| Popis: |
The Alpha field is located offshore of East Malaysia and operated by PETRONAS. Alpha-C development wells were often completed with the wire-wrap screens (WWS) for sand control over multiple zones separated by packers. This completion design was deployed in more than 20 wells which include the oil or gas producers and water injectors. Some producers were converted to injectors for reservoir pressure maintenance with several attempts to convert producers to injectors hampered by poor or no injectivity. Remedial efforts to restore injectivity, including stimulation, proved unsuccessful. This prompted the Alpha production team to look for a way to investigate the possible causes of inadequate injectivity, without the expense of retrieving the completion string. PETRONAS suspected that fines migration was either plugging the outer WWS or filling the annulus between the screen and the tubing, thus reducing the injectivity. A series of pulsed-neutron logs were recorded in five (5) problematic wells. The objective was to determine if fines migration was the cause of the blockage, using silicon activation and neutron-gamma ray spectroscopy. Silicon activation, also known as Gravel Pack (GP) logging is a technique traditionally used to evaluate the quality of the gravel distribution in a gravel pack. The measurement is sensitive to silicon around the borehole, hence a technique was adapted to detect sand blockage in the wellbore by logging the pulsed neutron tool inside the tubing. The technique is sensitive to silicon both outside the screen and inside the annulus between the screen and tubing. The inelastic and capture spectra measured by the near and far detectors of the tool were used to derive further details on the sand distribution in the wellbore. The difference in depth of individual measurements, with GP logging "seeing" deeper than the inelastic and capture spectra, has allowed for varying degree of sand buildup outside and inside the screen. The proposed method has helped to detect sand blockage around the wellbore, computing the height of the sand fill and inferring the depth of any damage to the screen. This paper discusses the innovative application of pulsed-neutron inelastic and capture spectra for detecting sand fill. The results successfully identified the location of the sand blockage in all five (5) wells. This has helped PETRONAS to decide on the ideal way to remove the sand blockage in the borehole before performing the next injectivity test. The remedial plans are to focus on removing the sand fill, rather than trying to treat plugged screens. This will include attempting to perform sand clean out operations (via coiled tubing), selectively performing tubing punches or opening sliding sleeves as appropriate and circulating out as much of the sand as possible. Obviously, the restoration of the injectivity rates will be highly dependent on the success of these attempts. |
