Multiphase Pumping

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    Multiphase Pumping as an Alternative to ConventionalSeparation, Pumping and Compression

    Mack Shippen, Schlumberger - Baker JardineDr. Stuart Scott, Texas A&M University

    Prepared for Presentation at the 34th

    Annual PSIG meeting

    Portland, Oregon

    October 25, 2002

    Abstract

    This study explores the application of multiphase pumps as an alternative to conventionalseparation using rigorous steady-state simulation models incorporating a newly developedmultiphase pumping model. The simulation results show that multiphase pumps areadvantageous in not only reducing facilities, but can also increase production rates by loweringthe backpressure on wells. Additionally, the complexities associated with multiphase flow througha single pipeline are compared to running dual single-phase pipelines and importantconsiderations observed with the steady-state simulation are highlighted.

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    1.0 Introduction

    Following its emergence from research labs a decade ago, multiphase pumping has become a viable

    solution to a wide number of field development plans. While the technology is seen to be particularlybeneficial in remote locations such as the deepwater Gulf of Mexico, pumps have also been deployed to a

    number of onshore locations ranging from Alaskan North Slope to Columbia and from West Africa to the

    Middle East.

    Multiphase production systems require the transportation of a mixture of oil, water and gas, often for many

    miles from the producing well to a distant processing facility. This represents a significant departure fromconventional production operations in which fluids are separated before being pumped and compressed

    through separate pipelines. By eliminating this equipment, the cost of a multiphase pumping facility is

    about 70% that of a conventional facility (Dal Porto, 1996) and significantly more savings can be realized

    if the need for an offshore structure is eliminated altogether. However, multiphase pumps do operate lessefficiently (30-50%, depending on Gas volume fraction and other factors) than conventional pumps (60-

    70%) and compressors (70-90%). Still, a number of advantages in using multiphase pumps can be realized,

    including: 1) Increased production through lowering backpressure on wells; 2) elimination of vapor

    recovery systems; 3) reduced permitting needs; 4) reduction in capital equipment costs; and, 5) reduction infootprint of operations.

    Interest in the subsea deployment of multiphase pumps has grown as operators search for methods toimprove recoveries and economics for subsea completed wells. While subsea completed wells enable

    development of deepwater resources as well as marginal fields in normal water depths, without some form

    of subsea processing, these wells are expected to experience poor ultimate recoveries due to the high

    backpressures. For example, conventional production operations routinely drawdown wellhead pressures to100-200 psig. A subsea completed well, however, may have abandonment wellhead pressures of 1,000-

    2,000 psig due to the backpressure added by the long multiphase flowline. In addition, operating as such a

    continual high backpressure has been shown to have a direct impact on production decline behavior, acting

    to reduce ultimate recovery (Martin & Scott, 2002). Maintaining a high backpressure can be viewed as aproduction practice that wastes reservoir energy. Energy that could be used to move reservoir fluids to the

    wellbore and out of the well is instead lost to flow through a choke or a long flowline. It is anticipated that

    some form of subsea boosting and/or processing of produced fluids will be necessary to improveefficiencies, allowing longer production from these wells and better recovery factors. Subsea processing

    covers a wide spectrum of subsea separation and boosting scenarios. Subsea multiphase pumpingtechnology is perhaps a decade ahead of subsea separation and provides many advantages in terms ofintervention when compared with wellbore artificial lift methods.

    Multiphase pumping is a relatively new technology and acceptance has been hampered by a lack of

    engineering design tools. Recently, pipeline simulation codes have incorporated the ability to modelmultiphase pump performance as part of the overall multiphase production system. This paper illustrates

    the use of such a model to evaluate the benefits of subsea multiphase pumping.

