An AVO Study And Application On Coalbed Methane Reservoir

Coalbed Methane is also known as "gas", it mainly comprise CH4 (methane), and is generated in the evolvement course of coal. In modern time, energy crisis has affected the whole world. Being clean and efficient, Coalbed Methane has attracted people's attention. American began exploitation test on Coalbed Methane in the latter 1970's, and realized commercial exploitation in the middle and latter 1980's, which made Coalbed Methane the new energy source after coal, oil and natural gas. In China, the prospecting of Coalbed Methane take a late start, and the research on exploitation of Coalbed Methane is only more than a decades. The problem reside is can not accurately predict the section where Coalbed Methane enriches, and the predicted favorable region is very insufficient.The exploitation of Coalbed Methane is greatly affected by its adsorption-desorption features. Generally more than 90% of Coalbed Methane is adsorbed in tine holes of coal, the adsorbed methane cannot move freely, and only when the pressure decreases, can the Coalbed Methane be desorbed and extracted out. If the exploitation region is not rightly chosen, the fractures do not develop in coal bed and the penetrability is too small, then the desorption of Coalbed Methane will be greatly affected, and cause a very low production rate.The theoretical basis of AVO technique is Zoeppritz equation which describes the rules and regulations of reflection and transmission of seismic wave on a horizontal interface. This paper first studied how different differences of different parameters can affect the accuracy of the Zoeppritz approximation equations, and then studied the effect of variation of different parameters on the reflection coefficient curve. Based on this study, chose a suitable approximation equation to do AVO intercept and gradient crossplot analysis on a ray tracing generated seismic record of a coalbed model. After that, designed a gas sand model and generated seismic record using ray tracing and apply NMO, and after that used geo-stack to do inversion on the seismic record, and generated pseudo-Poisson's ratio reflectivity section and a fluid factor section. After studying the reservoir characteristic of Coalbed Methane and its adsorption-desorption features, I believe AVO geo-stack inversion and fracture detection will be helpful in determine the suitable exploitation zone of Coalbed Methane. Based on the design of fluid factor in AVO geo-stack inversion, This paper put forward the primary condition and method of constructing a "coalbed factor". In this paper, AVO geo-stack was applied to model data, and after verified the inversion result, constructed the "coalbed factor" using well-logging data, and checked the coalbed using AVO geo-stack. Besides, tried to distinguish coalbeds with different Poisson's ratios using pseudo Poisson's ratio, and the inversion was good. Moreover, for verify the AVO geo-stack result, this paper used the first, second and third generation of coherence algorithm to do fracture detection, and the result was also fine.