A Cost Effective Technique for Production Well Testing
Authors: Nerby G. , Saether G. , Velle O.J. 
Published: October 2008

1 INTRODUCTION
In connection with subsea development of hydrocarbon reservoirs, selection of method for production well testing is one of several key issues. This article describes a cost efllcient technique for testing of production wells. The presented method utilizes well monitoring information interfaced to three-phase mechanistic and thermal models allowingcontinuous on-line calculation of well and transportline phase rates.
The presented computer system is named IDUN. It is a user friendly application running on standard UNIX workstations. IDUN is installed at Saga Petroleum's Tordis filed. IDUN is under preparation for Norsk Hydro's Troll Olje and Saga Petroleum's Vigdis field in the North Sea.

2 BACKGROUND
There are few test alternatives available for production wells. The procedures are based on testline, deduction testing and subsea multiphase metering. These methods are briefly described below.

2.1 Testline
Single testlines or testlines combined with production Lime permits sequent at lasting of well sonar regular basis.
A complex arrangement of operated subsea valves for rerouting of individual production wells is required, Long transport distance requires good heat insulation and/or large methanol consumption toensure safe operational conditions. The total cost of the required subsea facilities is dependent on the field location and configuration. However, a typical figure of welltest-costs may be around 2 MNOK/km per well.

2.2 Deduction Testing
Deduction testing is selected for several field developments, such as in Snore, Statfjord satellites and Tordis prior to IDUN installation. The procedure uses the production flowline(s) for welltest procedures. Initially, the total production from one flow line is measured. Subsequently, one well is shut down, while maintaining the inlet subsea manifold pressure at the originally value. After stabilizing the production conditions, the difference in produced rate reflects the contribution from the throttled or shut-in well.
This method allows the subsea production system to be kept to a minimum of components. However, the method introduces limited accuracy and introduces reduced production regularity.

2.3 Three-phase Meters
Several metering principles are available for topside and onshore production facilities. The reliability of subsea multiphase metering units have not been validated to date. No established cost figure is available for minimized multiphase meters, but 4MNOK per unit is anticipated as a realistic cost.
From an economical point of view, three phase meters are not attractive alternatives for well testing if reliability cannot be confirmed.

3 IDUN description
IDUN takes advantage of available sensor signals in wells, subsea pipe work and topside facilities. The goal of the IDUN system is to carry out comprehensive interpretation of sensor measurements and auxiliary process information by transforming analog or digital signals into measurable flow rates of oil, water and gas. IDUN calculations maybe applied on single or subsea commingled production systems.
The production rates of oil gas and water per well are defined as free and conditionally independent variables. The indivdual phase flow rates in the wells stashing the multiple measurable quantities represent the flow conditions at a given time. Several factors representing key-parameters for fluid mechanics, fluid-properties.