Theory On P-Wave Azimuthal AVO And Detecting Technique Of Coal Bed Cracks

As recent research, CBM (Coal Bed Methane) effusion relate with several geological factors. Among those factors, the tectonic damage or cracked degree are key factors. On the other hand, as the chief storages and main migratory channels of CBG (Coal Bed Gas), cracks affect its production directly. From those two aspects, the survey of tectonic damage or cracked degree is the key factor to prevent CBM effusion and optimize CBG production.By analyzing the technique of reflected P-wave AVO on coal beds, revised the traditional viewpoint on coal field's P-wave AVO, and considered that all coal beds, not only thick but also thin, are fitting AVO analyzing. Through the analysis of reflected P-wave AVO, tectonic damage of coal beds identified, especially soft coal detected. In addition, by introducing coal bed's approximation of P-wave AVO, improved approximation's precision of reflected P-wave AVO on coal bed, and by coordinate transforming, extended analyzable data range of P-wave survey. Thus making high precision's AVO inversion of coal beds becomes feasible. Through cross-plot analyzing of P, G attributes reflected on coal beds, finds that the scatter graphs of P, G attributes are regular spread according to different coal beds and their roof/floor.Most coal beds have massive cracks and cleavages, and when seismic wave propagating in them, seismic wave will exhibit anisotropy. By the introduction of effective media theory, achieved the elastic constant and velocity anisotropic curves of VTI coal, HTI coal, cracked HTI coal and ORT coal, and discovered that most coal beds have weak anisotropy. By importing anisotropic parameters, quantitatively achieved coal beds'anisotropies, and found that most coal beds adapt to weak anisotropic theory.By analyzing the AVO characters of P-wave on VTI coals, found that their P, G attributes are differ from primary coal obviously. Along with the enlargement of tectonic damage, they assume the orderly linear migration on P, G attributes'cross plot. By giving an AVO approximation of VTI coal, made high precision AVO inversion possible. Through the studying of azimuthal AVO/AVAz on HTI coal, discovered that the azimuthal anisotropy of azimuthal AVO/AVAz almost may neglect when incidence angle is smaller (less than 10o degree); along with incidence angle increasing, its scope of azimuthal anisotropy increases. When the roof of HTI coal is sandstone, its scope of azimuthal anisotropy is bigger than the situation of mudstone roof. Regarding cracked HTI coal, its azimuthal AVO/AVAz characters are basically consistent with HTI coal; the only difference is its azimuthal anisotropy obviously controlled by cracks. At the same crack density cases, the water saturated anisotropic scope of AVAz curve is bigger than the case of gas saturated. When cracks mutually connected, the permeability almost not affects the azimuthal AVO/AVAz characters of gas saturated. When cracks water saturated, along with the permeability increasing, the attributes of P, G and fluctuate scope of AVAz curves reduce. When coal bed is ORT coal, the distribution and fluid infilling of two group cracks decide the characters of azimuthal AVO/AVAz. When coal bed's roof is VTI medium composed with mudstone and sandstone, its P, G attributes of P-wave azimuthal AVO are situated between mudstone roof's value and sandstone roof's value; and its normalized AVAz scope is similarly situated between mudstone roof's scope and sandstone roof's scope, but it is more close to mudstone roof's scope. Finally, through the introduction of a physical modeling, has studied the influence of survey layout on the interpretation of cracked anisotropy, and has given the flow charts of anisotropic processing and interpretation.In brief, through analyzing reflected P-wave AVO's characters on coal roof/floor or thin coal beds, coal bed's lithology could be determined; through studying the velocity anisotropy and azimuthal AVO/AVAz characters of tectonic coal, the degree of tectonic damage and the development of crack distribution could be confirmed too.