Conventional nuclear fluid density gravelpack evaluation tool (GPPT) was found to cause misinterpretation when run in well containing fluid of different densities. As a result an alternative tool called silicon activation gravelpack tool (SGPT) was field tested. The principle of the tool is based on detection of activated gamma ray which is produced when silicon atoms are made unstable by striking with fast neutrons.
The trials were carried out in 3 wells in Bokor. In order to get conclusive results, GPPT and PLT were also run in these wells and the results compared. It was found that in wells with single phase fluid, the SGPT results were in agreement with the GPPT'S. When logging from water to oil, both tools were affected but SGPT was less affected than GPPT.
The effect of water on GPPT was almost the same as the effect due to top of gravel. For SGPT, the effect due to gravel was more pronounced compared to effect due to water. When logging from oil to gas, SGPT didn't show any shift in readings whereas the GPPT was badly affected.
In conclusion, SGPT was found to be more reliable than GPPT for evaluating gravelpacks when there were more than one fluid densities in the wellbore.
Bokor field is situated 45 km offshore Sarawak, North West of Miri. The stratigraphy of Bokor consists of a series of sand and shales, stacked on top of one another, deposited in a shallow marine to fluviomarine environment. There are 73 oil bearing reservoirs with depth between 1500 to 5950 feet subsea. The individual reservoirs are between 5 feet and 130 feet thick. In view of the large number of relatively thin sand with similar pressures, the wells were completed on sand groups rather than individual sand units.
Development in Bokor field started in 1982 and currently there are 40 producing wells. All of these wells, except one are completed as dual string and due to the unconsolidated nature of the formation, all the intervals are gravelpacked, Typical completion of Bokor well is shown in Figure 1.