Field Applications of Idun Production Measurement System
Authors: Berg Kjartan , Davalath Janardhan 
Published: October 2008

The Idun Production Measurement System provides 3-phase rate estimation in wells, based on stationary measurements along the flow trajectory. It provides functionality comparable to multiphase flow meters and additional capability for production optimisation. Idun has been in operation offshore for seven years. Initial installations used pressure and temperature measurements only, and did not have any redundancy for sensor fall-out. As a consequence, when downhole gauges failed, the Idun system failed as well, and had to be run as gas/liquid or even liquid measurement systems only (2-phase or even 1 phase, respectively). The reliability of the Idun system for 3-phase rate estimation became the same as the reliability of the downhole gauges.

In parallel with the continuous development of more reliable downhole sensors, the Idun System has been improved to utilise all measurements in the production system that can be modelled. This allows for installation of additional sensors subsea instead of downhole, making the Idun measurement system much more robust and reliable. Since the analysis of the signals is performed in a central unit, the measurement itself can be simple, hence focus can be on accuracy and reliability, and low cost of replacement, etc. In summary, use of the Idun system would result in higher availability at a lower lifecycle cost if Idun relies primarily on seabed sensors.

A major benefit with an integrated production measurement system that uses the available single point measurements from the oil or gas field is that the measured rates from the wells are internally consistent. In this way, the rates are consistent with all measurements in the production system, or knowledge of drifting sensors is established during analysis. Based on good knowledge of production status, production optimisation can be more focused, and the operator can ensure that the reservoir and production system is operated to its full potential. In a case study, the benefit from such an optimisation using gas lift is illustrated. The paper will also present a summary of the results from recent field applications of the Idun measurement system in the operation of subsea tieback projects.

Introduction

Idun is an operational support system for online or offline monitoring and rate calculation, production control and optimisation. It has been installed on offshore installations since 1995. The basis of Idun is the Idun Production Allocation (IdunPA) module, which is a software based multiphase metering program. In this module, the state of the production system is established. Based on this knowledge, other modules deal with production simulation, optimisation and production choke valve setpoint control.

IdunPA calculates three-phase oil, gas, and water rates based on interpretation of stationary measurements, many of which are already available in the production system, see Figure 1. Typical measurements are pressure and temperature measurements at various points along the well flow path. IdunPA works by guessing a phase split, calculating the response from the production system, comparing the calculated properties with the measured properties at the gauges; and iterating on the phase split for the best match with the measurements, as illustrated in Figure 2.