Wet Gas Compression – A Logical Step to Follow Multiphase Boosting

Abstract

Multiphase boosting in oil and gas production may typically be characterized by:Slug flow occurrence possible, andGVF at inlet conditions higher than 50 %. The physical challenge for the equipment in use is the strong variation of the GVF (Gas Void Fraction), leading to transient flow conditions at the pump's inlet. However, when the GVF during the entire operations constantly exceeds 98 %, control of heat generation becomes a further challenge. Suitable answers need to be found for the following questions likeHow can wet gas compression best be described?Is this an efficient way to compress wet gas?What about installation economics? A technical issue for wet gas compression is the shaft sealing. Available standard seals can either cope with liquid or gas, but not with varying fractions of both. Other limiting factors are differential pressure, lubrication characteristics of pumped liquids, solids, and circumferential velocity. Introduction During the last decade multiphase boosting in oil and gas production has considerably matured. It is widely recognized as a true alternative to traditional production scenarios. The twin-screw pump is playing the major roll, when the equipment has to be selected. Due to its volumetric character heavy slugging, varying water content and other typical multiphase operating challenges this pump type is well suited for this purpose. By its low speed the hydrocarbon is transported without considerable turbulence what strongly avoids emulsification whenever water is also present - a definite advantage for the later separation of the phases. The demand for an increased and more constant Gas Volume Fraction (GVF) only was a little step ahead, then to be called "wet gas compression". Once the average gas volume fraction increases to about 98 % and only few liquid slugs, if any do occur, multiphase pumping should be called "wet gas compression". At the same time customer's interest has turned from oil to gas, and the associated liquid phase is of low viscosity, i.e. condensate and/or water. As the flow regime to the pump still is transient with changing inlet pressure and temperature, the same mechanical design guidelines must be used for the wet gas compressor as for the multiphase pump. Thermal expansion and quick temperature changes especially have to be looked after. Definitions A typical working domain for a wet gas compressor may be described by the following set of parameters:Power installed: P = 1.0 MWDifferential pressure: ? p= 50 bar (5 MPa)Inlet pressure: pin = 50 barg (5 MPa)Total capacity: Qin = 1,000 m3/h, which is equivalent to 240 MSm3/d at 10 barg inlet pressure, or 1,000 MSm/d at 40 barg respectively. To make the applications harder, the viscosity range is around or even below 1 mm3/s. Technical Issues In a twin-screw pump two pairs of intermeshing screws, forming cavities between them are rotating in the housing. The fluid is thereby conveyed from inlet to discharge from both sides. Thus axial forces are fully balanced. As no metal-to-metal contact is permitted, the clearances occurring need to be sealed by liquid. The sealant characteristics increase with higher viscosities. With little liquid occurring in the wet gas stream, it needs to be made available. This can preferably be done by storing it in the pump housing itself with a cooling circuit attached. Hence no liquid is added to the production and does not need to be separated at a later stage. Heat Generation and Slug Flow Occurrence. Whenever liquid or gas is compressed, heat is generated. The main difference is the heat storage capability of the different fluids: good with liquid (Water = 4.2kJ/kg·K) and very poor with gas (= 1.0 kJ/kg·K), the difference in mass throughput has to be observed. The discharge temperature of the fluid is governed by the pressure ratio and the fluid parameters. The latter being described with the isentropic exponent ?. Heat Generation and Slug Flow Occurrence. Whenever liquid or gas is compressed, heat is generated. The main difference is the heat storage capability of the different fluids: good with liquid (Water = 4.2kJ/kg·K) and very poor with gas (= 1.0 kJ/kg·K), the difference in mass throughput has to be observed. The discharge temperature of the fluid is governed by the pressure ratio and the fluid parameters. The latter being described with the isentropic exponent ?.

Optimal design and numerical analysis of multi-field coupling in a screw pump under wet gas compression

“…Xu 4 proposed to inject liquid into the screw pump from the suction for the operating condition of wet gas compression, so as to improve the reliability and efficiency of the twin-screw multiphase pump. Müller-Link et al 5 found that a twin-screw pump with a reflux circulation system can work normally even when the GVF exceeds 99.5%. Yang et al 6 optimized the design of synchronous rotary multiphase pump and proposed that a wider but shorter cylinder can reduce the leakage caused by high gas content and is more suitable for the working conditions of higher inlet GVF.…”