A clastic reservoir in central Oman is to be developed with some 100 wells. The rock is sensitive to formation damage and experience shows that productivity is improved by a factor of 10 by perforating with deep-penetrating charges underbalanced and avoiding contact of brine with the formation. The wells will be horizontal to drain a 24.5m oil rim, and zonal isolation for future water and gas shutoff is desirable.
An interval of 600 meters was perforated with 4 ½" OD guns. The guns were retrieved under pressure and a lower completion packer was run to isolate the reservoir from the surface. The upper completion was run with crude in the hole to ensure that no brine should contact the formation.
To perforate underbalanced with a drilling rig, the well was constructed with a 7" liner at the start of the build section to total depth of the well. A 7" tie-back was run with a surface controlled downhole isolation valve (DHIV) as a barrier after perforating. The well was displaced to crude to create underbalance. A 7" predrilled liner joint was run beneath the DHIV in the work string to allow circulating out well bore fluids so that the guns could be retrieved safely. A rotating control head and underbalanced drilling surface equipment package was utilized for stripping with 8,000 kPa surface pressure bleed off volumes for tripping and manage the crude circulating system. The tie-back was retrieved and the upper completion was run after setting a plugged packer in the 7" liner. This method is unique because it utilized a conventional drilling rig for completion operations along with an underbalanced drilling package to perforate and complete the well underbalanced.
The field was discovered in 1979 by Petroleum Development Oman (PDO) and came on stream in 1981. By 2005, seven oil producers were drilled. The 8th well is the subject of this paper. Oil was discovered in two sandstone reservoirs at depths around 3000m subsea. Despite low porosities (11–15%) high permeability's are encountered (100–500 mD). The sandstone is strong and triaxial tests as well as production experience does not lead to the expectation of a sand problem.
The rock is prone to formation damage and early wells generally produced at low rates. The first horizontal well (W-6) was drilled with oil-based mud (OBM) and produced at an improved rate. The vertical well W-7 was also drilled with OBM and was perforated underbalance with deep-penetrating guns; it achieved 1.5 times the oil production of W-6 without (to date) excessive gas or water production. Similar results were achieved in a workover of W-4 where a new interval was perforated underbalance with deep-penetrating charges.
A field development study was carried out, resulting in a Field Development Plan (FDP) recommending development of the field with 100 horizontal wells. A component of the FDP study was definition of a well completion strategy for the field; the first step in this strategy was testing the completion concept of a horizontal well drilled with OBM, completed with a cemented liner perforated with deep penetrating charges underbalanced. This well, W-8, was drilled in the 3rd quarter of 2006. A production test of 4 times the rate of W-6 was achieved. This is one of several completion types to be evaluated; a second "concept prover" well (W-9) is currently being drilled underbalanced with the intention to complete it barefoot. A comparison of the costs and performance of the various completion trials will lead to concept selection for the main body of production wells to be drilled in the coming years.
The success of underbalanced perforating with deep penetrating charges experienced in W-7 and W-4 (workover) is believed to have been due to avoiding formation damage by not killing the well; this was relatively easy to achieve as these are vertical wells and the short gun sections (approximately 10m) could be dropped into the rat hole. However, applying the technique in a horizontal well does not have the same possibility, and in W-8 special measures to retrieve the 671m long gun assembly without killing the well needed to be taken.