Statoil ASA, Norway

Seismic diffractions have unique properties compared to reflections beyond potential in getting higher resolution. Through four different cases with different challenges and geological setting, we show the benefit from using diffractions to complement the information in the reflections, providing business critical geological information beyond what is possible with reflection-based imaging alone. Diffractions are found in standard seismic data, but are usually weak compared to reflections. Despite this, one of the examples shows improved detailed imaging of a carbonate reservoir from diffractions beneath several kilometers of salt and suggests such methods have a larger range of applications than what has been believed.

Presentation Date: Thursday, October 18, 2018

Start Time: 8:30:00 AM

Location: 204C (Anaheim Convention Center)

Presentation Type: Oral

Important properties are the pressure drop for high rates and liquid accumulation at low rates which contribute to determining the operational envelope of the field. The liquid content and pressure drop are important not only at steady operating conditions, but also during transient operations such as those caused by changes in flow rate or outlet pressure. The arrival time and the volume of the liquid surges following ramp-up may be critical factors when designing liquid-receiving and separation facilities and creating operational guidelines. Equally important is the behaviour during liquid accumulation, both correct prediction of the production rate where the liquid begins to accumulate and how long it takes for the different segments of the well/pipeline to accumulate.

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.

ABSTRACT

In subsea processing systems multiphase cooling is required to meet a variety of process needs and flow assurance challenges. Required functionality might include enhanced pipeline corrosion protection, pipeline temperature control, improved process efficiency, compressor anti-surge cooling, gas dehydration and hydrate formation control. The multiphase cooler is an integral building block of the subsea processing station and is now, after the first successful year of operation on the Åsgard Subsea Compression station, a proven technology with a solid track record.

This paper will present the development and qualification of the Åsgard Subsea Compression Station Inlet Cooler. In addition, operational experience and performance data from the first year in service will be analysed and compared to previously qualified CFD tools. The paper will furthermore focus on the multiphase tests carried out during project execution to qualify the liquid distribution philosophy undertaken to guarantee liquid distribution in the cooler inlet header. Significant effort was put into ensuring sufficient MEG distribution to all cooling pipes to avoid hydrate growth under prolonged operation within the hydrate formation envelope. Three phase testing was performed validating the CFD methods used to determine the functionality of the subsea product.

INTRODUCTION

The Åsgard Subsea Compression Station is located about 200 kilometres off the west coast of mid-Norway on the sea floor at a water depth of 260 meters. The station consists of two identical compressor trains receiving multiphase wellstream from the Midgard and Mikkel fields and producing towards the Åsgard B platform. The main purpose of the compressor station is to maintain production above the minimum flow requirement for the pipelines to avoid slugging and thereby increasing the field output by 280 million barrels of oil equivalents.

ABSTRACT

We consider the apparent roughness and increased pressure drop associated with the presence of a thin liquid film between the gas and the pipe wall in a two- or three-phase separated gas-liquid flow. The main objective is to improve the pressure drop predictions for near-horizontal gas-condensate flows with low liquid loading. However, in this paper, we focus on vertical (fully symmetric) annular flow to isolate the effect of the liquid film. To support the model development, SINTEF conducted experiments in a 4-inch ID 50 m-high riser at the Tiller test facility in Norway. The data revealed interesting and unexpected phenomena for high water fractions. Nevertheless, a new model for the film roughness based on dimensional analysis and simple but fundamental physics is able to give results in very good agreement with the data. The new model also provides a robust estimate of liquid entrainment. All liquid in excess of that which can flow in the liquid film is entrained into the gas phase through the action of interfacial turbulence.

INTRODUCTION

The Tanzania Gas Project aims to exploit reserves located offshore from Tanzania in East Africa. The narrow operational envelope associated with the extreme water depth underlines the importance of accurate flow simulations for design and production. A large data set was sampled at the Tiller high-pressure test facility in Trondheim, Norway in 2013 and 2014, to support the modelling of liquid accumulation in the Tanzania field (Holm (1); Kjølaas et al. (2); Biberg et al. (3); Staff et al. (4); Nossen et al. (5)).