Research On Detection Method Of Well Cementing Based On Characteristics Of Flexural Wave

In the exploration and development of oil fields,well cementing evaluation is a very important work.With the application of new cementing technology,especially the extensive use of lightweight cement in bonding well,the traditional logging method based on acoustic impedance is difficult to evaluate the cementing quality accurately.Therefore,it is very important to study a new effective evaluation method.Currently,the well cement logging method based on flexural wave has become a research hotspot.But there are still some theoretical and engineering problems need to solve,due to the complexity and particularity of the detected objects.In this academic dissertation,the further studies are carried out on some theoretical problems of flexural wave in cement bond logging,based on the work of the predecessors.The main works in this paper are following:1.The layered models of casing well with different cementing quality are established.An improved transfer matrix method is proposed according to the layered structure characteristics with the alternation of solid and liquid.By this method,the propagation characteristic equation of Lamb wave in casing well is derived;2.The concept of complex wave number vector is introduced in order to describe the amplitude attenuation caused by energy leakage in the propagation of Lamb wave quantitatively.Combined with the Snell theorem,the boundary conditions that should be satisfied in layered media are studied.So,the propagation equation of Lamb wave is extended to the complex wave number domain.The new characteristic equation can be used to describe the relationship among the frequency,phase velocity and attenuation rate simultaneously;3.Because the characteristic equation of Lamb wave is a complex transcendental equation,which is very complicated,an efficient numerical method for solving the equation is proposed according to the complex theory and the propagation characteristics of mode waves;4.The excitation conditions of the flexural wave are studied,and the relationship among the sound frequency,incident angle and excited mode waves is presented.Then one method is proposed to calculate the equivalent reflection coefficient for single interface by generalized ray theory.Based on the above and combined with CSP(complex source point)theory,the forward modeling of excitation,propagation and reception for flexural wave is established under conditions of different cementing quality.The boundary waves from all interfaces,including the leaky waves of the flexural wave,are obtained by numerical calculation;5.Based on the theoretical research,one experimental system is designed,and the relevant experiments are done to verify the theoretical research correctness.The main research conclusions in this paper are following:1.The research results of both the theory and the experiments show that theattenuation rate of flexural wave has a strong correlation with the cementing quality between casing and cement,i.e.,when the cement is ordinary cement or lightweight cement,the attenuation rate increases with the decrease of the thickness of the water layer between the casing and cement;when it is fast cement,the attenuation rate gradually increases with the decrease of the thickness of the water layer,and then decrease after reaching a certain peak.Therefore,it is theoretically feasible to detect cementing quality of the ordinary cement and the lightweight cement with attenuation characteristics of flexural wave,while the cementing quality evaluation to the fast cement should adopt the traditional acoustic impedance logging tool.2.The numerical calculation and experimental results show that the center frequency of the transmitting ultrasonic transducer should be 250 kHz and the bandwidth should be 60 kHz for the best effect for excitation and receiving of the flexural wave.The acoustic system for transmitting and receiving should adopt the pitch catch configuration with one transmitter and two receivers,the best incident angle for the excited transducer varies with the thickness of the casing.For the casing with 9 mm thickness,the best incident angle should be 33 degrees;the two receivers,with a distance of 10 cm,should have the same central frequency and bandwidth as the transmitter and are oriented with the same angle as the transmitter.Leaky wave's sampling rate should be greater than 1 MSPS,the digital quantization precision should be greater than 14 bit at least.The achievement and innovation of this paper are following:1.An improvement to transfer matrix method is made so that it better describes the layered structures of the cased well which have the characteristics with the alternation of solid and liquid.The propagating equation of Lamb wave in the layered structure is derived by the improved method,which avoids the pseudo-model phenomenon.At the same time,the concept of complex wave number vector is introduced in the sonic field analysis of the cased well,which help to better describe the attenuation phenomenon in the propagation process of Lamb wave due to energy leakage;2.A special numerical method for solving Lamb wave characteristic equation has been proposed.The method first predicts the region of the optimal solution in numerical calculation according to the propagation characteristics of Lamb wave;then determines the existence of the solution accurately near the predicted region based on the complex function theory;finally,the optimal solutions are searched in the solution-existed region,and the dispersion curves and attenuation curves are obtained,both of which can be used to describe the propagation characteristics of the flexural waves.This method improves the searching speed of the solution while reduces the rate of solution misjudgments;3.The computational formula of equivalent reflection coefficient for single interface of casing well has been derived according to the generalized ray theory.Theboundary waves from each interface are calculated according to the forward model,the full waves of the receiving point are superposition of these boundary waves.The forming process of the leaky waves of the flexural wave can be observed in the full waves,which can locate the boundary waves corresponding to the cementation interface and discriminate the composition of the longitudinal and transverse waves.It provides a feasible calculation method for logging interpretation and analyzing the physical state of the cementation interface.To sum up,this paper studies the cement bond logging method based on flexural wave.The research results not only lay a theoretical foundation for the cement bond logging,but also provide technical guidance for the development of the cement bond logging instrument based on the flexural wave.It is of great significant in the theory and practice.