Planning and Executing Long Distance Subsea Tie-Back Oil Well Testing: Lessons Learned
Authors: Harun Amrin Fadila 
Published: February 2016

Abstract
Three subsea oil wells in a water depth ranging between 5200 and 5400 ft in the Gulf of Mexico are tied back to a Tension Leg Platform via a flowline network of a single 2.3 miles pipe-in-pipe connected to a 17 miles dual active-heating flowline. During normal operation these wells flow to a common separator at the topsides. Production allocation to each well is done by subtracting the topsides separator measurement by the subsea multiphase flow meter reading dedicated to one of the wells and manually splitted between the other two wells. When the topsides separator started showing some water production, more rigorous well testing is required to know where the water comes from. The information is crucial for the depletion plan of the field. Since well testing would cause some production deferment, its objectives were expanded to include subsea pigging valve integrity testing, subsea multiphase flow meter validation, and rigorous fluid sampling. The program was executed smoothly and met all its objectives. The formation water was identified from the unexpected well; thus suggested re-visiting the reservoir model. Although not predicted by the transient simulation works, slugging was experienced when the weakest well was producing by itself but it was managed through topsides choking. The key success for the smooth operation was a well-thought plan developed by a multi discipline team and the guidelines based on transient simulation works.

Introduction
King and King West oil fields are 100% BP operated and located in the Gulf of Mexico (GoM). The fields consist of three subsea wells, i.e. King D5 and D6 wells and King West D3 well located in a water depth ranging between 5200 and 5400 ft. They are tied back to the Marlin Tension Leg Platform (TLP) located in 3200 ft water depth. D3 well is connected to D5 well through a 2.3 miles, 6 by 10 inches pipe-in-pipe flowline. D5 and D6 wells are then connected to the TLP via a 17 miles, 8 by 12 inches dual active heating flowlines (Figure 1). The flowlines connecting D5 and D6 wells to the TLP are designated as the West and East flowlines, respectively. A remotely operated subsea pigging valve located close to D6 well is normally open. D5 and D6 wells are producing from the same reservoir while D3 well is producing from another reservoir. For reservoir management purposes, D3 well is equipped with a subsea multiphase flow meter (SS MPFM) located in a jumper between the wellhead and the flowline.
Fig. 1- King/King West Field Layout
Marlin TLP processes production stream coming from 3 dry tree wells (two oil wells and one gas well) and 5 subsea wells (three oil wells and two gas wells). The well fluids go to three three-phase separator (HP, Test, and IP) set at the same pressure (Figure 2). The oil flows through the LP separator and the gas goes to the compression system before they both going to the export system. The produced water is treated and dumped overboard. D3, D5, and D6 wells are normally flow to the IP separator. However, the topsides piping and manifold systems allow any well to flow to any separator providing it does not upset the plant, mainly the water treatment process