Abstract
A downhole gas compressor and turbine expander are being developed to inject excess gas into North Slope fields. North Slope oil fields and some oil and condensate fields elsewhere in the world are limited in liquid hydrocarbon production because of surface gas processing facility limitations. These fields usually require costly surface facilities to process, compress and reinject the produced gas.
The Subsurface Process And Reinjection Compressor (SPARC) concept uses the excess production pressure (energy) that is usually wasted across a choke to generate power through a downhole turbo expander that runs a downhole gas compressor to reinject a portion of the gas stream. The system consists of a downhole separator, compressor, turbo-expander and other standard downhole equipment for the necessary plumbing.
Two basic machines are being developed: one operates in parallel flow and the second operates in series flow. The parallel flow machine maintains the highest pressure possible throughout in order to inject the maximum amount of gas. A separator divides the production into two streams. The first stream, which consists of liquids and a portion of the gas, is used for power generation. The second stream, dry gas at separator conditions, is compressed and injected into another reservoir or isolated zone that the wellbore penetrates. The amount of gas injection depends upon the amount of excess energy that is available. Typically, North Slope wells can reinject from 30% to 60% of the gas downhole without the gas being produced to the surface.
The second machine operates in series flow. The entire liquid and gas stream is passed through a turbo expander and then a portion of the gas is separated for reinjection. The series machine allows for the maximum cooling and pressure reduction to recover a significant fraction of the condensate from the gas stream prior to gas separation and reinjection. The series machine reinjects less gas than the parallel machine but it will recover a higher percentage of the condensate in the produced gas. Therefore the series machine may be the more economical machine to use in a condensate reservoir.
The development of the SPARC is requiring significant advances in compressor, turbo-expander, separator and packer technology. Current designs show that the SPARC can be made small enough to be placed through 3-1/2" tubing into liners as large as 7" via wireline. The primary advantage of the SPARC is that it can provide additional gas injection capacities at a fraction of the capital cost of traditional surface equipment.