Pig lift - a pneumatic artificial lift method that uses a mechanical interface, or pig, that travels back and forth in a U-shaped tubing - has been field-proved in Brazil in a variety of conditions.

Case histories show pig lift1-3 can boost production and solve difficult operational problems. An understanding of the general scheme of operation can be gained from Figure 1. Four pneumatic valves (1, 2, 3 and 4) at the surface control the operation. Valves 1 and 2 control gas injection, and 3 and 4 control production.
During an initial period of build-up, both gas valves are closed, both production valves are opened, the pig rests at the top of one of the strings between derivations A and B, and the liquid flows in at the bottom of the U-shaped tubing (U-adapter) and accumulates there. Then, the production valve of the string that hosts the pig is closed, and the gas valve of the same string is opened. The gas pressure pushes the pig downward until it passes the U-adapter and then pushes it upward through the other string until it reaches the surface. All the liquid previously stored is carried out to the surface. A conventional standing valve at the bottom of the U-adapter prevents the return of the liquid back to the well. Injection of gas during all upward pig displacement is not necessary. The energy required to push the liquid slug upward decreases continuously when the liquid slug reaches the surface since vertical flow is being transformed into horizontal flow. The pressure sensor commands the gas valve to close when the liquid slug reaches the surface. The pig stops between derivations A and B, and a blowdown begins, with the gas behind the pig being produced and, as a consequence, depressurizing the tubing strings. Then, both production valves are opened, starting a new cycle entirely similar, with the pig traveling in the opposite direction.
The pig is spherical and made of a relatively soft material. Its diameter is slightly greater than the internal diameter of the tubing. As a consequence, the insertion of the pig in the tubing is performed with dimensional interference, promoting sealing and cleaning. Additionally, the resulting static friction between the pig and the tubing holds the pig between the derivations A and B during the build-up period, preventing it from falling due to gravity.
Check valves 5 and 6 prevent the gas from bypassing the pig at the beginning of the gas injection period when it is between derivations A and B.
The pig is introduced into the tubing string through valve 7 and positioned between derivations A and B with a simple tool. When pig removal is required, it is positioned
in the opposite string and removed through valve 8, which is connected to a pig chamber. Valve 9 is shut to permit smooth removal. When the master valve is being opened for gas to push the pig, Valve 9 prevents gas from bypassing it.
Check valve 10 is employed in installations without a packer to prevent liquid from returning to the annulus when the annulus is vented to the flowline.
Selection criteria
The main advantages of pig lift are the elimination of liquid fallback and maintenance of system cleanliness even with sand, paraffin or scale in the fluid. Availability of high-pressure gas is the most important technical requirement for pig lift. Casing diameter may limit the tubing diameter and, as a consequence, the production rate that may be attained. Here are examples of well types in which pig lift is particularly appropriate.
Wells with a long sump. Some wells have a relatively long distance between the bottom of the perforations and the bottom of the well. Pig lift may be run with the admission of fluids below the perforations, and therefore, the highest drawdown may be attained (dynamic liquid level at the bottom of the perforations) with the reservoir producing almost its maximum rate. Among the pneumatic methods, only chamber lift could achieve a similar performance.
Difficult-to-handle wells. Some wells present operational problems that may prevent producing them economically by means of conventional artificial lift methods. Highly viscous fluids, paraffin, asphaltene, sand and scale are components that are costly to deal with. Corroded casing restricts pneumatic methods that require a pressurized annulus. Pig lift may easily handle these problems and be a cost-effective option. In some cases, pig lift may be the only option.
Deep wells. Intermittent gas lift in deep wells may encounter excessive fallback.
Even if a plunger is used, it may take a long time for the plunger to go back downhole. The pig eliminates liquid fallback without the plunger limitations.For a given injection pressure, pig lift can handle a greater column of liquid, with the U-shaped tube lending a chamber effect.
Field results
The first pig lift installation was run onshore in 1993 in Livramento field's well LV-28, northeast Brazil, replacing an inefficient
natural flow that induced small drawdowns. An increase of almost 100% was obtained in the average production rate.
The second installation was in Miranga field's well MG-394, onshore northeast Brazil. This is considered a standard installation. Several improvements were made, and the spread layout allows for an instructive and comprehensive view of how pig lift works (Figure 2). A strong production rate increase (70%) was obtained mainly due to a sump of 919ft (280m) below the production zone, submitting the reservoir to a maximum drawdown.
Since that time, more than seven wells (five offshore) have been converted into pig lift with improvements in the production rate from 10% to 130%. Pig lift has been applied in a variety of conditions: depths from 650ft to 10,000ft, rates from 20 b/d to 375 b/d, API degrees from 28 to 43, and water cut from 0 to 0.95.
Pigs are made of high-density polyurethane foam and nitrile rubber with an average life of 2,174 miles (3,500km).
What's ahead
Petrobras is interested in disseminating pig lift, and some efforts are being made in this direction.
A pig lift installation in a horizontal well is an important goal, especially when associated with steam injection.
Acknowledgements
The authors acknowledge Petrobras for permission to publish this paper and, on behalf of P.C.R. Lima (the inventor of pig lift), also acknowledge all those who contributed to the development of the technology.
References
Lima, P.C.R.: "Pig Lift: A New Artificial Lift Method," 1996 SPE Annual Technical Conference and Exhibition, Denver, Colo. SPE paper 36589.
Lima, P.C.R.: "Method and Apparatus for Intermittent Production of Oil with a Mechanical Interface," USA Patent 5,671,813 (1997).
l Almeida, A.R.: "Applications, Advantages and Limitations of Pig Lift," Rio Oil & Gas Conference, Oct. 5-8, 1998, Rio de Janeiro, Brazil.