At whatever location we proposed to drill, the shallow site survey invariably predicted the presence of potential shallow gas accumulations Our credibility with the driliers was very low. We decided to investigate AVO (amplitude versus offset) phenomena in an attempt to discriminate between seismic "bright spots" caused by lithological contrasts and those due to gas-bearing sands Case histories of three North Sea wells are presented with an analysis of how successful pre-drilling predictions have been. The deliberate search for AVO effects has had a major beneficial impact on our approach to shallow gas hazard detection Extensive pre-stack analysis has revealed shortcomings in acquisition and processing, and pointed the way forward. We conclude that AVO analysis is potentially an effective tool in the reliable prediction of shallow gas hazards.
In our experience, there has been a tendency to interpret any high-amplitude seismic reflector as a potential "bright spot" and to infer the presence of shallow gas accumulations on this basis Frequently, several shallow gas zones have been predicted at which appropriate drilling precautions were advised A trend soon emerged that at whatever surface location we proposed to drill a well, the shallow site survey seemed to predict the likelihood of encountering shallow gas Subsequent drilling invariably encountered no shallow gas zones and often, not even an increase in the background gas measured from drilling-mud returns We were in danger of losing all credibility with our drillers.
We present case histories for three North Sea wells and analyse just how successful pre-drilling predictions have been The results of these post-mortem analyses have had a dramatic impact on our approach to acquisition, processing and interpretation of shallow gas hazard seismic data
A high resolution digital seismic survey was carried out across the proposed well location A final site survey report was prepared by the contractor and In it, we read the following
"A number of anomalous high amplitude events were noted on the relative amplitude sections, particularly around the proposed drilling location
These bright spots occur at four levels, loom, 339–405m, 500–560m and 800–880m below the seabed respectively Elsewhere in the survey area occasional bright spots are found at these levels and at an intermediate level, 235–283 metres below seabed The uppermost bright spot level (100m) occurs within an acoustically transparent sequence (probably clay) and may represent a very strong lithological contrast (clay/gravel) or a shallow gas pocket
The lower four bright spot levels are associated with continuous medium-high amplitude reflect or sand are more likely to represent lithological contrasts, although due to their anomalous amplitude appropriate caution should be exercised " In conclusion, our contractor stated "Shallow gas may be encountered at 100m, 339–405, 500–560m and 800–880m and therefore precautions should be taken when drilling at these levels"
These conclusions predict a cumulative shallow gas hazard thickness of ca 220m, some 30% of the 26" drilling phase. As a direct result of these shallow gas predictions, a 12%" pilot hole was drilled with a MWD (measurement while drilling) gamma ray/resistivity tool before the hole was opened out to 26" diameter.
