Abstract
The Statfjord Formation (Fm.) in Gullfaks Sør (GF Sør) was discovered in 1978, and set in production in 1999 as part of the Gullfaks Satellite Development.
The STOOIP for the GF Sør Statfjord Fm. was estimated to 35 mill. Sm3 in the Plan for Development and Operations (PDO), with estimated recoverable reserves of 12.6 mill. Sm3 through an 8-slot subsea development and partial pressure support by gas injection. The start up of the first two wells showed that the production rate were lower than expected, due to severe lack of reservoir communication, hence influx rates were insufficient to maintain planned offtake rates of 1000-2000 Sm3/sd. The GF Sør Statfjord Fm. is a fluvial to shallow marine reservoir with large reservoir heterogeneities. There is also structural complexibility, which limits the reservoir communication. Further development of the Statfjord Fm. was therefore postponed after completion of the first two wells due to lack of reservoir understanding and need for a revised development strategy. The third well in the GF Sør Statfjord Fm. was drilled in 2001. The well experienced gas breakthrough after a short time. Based on the gained production experience the reserves were reduced from 12,6 mill Sm3 in the PDO to 3,0 mill Sm3 in the reservoir management plan.
The main focus for the GF Sør Statfjord Fm. since 2002 has been to qualify the use of smart well technology to increase the recovery. Since the production has been limited by reservoir heterogeneity and gas breakthrough, surface operated down hole inflow control and multilateral wells were identified as possible solutions for future wells.
This paper describes the solutions made for the fourth horizontal subsea well drilled in the GF Sør Statfjord Fm. late 2003, where a two zones surface controlled inflow device system (DIACS) was installed. The paper also addresses the need for cheaper well and drilling solution from floating drilling units, to maximize the number of drainage points in reservoirs with limited economical reserves.
