Subsea Sampling: Reducing Uncertainty From PVT by Utilizing Representative Sampling Subsea

Abstract

Inaccurate PVT information has been shown to contribute significantly toincreased uncertainty for subsea multiphase and wet gas meters when determiningstandard conditions flow rates. Typically, subsea meters have been set-up usingPVT information gained from samples taken during drilling. Rarely has this databeen updated, due to lack of access to quality representative samples. Thispaper addresses the issues regarding access to well stream fluids, the abilityto capture representative samples, sample handling and storage, and the dataneeded to recombine the samples, and possibilities to implement solutions tothese challenging conditions. Multiphase meters serve as the operator's eye into the subsea multiphase flowprocess and provide volumetric flow rates and split between the oil, water andgas phases along with a number of measured and calculated values. The dataprovided by the meters is used for flow assurance, allocation, and productionmanagement and to gain understanding of the reservoir structure, hence theimportance of maintaining the quality of the measurement over time. Themeasurements from any inline meters are reported at line conditions, where themeasurements are taken, usually near the subsea tree, but the operator usuallyrequires the meter to provide flow rates in Standard Conditions as well. Theseline conditions measurements are then converted to standard conditions usingeither a set of correlation (black oil model usually) or a tuned Equation ofState (EOS). Depending on the fluids, the method used can have an impact of upto +/- 30% uncertainty at standard conditions (Joshua Oldham, Exxon Mobil, atSubsea Tieback 2008). In order to reduce this source of uncertainty, the EOS must be updated asneeded. If wells are commingled subsea the only way to gather the informationneeded is to sample the fluids at meter conditions in a representative stateand recombine them in the laboratory. The remainder of the paper will describe the tools and methods needed toachieve this as developed by Schlumberger and Framo Engineering.

Subsea Business: Fiscal Allocation, a New Endeavour for a Better Wet Gas and Multiphase Measurement

“…Pinguet and alt (Ref [5] addressed this challenge about how to maintain this information over the life of a field or a well. This is the critical point, indeed allocation of produced hydrocarbons has major fiscal consequences for a variety of involved parties.…”