The simultaneous flow of gas, condensate and water through pipes is a frequent condition encountered in the production of high pressure gas fields. The multi-phase measurement of such streams by orifice meter has in the past been hampered by the lack of equations which relate the effect of the liquids on the gas flow calculation. A general relationship is developed herein for orifice metering which permits the calculation of the gas flow rate, or the total hydrocarbon well effluent rate, on the basis of multi-phase orifice meter data. In addition to the usual parameters required for gas flow calculations by orifice meter, a knowledge of the liquid/gas ratios is also required for the use of the proposed equations.
The orifice meter is widely used in industry for the measurement of single-phase streams. The theory and developed factors are well documented in the literature. Flow calculation procedures for gas and liquid streams are published2,3, and have been in use for many years. One area of application of the orifice meter which has received very little attention is its use for measuring multi-phase streams. Although orifice meters have been used in lease measurement of multi-phase streams in the oil and gas industry, no general procedure has been proposed to date which permits the calculation of the gas flow rate or the total well effluent rate, based on the multi-phase meter data. In high-pressure, sour gas operations, the continuous separation of phases for measurement requires a substantial capital investment and high operating costs, which could be avoided by multi-phase measurement. The advent of remote telemetery measurement of flow streams requires an instrument which is rugged, simple, and easily maintained. The orifice meter satisfies these requirements; however, its use for multi-phase measurement has been hampered by an absence of a simple calculation method which accounts for the effect of the entrained liquids on the flow measurement. Such a calculation procedure is developed herein and has been verified by a substantial amount of field data.
There are many gas fields in existence that are rich in the pentanes plus fraction. The gas normally exists in the vapour phase in the reservoir, but due to temperature and pressure reduction with flow through the tubing to surface, a two-phase mixture is formed. This condition is illustrated on Figure 1. At surface, it is desirable to measure the total hydrocarbon well effluent rate from a particular well, usually in terms of MMscd/d. This rate includes the gas flow, as well as the vapour equivalent of the hydrocarbon liquids. In addition to the liquid hydrocarbon phase, there is usually a water phase present, due 1:0 water of condensation. In some cases, free water is also produced from the reservoir.
Such multi-phase streams are frequently encountered in high-pressure, sour gas operations. The total hydrocarbon flow rate I or well effluent rate, is calculated on the basis of orifice meter data of the separated phases by: