RIASSUNTO
Abstract
Wet gas metering is rapidly establishing itself as a special problem area within the more general area of multiphase flow metering. In fact, while attempts are being made across the Oil and Gas Industry to define the boundaries between humid gas, gas-condensate and high GVF (gas volume fraction) multiphase systems, a few commercial wet gas meters are already available for specific field applications.
A major interest of Imperial College has been the development of metering schemes for three-phase (oil-gas-water) flows in oil-field production systems. Imperial College has already developed a flow meter (MIXMETER)1, which performs satisfactorily in the range of gas volume fractions of 0 to 90%. Imperial College has therefore been recently developing a new concept (ANUMET), which shows some promise for measuring two-phase flows at much higher gas volume fractions.
The technical description of the ANUMET wet gas meter is outlined in this paper. Preliminary experiments on this new metering concept have been carried out with air-water flows in a 32 mm diameter tube in the LOTUS (LongTUbeSystem) facility at Imperial College. Encouraging results have been obtained and they are presented here.
Introduction
The total world gas production has increased from 200.4 bcf/d in 1991 to 241.9 bcf/d in 2001 and the total world gas consumption has increased from 194.1 bcf/d to 232.4 bcf/d during the same period of time2. For the Gulf of Mexico, it is expected that deepwater gas production will peak at around 5.3 bcf/d in 20043. Compared with oil fields, gas fields tend to be more environment-friendly because of lower carbon dioxide emissions and a cleaner combustion. However, the production of gas is, in most cases, associated with the production of liquid hydrocarbons (that form in the reservoir, in the wellbore and at surface as pressure and temperature drop), free formation water and condensed vapour. This is why, from a metering point of view, the techniques normally implemented for dry gas metering cannot be always applied.
Now that operators have become aware of the potential benefits in using multiphase flow meters for metering the produced streams of oil fields, in terms of cost savings, production optimization, field monitoring and reservoir management4,5,6, it is almost straightforward to extend the same considerations to wet gas and gas condensate fields. However, before generalising the problem of multiphase flow metering and deciding which techniques can be applied for the case of wet gas metering, it is necessary to first define ""wet gas"".
Definition of wet gas and wet gas metering issues
To date, several definitions of wet gas have been proposed within the Oil and Gas industry, all of them equally important in covering the full range of possible stream compositions, operating pressures and temperatures, and flow regimes. Wet gas metering, in fact, may apply to gas condensate fields, high GOR fields and wet gases. Accordingly to the definition given in the SPE monograph volume 207 and as illustrated in Fig.1, a reservoir fluid is classified as dry gas when the reservoir temperature is greater than the cricondertherm and surface/transport conditions are outside the two-phase envelope; as wet gas when the reservoir temperature is greater than the cricondertherm but the surface conditions are in the two-phase region; as gas condensate when the reservoir temperature is less than the cricondertherm and greater than the critical temperature; and as an oil (volatile or black oil) when the reservoir temperature is less than the mixture critical temperature.
A general definition of wet gas is that of a stream with liquid volume fractions between 5 and 10% at metering conditions8. Following this definition, wet gas metering can be seen as the top boundary of multiphase flow metering (oils with high gas volume fractions) and the bottom boundary of gas metering (gases with high liquid volume fractions). This implies that by pushing either multiphase flow metering or gas metering to their extremes, wet gas metering solutions could, in theory, be found. However, due to the complexity and peculiarity of wet gas flows, wet gas metering may necessitate a dedicated effort.