    2.0 Multiphase Pumping Technologies

    Over the past decade, several multiphase pump technologies have emerged for gas-liquid multiphaseflow in the petroleum industry. As shown in Figure 1, these methods fall into the broad categories of thepositive displacement and rotodynamic pumps. Figure 2 shows that the number of multiphase pump

    installations has increased rapidly over the past 5-7 years (Scott, 2002). This figure also shows the

    breakdown between the different multiphase pump technologies. It should be noted that while the helico-axial technology only represents a small number of the total multiphase pump installations, they are used in

    the majority of offshore and subsea applications and have the capacity to pump much large volumes of

    fluids than the positive displacement technologies.

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    Piston

    Helico-Axial

    Single-Screw (PCP)

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    Positive Displacement

    Twin Screw

    Progressing Cavity (PCP)(Single Screw)

    Piston

    Diaphram

    Rotodynamic

    Helico-axial(Poseidon type)

    Multi-Stage Centrifugal(ESP type)

    Multiphase Pumps

    Figure 1: Types of Multiphase Pumps

    Figure 2: Worldwide Usage of Multiphase Pumps (MPUR Survey, Scott, 2002)

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    A good summary of the development of multiphase pumping technology is given by Cooper et al (1996)

    and Scott & Martin (2001). Recently, a transient model has been proposed to describe the behavior of a

    rotodynamic pump (Ramberg and Bakken, 1997). Ideas on modeling the twin-screw pump have been

    presented by Vetter & Wincek (1993) and Egashira et al. (1996) and these pumps have been successfullyincorporated into field use (Oxley & Shoup, 1994; Jaggernauth et al., 1996; Caetano et al., 1997; Guevara,

    1999; and Giuggioli et al., 1999)). The following sections discuss the most commonly used types of

    multiphase pumps.

    2.2 Positive Displacement Pumps

    Positive displacement pumps operate on the principal that a definite amount of fluid is transferred through

    the pump based on the volume created by the pumping chamber and the speed at which this volume is

    moved. The amount of differential pressure that develops in the pump is a function of the resistance toflow downstream of the pump - that is, the pressure losses that must be overcome to deliver the fluid to a

    set pressure downstream of the pump. For any positive (or near positive) displacement pump, the

    interaction between the pump and the adjacent pipeline segments determines pump performance.

    2.2.1 Twin-Screw Pumps

    The twin-screw is by far the most popular multiphase pump in use and is manufactured by Bornemann,Flowserve and Nuovo Pignone. Twin-screws are particularly adept at handling high Gas Volume Fractions

    (GVF) and fluctuating inlet conditions. These pumps remain functional even at GVFs of 95% and with re-

    circulation systems can function at 100% GVF for short periods of time.

    Figure 3 gives a schematic of a twin-screw pump. The multiphase mixture enters one end of the pump and

    split into two flow streams that feed into inlets situated on opposite sides of the pump a design that

    equalizes stresses associated with slugging. Flow then passes through a chamber (created by theinterlocking screws) that moves along the length of the screws to the outlet. The volumetric flow rate is

    dependent on the pitch and diameter of the screws and the rotational speed. As the gas is compressed, a

    small amount of liquid will slip back through the small gaps between the screws and the containment

    chamber wall resulting in a reduced volumetric efficiency.

    Figure 3: Twin-Screw Pump (after Bornemann)

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    2.2.2 Progressing Cavity Pumps (Single-Screw)

    Widely used in shallow wells as an artificial lift method, the Progressing Cavity Pump (or Moyno pump)

    has been adapted for surface multiphase pumping. Note that the number of PCP pumps listed in Figure 2

    represents only the surface installations of this technology. The PCP pump is comprised of a rubber statorand a rotating metal rotor (Fig. 4). This pump is effective for low flow rates (less than 30,000 bbl/day total

    volume of gas, oil and water) and for lower discharge pressures (maximum of 400 psig). This pump hasthe unique ability to tolerate considerable amounts of solids (sand). However, high sand production ratesresult in the need to replace the stator on a regular basis.

    2.2.3 Piston Pumps

    One of the simplest forms of multiphase pumping is the use of a large double-acting piston to compress themultiphase oil, water and gas mixture. This approach is effective in the low and moderate flow rate ranges

    with a maximum capacity of approximately 110,000 bbl/day (total volume of gas,