Using time lapse seismic to make a reservoir beneath dirty salt visible
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Imaging salt bodies in the Nordkapp Basin has shown to be a very challenging task. Due to this, it has not been possible to produce from the reservoirs trapped beneath the salt anks. Dirt within the salt bodies is assumed to cause diractions, which disturbs the im-age signicantly and creates a seismic shadow zone beneath the salt.This hypothesis is mainly based on petrophysical data acquired in the Uranus well drilled in 2005 and previous research. In this Master's thesis in Petroleum Geophysics, the hypothesis regarding dirt and diractions is examined, by attempting to recreate the noise observed on real data. Previous work has shown that it is not possible to reproduce the poor seismic quality, by adding a realistic amount of dirt in the 2D plane. However, adding diffractions out of the plane could be the solution. Finite Difference modeling of a salt body with anhydrite inclusionsand additional dirt is performed, and diffractions both within and out of the 2D plane are added. By doing this, it has shown possible to recreate the noise and seismic shadow zone observed on real data. This concludes that dirt within the salt body could cause the poor seismic quality observed in this area, and that out-of-plane diffractions mustbe considered when doing 2D modeling. With this new knowledge of what makes the imaging of the salt so difficult, it is easier to find a way to remove some of the noise and get better images of the reservoirs. In this research a method using time lapse seismic to remove some of the noise is investigated. This is done by modeling and processing both cases with and without hydrocarbons, which will represent a base and a monitor case. To simulate acquisition differences between thetwo cases, which often occur, an error of 12.5 m in receiver positioning is constructed. In addition non-repeatable random noise is added. By subtracting these two cases, constant geology and repeatable noise will be removed, and it was expected that the reservoirs would become more visible. The results show that in cases with both diffractions anda 12.5 m receiver positioning error, the 4D differencing did not make the reservoirs appear. However, by either removing the diffractions or with an ideal seismic acquisition, the method shows positive results.This concludes that for the data acquired in the Nordkapp Basin,where the amount of diractions is distinct, the acquisition accuracy must be high in order to see reservoir changes when doing time lapse seismic. Examining how accurate the acquisition must be is suggested as future work. To reduce the impact of acquisition differences, a match filter was designed and applied to the datasets. A match filter forces the amplitude and phase of one dataset to match those of another, so when subtracting the two cases some of the non-repeatable noise will cancel out. However, the effect of match lters has shown poor results in thiscase. Match filters will probably work best in cases with a constant change in time -or phase shift, but when there is a spatial shift there will be no consistent change in the wavelet spectrum from trace to trace. This is probably the cause of the poor results. In addition, the diffractions were included in the design window, which could have a negative eect on the lter, since it can be very sensitive to the presence of noise.