A downhole Splitter wellhead system was successfully introduced and field tested offshore in the Valhall Field, an upper Cretaceous chalk oil reservoir in the south end of the Norwegian sector of the North Sea (Figure 1) operated by Amoco Norway Oil Company on behalf of Amerada Hess Norge A/S, Elf Petroleum Norge A/S, and Enterprise Oil Norge Ltd. This is the first Splitter well in the North Sea area, and 3rd in the world. Extensive pre-planning and equipment function testing has been performed to meet the challenges posed by the application of this technology. The Splitter system is used in an innovative approach to drill, case and complete two independent wells in one wellhead housing. Each well can be operated, serviced and worked over as a complete separate unit.
The 3 objectives with this project were:Test of new technology for possible better use of slots in a slot restrained environment.Explore near Valhall exploration area (Mjød Field)Provide a long term waste disposal solution for Valhall
Due to hole stability problems, the second objective was canceled during the operation. Instead, a preplanned fallback production location on the Valhall field was drilled. Objectives 1 and 3 were successfully completed July 1997. One well is a 4700 m MD horizontal producer (850 m horizontal section) to the North of the Valhall Field. The second well is a high angle (68 deg) well with a long section of slim hole drilling prior to completion of a dedicated waste injection well, capable of taking roughly 4 MM bbls of waste at approximately 2200 m TVD in the overburden at the Valhall Field. Figure 2 shows a map of planned vs actual well locations.
Introduction
In January 1993 the Norwegian State Pollution Control Agency (SFT) reduced the allowable oil content on cuttings disposed to sea from 6% to 1%, a limit not achievable at the Valhall field with current technology, where there in addition to drilling waste also is generated large volumes of production waste (oily chalk and water). With cooperation from SFT, Amoco Norway and Amoco Production Research developed a unique through-tubing cuttings injection method in a dedicated injection well, capable of handling very large volumes of waste. This solution was successfully implemented at Valhall in January 1991.
However, in the slot restrained environment at the Valhall platform, with a low-permeability reservoir, the cost for a non producing slot is high, and the Downhole Splitter wellhead system was identified as a potential way of optimizing slot use. The old waste injection well would be plugged and abandoned, the slot would be reclaimed, and a splitted slot installed. Another argument for implementing a new waste injection solution was the fact that the existing waste injection leg had several deformations in the wellbore above the reservoir, and there was some uncertainty with respect to future lifetime for this well.
Mjød, an identified exploration opportunity some 4 kilometers south-east of the Valhall platform (Figure 2), had already been discarded due to too high dry hole cost. However, if the Splitter concept was used, a potential dry hole could be utilized for waste injection purposes. So although drilling the exploration well from an offset platform increased the drilling difficulties somewhat, it reduced the potential dry hole cost by as much as 90%.
