Summary
Downhole gas separators are often the most inefficient part of a sucker-rod
pump system. This paper presents laboratory data on the performance of five
different gas-separator designs. Only continuous flow was studied. Field data
are presented on two of the designs. The field data indicate that success or
failure of the gas separator is dependent upon the fluids and wellbore
pressures as well as the mechanical design of the gas separator. Successful and
unsuccessful examples of gas-separator performance in the field are shown along
with field fluid data properties.
Introduction
Gas interference in downhole plunger pumps has been studied for several
years. The first comprehensive analysis was presented by Clegg (1963), who
developed a theoretical analysis of separator performance and set some of the
rules of thumb that are still in use today. These guidelines were applied in
subsequent studies that developed practical methods for matching separator
performance to specific well producing conditions (Campbell and Brimhall 1989;
Dottore 1994; Ryan 1992). Poor performance of downhole rod pumps and problems
with progressing cavity (PC) pump operation owing to gas prompted the
undertaking of laboratory experimental studies by Robles and Podio (1999) that
included visual observation of separator-fluid mechanics using a full-scale
plexiglass wellbore and a conventional rod pump. The problem of downhole gas
separation recently has become of further interest in relation to dewatering
low-pressure gas wells and operating coalbed-methane wells. Patterson and
Leonard (2003) studied some different downhole gas-separation designs for
coalbed-methane operations in Wyoming. In these designs, the inlet to the gas
separators was smaller than normal and, along with some baffles, was thought to
allow gas to vent from inside the gas separator, obtaining good gas separation
in the field installation. While field installations provide the ultimate
validation of gas-separator performance, it is extremely difficult to isolate
the influence of each design parameter. It was these installations that
prompted the laboratory study of the gas-separator geometry to determine
whether the rules-of-thumb used by the industry for gas-separator design were
valid (Lisigurski 2004).
One of the most common sources of inefficiency in oilwell pumping
installations (rod pumps and ESPs of PC pumps alike) is gas interference, which
prevents the pump from delivering liquid at the design rate. Although this is a
well-known effect, there seems to be limited understanding of the mechanisms
that control gas interference, and this often results in the use of remedies,
such as installing downhole gas separators, that are ineffective or even
detrimental to the pumping-system performance.
The objectives of this paper are to give a clearer insight on the mechanisms
of gas interference in pumping wells and to present the results of recent
laboratory and field studies on the flow characteristics and performance of
some downhole gas separators.
In a pumping installation, one of the principal functions of the wellbore is
to operate as a two-phase (gas/liquid) separator so that the pump (which is
designed to pump liquid) can operate efficiently. Although this concept appears
to be obvious, it seems to be totally ignored by most operators when they
design completions and install hardware (gas anchors and the like) to combat
the effects of gas interference.
In these applications, the separation of gas from liquid is achieved through
gravity separation without the introduction of other mechanisms (centrifugal
forces, nozzles, etc.). Thus, the difference in density between the gas and
liquid is the main driving force to be used for separation. This also implies
that forces that oppose the effect of gravity, such as viscous drag caused by
high fluid velocity and turbulence, will be detrimental to the separation
process. Thus, high velocity of liquid or gas should be avoided if
possible.
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
17 June 2005
- Meeting paper published:
9 October 2005
- Revised manuscript received:
17 May 2006
- Manuscript approved:
27 May 2006
- Version of record:
20 February 2007
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