Gas E&P companies have identified that pressure boosting in closer proximity to the source reservoir can accelerate and increase hydrocarbon recovery beyond that possible by conventional central gas compression. Downhole pressure boosting significantly improves the vertical lift performance of the production tubing and stimulates reservoir flow by increased suction. The greatest economic benefit is gained when downhole compression is introduced during the 'plateau' production period. From the reference fields studied during a recent comprehensive feasibility study, incremental production gains of between 32% and 41% representing additional recovery of 14 to 20 Bscf per well over 5 years were found to be possible. By accelerating rates of production, downhole compression may also present an opportunity for the development of otherwise sub-economic fields.
This paper presents the findings of this feasibility study, which concluded that adapting existing technologies from other industrial sectors to the downhole environment is both technically and economically viable. For the first time a feasible concept exists, that when fully developed, will enable operators to extract significantly greater value from their gas assets. A description of the key reservoir and well drivers that determine the level of production acceleration and enhanced recovery is also presented.
To maximise well productivity and recovery, there are a number of techniques available to stimulate flow from a hydrocarbon reservoir. Some act directly on the formation, such as fracturing; others seek to maintain the driving reservoir pressure by injecting water. In gas fields central gas compression and/or wellhead compression are generally utilised to extend productive life. When the most significant 'bottleneck' to higher productivity is below ground, compression directly in the wellbore offers the biggest prize.
Compression close to the source reservoir is more effective in lowering the field abandonment pressure and hence increasing ultimate recovery. Furthermore, flow will be significantly accelerated by the use of such a downhole booster. Downhole compression has eluded the industry to date for many reasons.
Gas compressors need to be driven at rotational speeds well in excess of that possible with oil filled electrical motors, which would otherwise suffer from unacceptable churning losses. To achieve the necessary speeds with oil filled motors requires gearing and resolution of attendant high-speed shaft sealing issues. High-speed hydraulic drives may also be ruled out because of the same need for high-pressure high-speed shaft seals resulting from the high static head from the wellhead to the downhole compressor location. Such sealing technology does not currently exist.
Recognising that appropriate compressor, bearing and permanent magnet motor drive technologies now exist, their is today a practical design concept for downhole gas compression.
The value of this technology for enhancing recovery from natural gas wells is the primary subject of this paper.
In addition, the paper sets out to:
guide the operator towards the appropriate application of downhole gas compression (DGC) by showing a number of field development scenarios;
present the design concept for DGC derived from the feasibility study;
describe the forward development programme to bring this game changing technology to the marketplace.
