Pig Lift: A New Artificial Lift Method
Authors: Lima P.C.R. 
Published: December 2007

Artificial lift of oil wells is a fairly broad subject. There are many different methods available but in a few cases none of them turns out to be a fit option. Some specific situation such as high viscosity or waxy oil, high gas-to-liquid ratio (GLR), horizontal and/or very deep well generate artificial lift problems that causes high reservoir back pressure, and, consequently, low production rates.

Pig lift is a new novel artificial lift method developed to solve some of these problems. It uses a U-shaped double completion string in the wellbore, with a full bore bottom hole connector, and a surface piping and control system. This physical arrangement is put together to allow the cyclic and automated launching of a low density foam pig from the surface, pushing along with it the liquid phase accumulated into the tubing string. The method is, therefore, cyclic. High pressure gas is used to displace the pig.

The system was successfully installed in five wells in Brazil, increasing the production flow rate significantly, as compared to conventional artificial lift methods. This paper presents the description of the pig lift method, and reports the results obtained in these field trials. Discussions of its technical and economical advantages and potential areas of application is also given.

Introduction

A widely employed artificial lift method is pneumatic pumping, better known as gas lift, which consists of injecting gas into the annular, and allowing it to pass into the production string through appropriate valves installed downhole. Basically, there are two types of gas lift systems, namely the "continuous gas lift" and the "intermittent gas lift". In the "intermittent gas lift", the well is first allowed to produce with no gas injection into the tubing. A column of liquid accumulates in the bottom of the tubing. Next, gas is injected into the production string at the bottom of the liquid column, pumping the liquid column to the surface as a slug. Gas lift has some advantages over other pumping systems, because of the lower number of components prone to failure. It is very simple, and it has great operational flexibility too.

In spite of these advantages, gas lift also has some serious disadvantages. One of them is the high reservoir back pressure, which is a limiting factor in oil production. Another one is the slippage between the gaseous and liquid phases, a situation in which the gas flows to the surface faster than the liquid oil. In intermittent systems, this causes liquid fall back, with part of the liquid column remaining in the interior of the production string in each cycle.

A solution for the slippage problem is the utilization of plungers. In moving upward the plunger pushes the oil accumulated in the string, reducing fall back. This method, however, also has some restrictions. It is necessary to shut the valve existing at surface after the plunger reaches its highest position so as to make it possible for it to descend back to the bottom of the well. For deep wells, the waiting time for the plunger to get back downhole can be very long. Thus, the intermittent gas lift with plunger is applicable only to low flow rate wells, and it is not recommended for high productivity wells. Application of plungers in highly deviated and horizontal wells is not recommended either.

Another method utilizes synthetic rubber spheres to cause the flowing of the oil accumulated in the production string. This method, named ball pump, failed to gain widespread industry acceptance, perhaps due to the complexity of the facilities at the surface and of the subsurface components, the risk that the sphere may become stuck, and the cost/quality of the spheres.

Many wells now being lifted are reaching to an economic limit, and in the ongoing pursuit of lower lifting costs, creative engineering techniques have to be applied. It was in this context that the pig lift method was developed.

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