Tanzania gas development - flow assurance challenges

Holm, H. (Statoil ASA, Norway)

OnePetro 

ABSTRACT

The subsea gas development in Block 2 offshore Tanzania described in this paper is characterized by water depths of up to 2600 meters and tie-back distance to shore of around 100 km. The seabed outside East Africa consists of deep, large scale canyons and steep inclinations towards shore. The reservoir fluids contain very little condensate and the pipeline flow is typically low liquid loading conditions at high water fractions. The key focus of the work presented at the previous BHR conference in 2015 was related to liquid accumulation. However, this work also revealed that

  • frictional pressure drop increases significantly with high water fractions,
  • existing flow models severely under predicts frictional pressure drop at high water fractions, and they are not able to predict the effect of water fraction on the frictional pressure drop
  • little experimental data exist for such conditions (low liquid loading, three-phase flow at high superficial gas velocities).

The key focus of these presentations is hence related to frictional pressure drop in low liquid loading at high water fractions.

To support model development and model verification experiments were conducted in a 4-inch ID 50m-high riser at the Tiller test facility in Norway. The data revealed interesting and unexpected phenomena with respect to frictional pressure drop for high water fractions.

Also, as part of value improvement process the Tanzania project has evaluated replacement of the subsea Wet Gas Meters with a Virtual Metering System only. A study was conducted to evaluate the expected accuracy and uncertainties of a model based Virtual Flow Metering system (VFM) for Tanzania specific operating conditions. Reliable prediction of pressure drop is crucial for such a system.

This paper gives an overview of the Tanzania deep water gas development with focus on the flow assurance challenges relating to a potential subsea to beach concept and the background, motivation and high level results from the conducted work, while the “three-phase vertical flow experiments (SINTEF)”, the model development and verification (Schlumberger) and the Virtual Metering study (FMC) are presented in detail in separate papers.