Subsea Who Dat Project - Producing Light and Heavy Oil in a Deepwater Subsea Development
Authors: Simms Gerard John , Fowler Richard Lee , Cooley Bruce Dale , Leontaritis Kosta J. , Krishnathasan Kana 
Published: April 2012

This paper will discuss the viability of producing multiple deepwaterreservoirs with vastly different fluid properties from a subsea development in3,100 feet of water in the Gulf of Mexico. A summary of the diagnostic anddetailed flow assurance (FA) testing of different density oils will bepresented, addressing viscosity including non-Newtonian behavior, wax, asphaltene and hydrate behavior, downhole and flowline commingling, and fluidcompatibility tests.

The paper will discuss the transient behavior in a deepwater subsea developmentusing the results from the transient simulation model. Operability of theinfield flowline system as it pertains to hydraulics, pressure, temperature, wax appearance, hydrate formation, slugging, commingling of zones, shutdown, and restarts will be addressed. Provisions for pigging and a chemical programwere added for extra assurance. A larger size oil export line was selected toallow sufficient capacity for blended and/or heavy oils after restart. Finally, the steady state model was used to link the reservoir tanks, wellbores, andflowlines to confirm the hydraulics of the production system (i.e. flowlinesize and facility capacity).

Background

The Who Dat field is located in the Mississippi Canyon protraction area of theGulf of Mexico in blocks 503, 504, and 547 (Figure 1), and is being developedusing the Opti-Ex semisubmersible floating production system (FPS) which has acapacity of 60 MBOPD and 150 MMCFD. The Who Dat discovery is primarily oil andconsists of ten stacked amplitude-supported reservoirs in a salt withdrawalmini-basin. Three wells have been drilled to date, penetrating over 700' of netpay in nine distinct reservoirs ranging in depth from 12,000' to 17,000' TVD. Both gas and oil reservoirs were found with varying fluid properties (Table 1).A significant amount of fluid data were acquired in the open hole programresulting in over 60 downhole fluid samples. Twelve full PVT studies and eightdiagnostic flow assurance studies were performed for early fluid understanding. Later, flowback oil samples were acquired, rechecked, and furtherevaluated.

The field will be developed with 12 subsea wells (i.e. wet trees) flowing tothree four-slot subsea manifolds. Drill center A has one (1) manifold and drillcenter E has two (2) manifolds. Each drill center's manifolds are connected tothe production facility (FPS) via dual 6" nominal wet insulated flowlines andflexible risers with the capacity of roundtrip pigging. The length of theseinfield flowlines are about 3 miles and the geometry of the seafloor results ina down sloping flowpath. The project includes a 10" gas export line and 14" oilexport line also using flexible risers. The field subsea layout is shown inFigures 2 and 3.

The key inputs into the flow assurance process are non-contaminated fluidsamples and detailed lab analysis. In the case of system selection for the WhoDat field, fluid samples were available from three wells and multipleformations. There are some uncertainties about key formation fluid propertiesdue to drilling fluid contamination, such as viscosity, foaming, and waxcontent. Project challenges from a flow assurance perspective are summarizedbelow:Wide variation in the physical properties of the fluids in each reservoir(density from 15° to 45° API)High viscosity of produced fluidsFoaming of fluids and affects on meteringCommingling productionDownward sloping flowpath of oil and gas (flowlines)Non-Newtonian fluid behavior (oil export pipeline)