Sand production on water injectors: How bad can it get?

In this paper we present a field case based upon a reservoir operated by St atoil in the Norwegian Sea. The case concerns a series of water injectors-i .e., both subsea and platform-that underwent extreme losses of injectivity over short periods of time. When worked over, the wells showed extreme amou nts of sand till that sometimes were several hundreds of meters above the t op perforation. The link between well shut-ins and injectivity losses was c lear right from the onset of the study. The life of the injectors is thoroughly reviewed and the reasons for the in jectivity losses are established. 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. Second, it is established that, because of permeability heterogeneity, the wells crossflow during shut-in periods, hence allowing sand to be produced in front of the perforated interval. Third, it is shown that under routine operation conditions the particles pr oduced in front of the perforated intervals are not able to settle in the r athole before injection restarts and hence plug the perforation tunnel upon injection restart. Finally, it is demonstrated that, during a standard shut-in, pressure waves as large as 80 bar are generated because of the so-called water hammer eff ect that hits the formations as a seismic wave would do. As a consequence, the formation already weakened by sand production undergoes liquefaction th at triggers large amounts of sand to be released in the well, thus killing totally its injectivity. Finally, we present how the operating conditions of the wells were successf ully changed to avoid repetition of the problems experienced previously.