Quantitative Gas Hazard Analysis: Present Limitations and the Way Forward
The analysis of high resolution seismic data for shallow gas has made significant advances in recent years but there still remain major limitations in the ability to positively identify gas, let alone be able to accurately estimate concentration or pressure. A number of features which are commonly related to the existance of gas have been identified and when two or more such features are present this is commonly taken as being a positive gas indication and a probability of gas presence is often estimated. Such probability estimates are largely arbitrary (often based primarily on lateral amplitude variations) and the accuracy of such estimates is dependent on the skill of the analyst and knowledge of local geology. This paper discusses a series of techniques which should lead to more reliable estimates.
There are several ways in which gas identification could be improved given our current knowledge: more careful processing of site survey data including better amplitude control in the shallow section, detailed velocity analyses in the vicinity of well locations and migration of key lines; use of extra analysis techniques such as residual wavefield and forward modelling for Amplitude Versus Offset; use of numerical analysis rather than qualitative estimation.
Implementation of some or all of the above techniques enables a better estimate of the probability of gas presence to be made but there still remains uncertainty especially in areas of irregular subsurface relief. It is proposed that in order to ‘calibrate’ gas probability estimates more research is needed relating known occurrence of shallow gas to the presence of gas related attributes in seismic data.
KeywordsInstantaneous Frequency Site Survey Amplitude Versus Offset Offshore Drilling Shallow Section
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