Improvements to Deepwater Subsea Measurements RPSEA Program: Evaluation of Flow Modelling

Abstract

Abstract This report documents a project to improve subsea flow measurements to allowthe flow rates of individual wells to be known more accurately. Becauseproduction revenues are determined from flow rates, improved flow measurementreduces the financial risk to stakeholders, including producers and the U.S.government. Improved measurement also helps to improve reservoir recovery. Inthis project, commercially available flow models were evaluated for theirsuitability as virtual flow meters, to augment or replace hardware devices forthe determination of flow rates in deep water wells. The goal of this projectwas to develop methods for evaluating flow models, and to report on theefficacy of these models to predict flow rates. Several flow models were usedin the study, which also includes a survey of the available models. 1. Introduction 1.1. What is a Virtual Flow Meter? Virtual flow meters (VFM) are a method for determining the flow rate of a wellby means of modeling the flow. A VFM model may consist of a single well to anentire field of co-mingled wells. The models are fed data from sensorsinstalled at various measurement points (nodes). Typically pressure andtemperature sensors are used, which are available downhole (perhaps at multiplepoints), at the wellhead, and at the surface. With the pressure and temperaturedata at each node provided as an input, the VFM model computes the flow rate ofthe well. Typically, the model is tuned periodically using available flow data. Once tuned, the model is capable of determining the well flow rate with onlythe pressure and temperature nodal data. The flow models employed with VFMsystems may be built from first principles, but this is not the only method forVFM modeling. Flow models may also be derived from statistical analysis ofmeasurements acquired at different nodes along the production system. An important observation is that VFM software is not something that must beinstalled at the same time as the physical multiphase flow meters, as long ascare is taken to install proper sensors to be used at the various nodes. Thefault tolerant nature of the VFM system is an advantage, since a failed "hard"sensor will reduce the number of nodes, but will not cause a complete loss ofinformation. While VFM technology appears to be maturing rapidly, it is not yet generallyaccepted an equivalent replacement for good physical metering. VFM models areemployed as a back up to a physical meter to be used in case the primarymetering system is unable to function, or as an augmentation of physicalmetering to reduce measurement uncertainty. Some of the commercial VFMs use the technique of Nodal Analysis for the flowrate prediction. This method utilizes available transmitters (pressure andtemperature) and a mathematical model of the complete flow path from bottomhole to the topside facility. This method requires considerable input data, including well and pipeline profiles, the diameters of all components ofsections, sensor locations, fluid properties, choke geometry, etc.