Production Technology

Statoil to boost recovery with subsea production system

Using subsea separation, de-sanding and injection, Statoil expects to reach 55% recovery and generate 35 million bbl of additional oil.

Alexandra Pruner, Publisher, and Perry A. Fischer, Editor

Located in Block 34/7 of the Tampen area in the Norwegian North Sea, Tordis field will soon achieve the distinction of having the industry's first full-scale commercial subsea processing system in operation. This Statoil-operated field lies in 650-ft water depths, and the new subsea system will extend the field's life about five years to 2020. By fulfilling Statoil's goal of improving oil recovery to 55% from present 49% levels, Statoil and partners are expected to gain 1.2 billion bbl of oil across all its subsea fields. Tordis will add an incremental 35 million bbl of oil. The new system is scheduled to be completed during 2007.

BACKGROUND

Tordis field comprises Tordis East, Tordis Southeast and Borg reservoirs. Discovered in 1984, production began in 1994 with seven subsea-completed satellite wells connected to a manifold. Six of the wells are protected by a conventional steel framework, and one water injection well is in a silo sunk into the seabed.

The manifold had additional capacity, and two, four-slot templates were later connected: the J-template (production) and the K-template (injection). Production from J-template started in 1998 (Tordis East), and injection through K-template started in 2000 (Borg). Produced liquids go through two 10-km pipelines to the Gullfaks C platform for processing, Fig. 1.

Fig. 1. Tordis field layout (not to scale).

Now in its twelfth year of production, 270 million bbl of oil have been produced from Tordis. The field now has nine producing wells and six injectors. Initially, the producers flowed at rates of up to 24,000 bopd, but decline has reduced average well production to 12 – 13,000 bopd, with water injection used to maintain reservoir pressure. Operated by Statoil (28.22% working interest), Tordis' other working-interest partners include Petoro (serving as the state's direct financial interest, 30%), Norsk Hydro (13.28%), ExxonMobil E&P Norway AS (10.5%), Idemitsu (9.6%), TOTAL (5.6%) and RWE Dea Norge (2.8).

THE IOR PLAN

In late 2005, Statoil filed an amended plan for Improved Oil Recovery (IOR) that entails a two-phase project costing an estimated $285 million. The IOR plan will improve recovery by: reducing wellhead pressure to increase production; increasing transport capacity through subsea separation of water and sand from the wellstream; injecting produced water and sand into a subsurface formation; installing multiphase pumps to expand transport capacity; and maintaining regular production with retrievable, modular, components. Phase one of the plan will convert Gullfaks, in late 2006, to low-pressure production and is expected to recover about 16 million bbl of additional oil. The second phase will entail the installation, in 2007, of a subsea separation boosting and injection station, which is expected to recover an additional 19 million bbl of oil.

This planned switch to a low-pressure environment will also result in increased water production of up to 70%. Scheduled for installation in late 2006, a new Pipeline Inline Manifold (PLIM) was delivered to Statoil in July to allow for the interconnections of the flowline from the Tordis Subsea Manifold to the Gullpaks C platform, Fig.1.

Phase two of the IOR project will entail installation of a Subsea Separation Boosting and Injection (SSBI) station, and a water-injection tree. This system will be provided by Kongsberg FMC.

THE TECHNOLOGY

The unprecedented subsea system will entail separating produced water and sand from the wellstream and then injecting the water and sand into a subsurface formation for disposal, with gas and oil flowing to the Gallfaks C platform. The foundation structure will support the SSBI and its component parts, including the manifold, separation module, sand removal system, pumps and others. The SSBI station will weigh about 900 t, and the foundation structure will feature four corner-based suction anchors, allowing for leveling.

The SSBI will have one control module with 51 distinct functions and is expected to have a processing capacity of about 200,000 bpd. The untreated produced water will be reinjected into a separate well, reducing backpressure at the Gullfaks platform and thus allowing more hydrocarbons to be processed. The separator and sand-management system was part of a DEMO 2000/ Statoil-funded project carried out by FMC Technologies and CDS Engineering.

Referring to Fig. 2, well fluid will be routed initially through the SSBI's separation module. The separator contains an inlet cyclone gas knock-out, diverting most of the gas into a pipe outside the tank, minimizing the required size of the separation tank. Gravity-based separation partitions the remaining gas, water, oil and sand in the tank.

Fig. 2. Schematic of subsea processing module.

Within the separator, a sand removal system entails a flushing system, with specially designed nozzles to wash out lingering sand at regular intervals. Sand is then transported into the de-sander, batch-wise. Periodically, the de-sander empties the sand by creating a slurry with water, which is then pumped by the WIP into the injection well to the Utsira reservoir – a saline aquifer at ambient pressure. The water injection tree will feature a 12-in. ball valve for isolation and a 13-5/8-in. casing for volume. Sand volume design is 1,100 lb (500 kg) per day, with water production designed for up to 150,000 bbl (24,000 m³) per day and oil production up to 75,000 bbl (12,000 m³).¹

The remaining oil and gas in the separator tank will be remixed and pumped via a Multiphase Pump (MPP) to the Gullfaks C platform.

The pumps, including the water injection pump and the multiphase pump, will be powered via an HV electrical power cable from the Gullfaks C platform. The pumps can be retrieved for maintenance via a separate pump-running tool. The SSBI will also be equipped with two multiphase flowmeters to measure the composition of the well flow and adjust the separation system settings – given the make up of the water, oil and gas – and set the pump speeds accordingly.

Ensuring maximum operational flexibility and maintenance reliability are of paramount importance. The modular design of the SSBI will facilitate timely and efficient retrieval for maintenance. Process contingencies such as mixing sand with the oil for processing at the platform rather than reinjection, also provide alternatives for running the system.

In addition to gaining incremental oil recovery, the SSBI system means that additional water-handling facilities will not have to be built on the space-constrained Gullfaks platform. Further, this subsea system offers an environmentally benign solution for processing, fulfilling government-mandated zero discharge requirements. The overtrawlable design of the foundation structure will also ensure local fishing is not disrupted. Although the Tordis field is the first planned installation of the SSBI system, future applications include potential deepwater applications as well as boosting production and recovery in other offshore environmentally sensitive areas. 

LITERATURE CITED

¹ Fantoft, F and T. Hendriks, J. Elde, "Technology qualification for the Tordis subsea separation system, boosting and injection system," Paper OTC 17981, OTC, Houston, 1 – 4, May 2006.