Metrology of a dataset acquired with an optical fiber in a horizontal well
Acquiring a seismic profile data in a well using conventional geophone technology requires sensors to be lowered down the well. Unfortunately it is expensive and it involves the production interruption. The Intelligent Distributed Acoustic Sensing (iDAS) is a known alternative to this dispositive for vertical seismic profiles, with the benefit that it can stay after the acquisition, even while producing. The quality of this type of measurements for reservoir monitoring is studied here.
Methods, Procedures, Process:
In 2014, several lines were shot around a horizontal well in a carbonate area (Al Khalij) with the iDAS technology (walk above and lines in different directions from that of the well, some of them shot several times, some of them while producing, some with the production shut down). First this dataset was used to provide a 2D image of the reservoir below the horizontal part of the optical fiber. It also enabled a comparison of the measurement while producing or not and gave a dataset for repeatability evaluation.
Results, Observations, Conclusions:
Unfortunately, this dataset is not usable as a VSP for the vertical part of the well, due to coupling issues and directivity of the signal. Nevertheless, in the horizontal part of the well, dense spatial sampling opens the door to detect very small details in the reservoir, below the conventional seismic imaging capacities. However, separating the up-going wavefield from the down-going wavefield is challenging. The comparison of the repeated lines shows whether this technology is satisfying in terms of repeatability for 4D surveys or not. It is also possible to measure the difference of the direct and reverse paths which are both recorded. Since the equipment varies along the well, it gives information about the quality of the coupling, for e.g. with or without the packer. Finally, it shows whether it is necessary or not to shut down the production while acquiring the dataset.
It is already known that optical fibers can be used for VSP and reservoir imaging, but this experiment is the perfect dataset to evaluate the quality and to extract all the potential of this type of measurement especially in terms of reservoir monitoring. Thanks to the lessons learnt from this experiment, it will be possible to optimize the acquisition design of future surveys to improve the signal quality.
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