Theory And Method Of Transient Electromagnetic Detection In Production Well

Recent years witness the ever soaring international oil prices, which draws a lot ofattention of oil supply and demand worldwide. Considering the huge amount of thecased holes of the old oilfields and oilfields that are being explored, conducting theresearch of identification technology for formation resistivity is the top priority ofpetroleum researchers and workers. For a long time, the frequency domainelectromagnetic method is often used in the field of electromagnetic logging. However,this approach has some inherent defects. For example, the direct coupled signal thataliasing in the secondary field in induction logging, which need to take a number ofmeasures to eliminate, thus increasing the difficulty and complexity of the instrumentdesign. In the through casing resistivity logging, the poor contact between electrodesand the casing wall caused the signal to noise ratio is low, which makes the instrumentresolution is poor and the working efficiency is not high. Furthermore, due to the strongshielding effect of the metal casing on the electromagnetic wave signal infrequency domain electromagnetic logging, which is limits the further application ofthis method.The transient electromagnetic method use step wave or pulsed current field sourcesas an excitation, and measure the response signal in the interim period of the primaryfield. The method has some advantages such as sensitive to resistivity, has no directcoupling, the detection range is corresponding to time, and so on. Thus the resolution tothe target body is significantly superior to the frequency domain electromagneticmethod. In addition, the low frequency components of the transient electromagneticsignals can also improve the ability of the signal penetrate the casing. These advantagescan be better way to solve many problems in the traditional resistivity logging. Atpresent, the domestic and foreign researchs on the transient electromagnetic method aremainly focused in geothermal and crustal structure survey, geophysical and engineeringinvestigations areas. The transmitting frame in these applications is laid on the ground,and its effective detection range is the half space underground. But the distribution oftransient electromagnetic logging response is synthetically affected by the groundconductivity within the effective detection area around the borehole, which is a wholespace geophysical field problem. In view of this, for the actual work environment andcharacteristics of the oil and gas production wells, the dissertation proposed theproduction wells resistivity logging method based on the transient electromagnetictheory. And mainly surveys basic theory, methods and key technologies for oil and gas production wells transient electromagnetic detection. With the support of national863plan, the author conducts an experimental research of various media of differentresistivity characteristics outside the casing based on transient electromagnetic method.The main research results obtained are as follows:1. When electromagnetic wave spread in inhomogeneous media, it will produce thepropagation effects at the interface of the conductive medium. Traditional analyticalmethods lack the intuitive description for this phenomenon. This dissertation in depthstudies the propagation effects produced in the spreading process of electromagneticwave in inhomogeneous medium, and gives the its figurative representation. Which isprovides a basis for further reveals the propagation characteristics of electromagneticwave in production wells. In this dissertation, three methods, which are geometric factor,geometric factor theory of skin effect, theory of electromagnetic wave propagation,have been used to calculate the apparent conductivity curve of vertical layeredconductive media. The research results show that in the analysis of electromagneticfield distribution in inhomogeneous medium, the theory of electromagnetic wavepropagation not only allows for amplitude attenuation and phase shift produced byelectromagnetic wave, but also considers the reflection and refraction which take placein media cross interface, therefore it is the most accurate method in analyzinginhomogeneous medium, thus providing some ideas and methodologies for thetheoretical calculation of transient electromagnetic method.2. Based on the principle of electromagnetic induction, this dissertation put forwarda transient electromagnetic resistivity logging method for production wells. It can avoidthe problem of the low signal to noise ratio and the poor resolution in through casingresistivity instrument. This method uses non contact manner to achieve production welllogging and applys transient signal as the excitation. The received signal is not onlysensitive to the resistivity, and the time delay characteristics of it is corresponds to thedetection depth. According to the basic theory of frequency domain electromagneticfield, this dissertation analyzed the electromagnetic response characteristics of electricdipole source in detail. The simulation results show that the eddy current loss insidemandrel is proportional to the transmit signal frequency and the mandrel conductivity.Only low frequency electromagnetic waves and even ultra low frequencyelectromagnetic waves can effectively penetrate the casing into the formation. In orderto make electromagnetic wave penetrate the casing into formation, one should adjust thetransmission frequency in time according to casing thickness. But the changes of casingradius have no similar problems. These conclusions have laid a solid foundation for further study the transient electromagnetic response characteristics in production wells.3. For the actual characteristics of the oil and gas production wells，this dissertationestablished the calculated model for transient electromagnetic method in borehole. Atpresent, the reports on the numerical simulation of transient electromagnetic method inproducing wells in domestic or foreign are close to the blank. In view of this, the authordeduces the electromagnetic response expression of transient magnetic dipole source inaxisymmetric layered medium including borehole mud, metal casing, cement sheath andformation. In addition, the also author calculates the transient electromagnetic responsecharacteristics of the layered midium. The research shows that, at the point wherecasing thickness varies in some sections of the well, transient electromagnetic responsesare abnormal, in this way, the thickness of casing and casing collar can be identified.The higher the relative permeability, the more difficult the electromagnetic signals cango through the case, the lower the amplitude of the response signal. And the transientelectromagnetic response characteristics of the casing diameter are opposite to it. Withregard to low conductivity formation, low resistivity cement sheath can result in greaterinaccuracy, and as for the high conductivity formation, the influence of cement sheaththickness on response signals can be neglected. The transient electromagnetic responsesignal amplitude in borehole is higher than the through casing resistivity instrumentsignal for three to four orders. This has provided theoretical base for the parameterdesign of transient electromagnetic detection devices and logging signal interpretation.4. In according to the above theoretical results, this dissertation builded a transientelectromagnetic detection devices experimental system in borehole, and carried out asimulation study for key technologies. The parameters design principles of transientelectromagnetic devices are proposed and the range of emission parameters is also given.Moreover, the author has designated the optimum observatory devices in oil and gaswells. The transmitted signal waveform is determined. And the impact of transmit coilwire, coil turns and the emission current amplitude on the excitation signal off time arealso analyzed. What’s more, the author, by analyzing receiving coil equivalent circuit,also discusses in detail the transitory process of full induced electromotive force byramp wave excitation. In order to shorten the intrinsic transitory process of coil, theauthor believes that adding matching resistor and increasing damping should be utilizedto make the receiving coil in critical damping. In addition, the estimating methods ofmatching resistor and distributed capacitance have already been given, thus paving theway for parameter research of transient electromagnetic detection devices andunderground experiments. 5. A prototype of transient electromagnetic detection device in production wells isdeveloped in this dissertation. And the author also conducts indoor and outdoor researchfor through casing resistivity transient electromagnetic detection. Indoor simulationexperiment proves the effectiveness of the ring current source around the mandrel ondifferent conductivity media. Artificial abnormal body detection experiment shows thatthere is a greater difference of detection devices on outside casing artificial abnormalbody, illustrating that this device can effectively detect low resistance abnormal body.Indoor pool detection experiments shows that when adjusting outside casing liquidconductivity, the response signals of detection devices possess greater judging ability.By simulated well experiment, the author further compares and researches theperformance of oil and gas wells transient electromagnetic detection devices on outsidecasing medium. In this way, and the rationality of the transient electromagnetic methodcoil design, and the correctness of forward model and calculation for it have alreadybeen proven.The research findings lay a theoretical foundation for the application of transientelectromagnetic method in exploration and detection of oil and gas production wells.Furthermore, this dissertation also provides the way and basis for the research anddesign of transient electromagnetic detection devices in oil and gas wells.