Abstract

This paper presents a field case based upon a reservoir operated by Statoil in the Norwegian Sea. The case concerns a series of water injectors - i.e. both sub-sea and platform - that underwent extreme losses of injectivity over short periods of time. When worked over, the wells showed extreme amounts of sand fill that sometimes were several hundreds of meters above the top perforation. Furthermore, the link between well shut-ins and injectivity losses was clear right from the onset of the study.

The life of the injectors is thoroughly reviewed and the reasons for the injectivity losses are established.

  1. First, it is shown that even under no flow conditions corresponding to shut in periods, the rock around the wells is too weak to sustain the stresses and fails.

  2. Second, it is established that because of permeability heterogeneity, the wells are cross flowing during shut in periods hence allowing sand to be produced in front of the perforated interval.

  3. Third it is shown that under routine operation conditions, the produced particles in front of the perforated intervals are not able to settle down in the rat-hole before injection restarts and hence plug the perforation tunnel upon injection restart.

  4. Finally, it is demonstrated that during a standard shut in, pressure waves as large as 80 bars are generated because of the so-called water hammer effect that hits the formations as would a seismic wave do. As a consequence, the formation already weakened by sand production undergoes liquefaction that triggers large amounts of sand to be released in the well, hence killing totally its injectivity.

Finally, the paper presents how the operating conditions of the wells were successfully changed to avoid the repetition of the problems experienced previously.

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