4.8 The Grane PRM Project Guest Contributor: Rigmor Elde


Sometimes one sees things clearly years afterwards than one could possibly at the time. Agatha Christie, 'The Mysterious Mr. Quin'


Te main objective of 4D seismic monitoring at the Grane field is to increase oil recovery. Te PRM system was installed in order to better understand the dynamic behaviour of the reservoir by getting more frequent and fresher 4D seismic with even better quality than could be acquired using towed streamers. Additional objectives such as the monitoring of overburden integrity around shallow injectors can be considered a benefit of PRM technology (Roy et al., 2015). Improved repeatability between surveys and fast turnaround


for seismic data processing are important success criteria when using PRM technology. In the start-up phase these criteria have fulfilled expectations. Preliminary measurements in a window right above the reservoir zone show a reduction of 50% in NRMS (Normalised Root Mean Square error), from ~24% for streamer vs streamer to ~12% for PRM vs PRM. By the autumn of 2017 seven PRM surveys had been


acquired on Grane and production effects observed confirm the usefulness of frequently acquiring high quality 4D seismic data. Below we give two different examples of the application and value of 3D and 4D PRM seismic data respectively.


4.8.1 Better Imaging Within Platform Shadow Area


Data quality in the platform shadow area has been a problem with towed streamer acquisitions but this has improved with PRM datasets. Te monitoring of shallow waste injectors is part of the


scope of PRM, in this case looking for ‘out of zone injection’ and potential leakages to the seafloor. When the initial preliminary fast track data from the first PRM survey arrived in February 2015, the overburden was scanned for potential differences in the seismic signature that could relate to production/injection changes. Te overburden around a specific water injector was


Acknowledgement: Te authors thank the Grane licence partners for permission to publish this section, and the many persons who have contributed to this work. Te views and opinions expressed here are those of the operator and are not necessarily shared by the licence partners.


of particular interest and concern, as this well has had a very complex injection history. Previous 4D streamer seismic surveys (2005, 2007, 2009,


2011 and 2013) had not been able to monitor the area properly because of missing or highly uncertain data, as they were undertaken very close to the Grane Platform (i.e. in the platform shadow area) where towed streamer seismic data is difficult to acquire and image. PRM seismic data, however, is less affected by the platform shadow since the seismic stations are situated closer to the structure, and yield better results near the platform than the corresponding streamer data. During the first PRM survey (PRM0), in autumn 2014, a


pronounced depression in the Top Utsira reflector, about 75m in diameter and 10ms in vertical extent, was observed (Figure 4.64) just above the injection point of the shallow injector. On the seismic data acquired before the start of production (1993) the Top Utsira reflector did not show this depression. Based on this observation on the PRM data, together with the injection history, it was decided to stop water injection in this well. A detailed seabed survey concluded that there was no leakage to the seabed, though due to uncertainty about the nature of the overburden damage, and how it might develop if injection was continued, it was decided to keep the well shut in. In order to explain what had happened in the overburden


around this injector a seismic modelling project was undertaken, the aim of which was to test the most likely scenarios leading to the overburden change.


4.8.2 PRM After a Few Days of Gas Injection


Te value of ‘fresh’ 4D data was demonstrated by monitoring the gas expansion resulting from a newly drilled gas injector, drilled shortly before of the second PRM survey, undertaken


Figure 4.64: (a): Close-up of the injector (white line) on the Top Utsira TWT map. The estimated diameter (~75m) of the depression and the distance to the platform (~450m) are superimposed. (b): PRM0 seismic data along the shallow injector well path. The Top Utsira reflector interpretation is shown in yellow, and the area of the anomalous depression is outlined with a question mark. Note the injection point just below the Top Utsira depression.


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