Qualification of a Nonintrusive Multiphase Flow Meter in Viscous Flows

Abstract

Abstract New requirements have recently appeared for accurate and reliable flow rate measurements in various operational areas where viscous fluids are produced. Heavy oil as produced in Venezuela, Confederation of Independent States (CIS), Brazil, Angola and China and lighter oil emulsified with large water cuts (worldwide) present new challenges to multiphase metering. The compact, cost-effective multiphase meter described in this paper is intended to replace large, costly test separators designed for heavy oils. The meter is non-intrusive and combines a classical venturi to measure total mass flow rate and a dual energy gamma-ray composition meter to measure oil, water and gas fractions. A benefit of this compact design is that the meter can be switched from one well to another and provide robust, consistent flow rate measurements without any flow recalibration. According to published single-phase flow literature, liquid viscosity plays an important role in meter performance. Oil companies are becoming increasingly aware of these problems and want more evidence that a multiphase meter will perform regardless of fluid properties. To qualify the multiphase meter in high-viscosity flows, we conducted comprehensive flow loop tests in Venezuela. The challenge was to gain a solid understanding of viscosity effects even in the presence of large amounts of gas, and for the entire operating envelope of the meter. These test results have been used to refine the meter interpretation model to maintain the metering accuracy in these conditions. The interpretation includes a mixture viscosity model based on the dead-oil viscosity; for permanent monitoring and periodic testing, the effluent viscosity variations are updated when pressure, temperature or water cut is changing. To demonstrate that the specified measurement accuracy was achieved up to the highest viscosities tested (several thousand centipoise), the meter was field tested in comparison with a test separator. We present the results of 54 jobs. Until recently, the only multiphase meters available to measure viscous oil flowing with gas were intrusive. Positive displacement meters, such as oval gears, twin helical screws or vane types, were used to measure the total gas-liquid volume flow rate. These meters are very sensitive to severe slugging in the upstream pipeline and prone to mechanical damage from produced solid particles. The lightweight, small-footprint meter described here solves these problems. Consistent flow rate information can be obtained even when the meter is switched from well to well. Fluid property or flow regime changes do not affect the flow rate measurement. Introduction One of the main objectives of multiphase flow meter design is a low-cost system that will accurately measure the flow rates of oil, water and gas in difficult multiphase flow regimes. These include emulsions, high-viscosity oils and foaming conditions for which conventional metering systems based on phase separation have great difficulties or cannot successfully perform the tests. Even during efficient separation conditions in a conventional metering system, there is a significant advantage in continuously monitoring a well stream. Whereas a conventional testing system may take hours for stable measurements, a multiphase flow meter can yield good test results minutes after the well is opened. Elimination of a test separator, manifold and/or flow line is ordinarily the single most important reason for choosing to use a multiphase meter. Test separators and the associated metering equipment are expensive and require additional platform space on offshore topside installations. In satellite fields, a test line back to a test separator on a platform is a significant capital expense. For subsea applications the advantages are even greater as separate test lines and, in some cases, entire platforms, may be eliminated.