Reducing Uncertainty from PVT by Representative Sampling Subsea

摘要

Inaccurate PVT information has been shown to contribute significantly to increased uncertainty for subsea multiphase and wet gas meters when determining standard conditions flow rates. Typically, subsea meters have been set-up using PVT information gained from samples taken during drilling. Rarely has this data been updated, due to lack of access to quality representative samples. This paper addresses the issues regarding access to well stream fluids, the ability to capture representative samples, sample handling and storage, and the data needed to recombine the samples, and possibilities to implement solutions to these challenging conditions. Multiphase meters serve as the operator's eye into the subsea multiphase flow process and provide volumetric flow rates and split between the oil, water and gas phases along with a number of measured and calculated values. The data provided by the meters is used for flow assurance, allocation, and production management and to gain understanding of the reservoir structure, hence the importance of maintaining the quality of the measurement over time. The measurements from any inline meters are reported at line conditions, where the measurements are taken, usually near the subsea tree, but the operator usually requires the meter to provide flow rates in Standard Conditions as well. These line conditions measurements are then converted to standard conditions using either a set of correlation (black oil model usually) or a tuned Equation of State (EOS). Depending on the fluids, the method used can have an impact of up to +/- 30% uncertainty at standard conditions (Joshua Oldham, Exxon Mobil, at Subsea Tieback 2008). In order to reduce this source of uncertainty, the EOS must be updated as needed. If wells are commingled subsea the only way to gather the information needed is to sample the fluids at meter conditions in a representative state and recombine them in the laboratory. The remainder of the paper will describe the tools and methods needed to achieve this as developed by Schlumberger and Framo Engineering.