotation matrix to transform stratum space Cartesian coordinate into instrument space Cartesian coordinate, then transform it into cylindrical coordinate system, thus conductivity matrix tensor of anisotropy formation in cylindrical coordinate can be expressed. To the partially filled grid, the eccentered interface indicated between the borehole and the formation is non-conformal to the cylindrical grid, we employ an area-weighted average of quasi-static approximation to compute effective conductivities and approximate them as uniformly filled cells, so as to improve the accuracy of the simulation. To the problem of limited memory of computer, we use UPML absorbing boundary conditions to reduce the reflection of the outer boundary and truncate the computational domain, so as to improve the efficiency of the simulation. In order to validate the proposed algorithm, we compare the proposed algorithm with NMM based on model of simple homogeneous formation, and results show that the algorithm is correct. By studying the numerical simulation algorithm of azimuth electromagnetic wave resistivity LWD, we set up an algorithm to express the conductivity tensor in dipping and anisotropic earth formations with rotation matrix and area-weighted average, this algorithm can be extended to numerical simulation of other electrical LWD.3. Forward modeling on response characteristics of loggingBased on the research of the calculation methods and numerical simulation algorithms, we study the response characteristics of azimuth electromagnetic resistivity LWD in complicated conditions. By simulating for different model, results are shown as follows.1) In anisotropic formation, the measured apparent resistivity is a comprehensive reflection of the horizontal conductivity and vertical conductivity.2) The apparent thickness is widened with the inclination increases, and the horn effect can be seen in the phase difference response and amplitude ratio response for larger dipping angles, as the dipping angle is increased, the horn effect is more obvious.3) In general, the effect of eccentricity on the tool response is smaller when the conductivity contrast between the mud and the surrounding formation is low. Nevertheless, when the conductivity contrast between the mud and the surrounding formation is high, the effect of eccentricity on the tool response is larger, what’s more, with the eccentricity increases, and the amplitude attenuation ratio is also increased.4) At the same dipping angle, with the eccentric distance increases, horns effect of the boundary is more obvious.5) For the electromagnetic LWD tool that surface of coil is perpendicular to the mandrel, when tool penetrating through an the boundary with large dipping angle, in the logging response curve, horns are appeared on both upper and lower boundaries, moreover, the shape of horn is similar and width of horn is narrow, according to the shape of horn, it can indicate the presence of formation boundaries. However, for the titled coil electromagnetic LWD tool, when tool penetrating through the boundary with large dipping angle, horn of logging response curve is asymmetric, what’s more, the shape of horn is wide and deep, this horn is not only used to indicate the presence of formation boundaries, but also indicate the resistivity of the undrilled formation is high or low in advance, this allows for a proactive adjustment of the dipping angle and azimuth direction of the drill, hence, geosteering capabilities. By studying characteristics of logging response in forward modeling, we are familiar with the response of azimuth electromagnetic resistivity LWD in complex conditions, it provides a theoretical basis for the inversion calculation, development of instrument independently and interpretation of logging data for the future.4. Dynamic behavior of Duffing oscillator and impact of noiseIn this dissertation, dynamic behavior of Duffing oscillator and impact of noise are researched. We establish a detection model of Duffing oscillator based on one-dimensional nonlinear oscillator model, study the influence of movement state by the cycle perturbation and noise, theoretical analysis and numerical simulation show that:1) With the cycle perturbation increases, the phase trajectory of Duffing oscillator will be followed in accordance with the state of the homoclinic orbit, bifurcation state, chaotic state, the state of large-scale period, and the nonlinear dynamic characteristics is very obvious.2) Duffing oscillator is immune to any distribution of random noise and is sensitive to initial value of signal, the smaller amplitude of the input signal, the more sensitive to the signal for the system, the better anti-noise performance of the system, and the lower SNR.3) The impact of noise to the movement state of Duffing oscillator is relative to the intensity of noise, the presence and strength of noise effect the performance of detection system. By researching the dynamic behavior of Duffing oscillator and impact of noise, providing the support to weak signal detection based on Duffing oscillator, and revealing the weak signal detection system based on Duffing oscillator has the lowest detection threshold.5. Discriminate algorithm on phase state of system automaticallyIn this dissertation, discriminate algorithm on phase state of system is researched. Based on the conventional discrimination method, we study the automatic discrimination method from the view of engineering, and present two automatic discrimination methods which are energy factor of the power spectrum and crossing zone of the phase diagram. On the basis of the power spectrum calculation, we use some spectral components within the frequency window to quantitatively describe the energy factor that express the energy characteristics of the motion system, and main spectral components is the center of window. By setting a zone, the phase diagram is divided into two regions, one is inside, the other is outside, and the state of system is determined by the count of crossing. The presented methods have characteristics of low calculation amount, real-time, and it is easy to be ported to hardware, results show the methods are effective. The research provides effective means to discriminate state of system for the Duffing oscillator applied to the detection of weak signals actually. 6. Design of detection circuit for azimuth electromagnetic resistivity LWD based on Duffing oscillatorWe design a detection circuit based on Duffing oscillator. On the basis of weak signal detection with Duffing oscillator, we analyses the impact of measurement by phase difference and frequency difference between the signal and cycle perturbation. With the method of time scale transformation, Duffing oscillator is extended to detect the signal which frequency is2MHz. And the measurement principle of amplitude ratio and phase difference based on Duffing oscillator is studied, we design the overall block diagram of detection system, moreover, we use specific hardware circuit to realize the Duffing oscillator model. By inputting the thermal noise and simulating, results show that the designed Duffing circuit can detect signal with frequency of2MHz, circuit is very sensitive to parameters, meanwhile, circuit is very immunity to noise, with the accuracy of the critical threshold improves, the detectable SNR decreases. This research provides technical support to detect electromagnetic weak signal LWD based on Duffing circuit.This dissertation consists of five chapters, it’s organized as follows. In Chapter I, we describe the purpose and significance of research, status of relative technology, as well as research task, research methods and innovation. In Chapter II, we describe the method of FDTD, discuss the problem of calculation in Cartesian and cylindrical coordinates, it is supported to simulate the logging response for azimuth electromagnetic resistivity LWD. In Chapter III, we describe the principles of azimuth electromagnetic resistivity LWD, discuss the algorithms of forward modeling, and simulate the logging response for different model. In Chapter IV, we discuss the method of logging weak signal detection and Duffing circuit design. In Chapter V, we briefly summarize the work of dissertation, and present the proposal for future research.