4D Monitoring With Permanent Borehole Seismic Sensors: Progress, Challenges, and The Road Ahead

We examine time-lapse data recorded using wells instrumented with permanent fiber-optic borehole seismic sensors in two separate locations, the Valhall field in the Norwegian North Sea and the Mars field in the deep water Gulf of Mexico. The Valhall surveys showed significant noise in the injection well for long periods after the well was shut-in. Analysis of downhole pressure and temperature data as well as accompanying RMS noise analysis of the borehole seismic data showed that the well was not stabilized even 12 days after the well was shut-in. Analysis of the borehole seismic data showed that the best data was acquired after 10 days or more shut-in time. On the other hand, data acquired from the Mars well for the two surveys in 2007 and 2010 during full production of the well was of acceptable quality and supported imaging of near-borehole structure. In addition, pressure and RMS noise analysis showed that the Mars well stabilized in a much shorter time than the Valhall well, in a matter of 4-5 hours versus an estimated 2 weeks. Imaging results for the Valhall well showed higher-resolution images than seabed OBS results below the tool while timelapse process showed consistent images but little difference. Imaging results for the Mars wells showed superior imaging for key reservoir sands below the borehole when compared to the corresponding 2010 OBS seismic. In particular, interpretation of the 3D VSPs suggests the main reservoir Upper and Lower Yellow sands are present updip to the well, in contrast to the OBS results which hints thinning or possibly pinching out of the Lower Yellow sand. Time-lapse comparison of the two Mars 3D VSPs shows minor differences with current processing.