Downhole Gas Compression – Game Changing Technology
Using conventional extraction methods, up to 40% of the gas initially in place can be left stranded in abandoned gas fields, Downhole gas compression, i.e. actively compressing the gas at the base of the tubing, offers the potential to recover this gas. However, it requires the application of novel and innovative technologies to overcome the environmental and physical constraints imposed on any equipment subjected to downhole conditions. Corac Group plc, in collaboration with Dynex Semiconductors ltd, Nottingham; and Sheffield and Newcastle Universities are working on key enabling technology areas that will facilitate up to 500kW of compressor power to be achieved in the tubing downhole. Julian Reed (joreed@corac.co.uk), Engineering Director of Corac Group plc, presents the conceptual basis of this research which receives some funding from DTI.
Background
To maximise recovery of gas from a well, there are a number of artificial methods currently available to stimulate flow from the reservoir. These include fracturing the formation to unblock the porous strata and installing central and well head compression (placing large compressors at the sea bed). However, all of these are rendered ineffective where the principal resistance to gas flow is due to the friction effects generated by the substantial lengths of delivery production pipework, sometimes of the order of 2 – 3 km.
Downhole gas compression uses a compressor placed as near as possible to the point where the gas is flowing into the wellbore from the formation. It has been predicted to improve the vertical lift performance of the subsurface tubing and can stimulate reservoir flow by increased suction (Figure 1). The maximum benefit is gained by early deployment in prolific flowing reservoirs where the rate of production can be increased, or by positioning the compressors in backed out/low pressure reservoirs where stranded gas can be recovered. Typically in backed out reservoirs, 20 – 40% of gas initially in place is stranded or inaccessible by conventional techniques. Downhole gas compression gives the possibility of recovering these resources and extending the productive life of a well.
Figure 1: Potential yield improvements from utilising Downhole Gas Compression
Constraints
In order to achieve downhole gas compression, several key enabling technologies have to be applied or developed. Fundamentally 250 – 500kW of compressor power has to be deployed 2 – 3 km below ground within the constraints of typically 7 – 9 5/8 inch diameter production tubing. The environment is typically 20 – 60 bara and the gas is typically in excess of 100ºC. The flow rates essentially require high speed compressor technology which in turn requires high speed motor development. Conventional lubricated systems will not survive in this environment as seals and lubricants will be degraded by the gas flow and entrained contaminants, temperatures and rubbing speeds of the seal surfaces.
Concept and Solution
Corac have developed high speed centrifugal compressors based on permanent magnet high speed motor technology, augmented by gas bearings throughout and employs state of the art aerodynamics for optimum efficiency. Adoption of these technologies enables the most compact, high power density machines to be developed and there is an obvious synergy with these technologies and their application into a downhole environment (Figure 2).
Figure 2: Downhole compressor module
As the motors require controlled amplitude and frequency alternating current electrical supply, the delivery of this energy to the compressor presents a formidable technical hurdle to be overcome. Supplying alternating current from the surface will result in significant losses due to the distance from the supply to the base of the tubing.
Therefore, the route adopted is to utilise a 3.3 kV direct current supply down the well with the whole motor drive and inverter technologies having to be developed to operate at the base of the well, inside the envelope of the delivery tubing. To achieve this target, Corac, Dynex and Nottingham University are developing a complete drive and inverter module (Figure 3). This will initially deliver 250kW of motor drive power and operate in an environment in excess of 105ºC, using the gas flow to cool the electronics components and auxiliary systems.
Figure 3: Prototype drive module
DIPEC (Downhole Integrated Power Electronics and Control)
The DIPEC programme sets out to develop a motor drive which will survive in a 100ºC gas stream and provide power for a compressor string requiring over 250kW. The power switching elements are to be cooled by the gas flow and therefore the project consists of:
- Simplification of the switching duties of the IGBTs to reduce their operating temperatures.
- Development of a unique layout of the power switches, gate drives and controllers on a structural element that can be inserted into a 7 inch diameter production tube.
- Optimisation of the thermal conductivities of the switch substrates and mountings to ensure adequate heat dissipation to the gas stream.
- Integration of the direct current bus bars and motor feeds into the structural element whilst still satisfying the overall space limitation from the production tubing.
An additional aspect of the project includes
- Development of an industrial drive based on the topology derived above with an integral heat sink coupled to the motor cooling system, the main benefits being the dramatic reduction in footprint of the electronics package and integrated cooling.
The concept utilises existing 3kV switch technologies and is therefore limited to the maximum temperature of the switching devices before they fail. Typically the surface of the switch can reach 180ºC, but is then limited by the current that can pass at this temperature. For a realistic drive, the combination of switching speed and useable current reduces this to about 140ºC. Preliminary studies have indicated that this puts a constraint of the bulk gas temperature of 105ºC which in turn limits to general applicability of this technology as many wells operate in excess of 150ºC. Clearly if the operating temperature of the switching devices can be increased, the market opportunities similarly increase.
Power Electronics Adverse Temperature Environments (PEATE)
Corac, in collaboration with Sheffield University, Newcastle University and Nottingham University have been successful in gaining a DTI sponsored project to explore the possibility of Silicon Carbide (SiC) switching devices to offer increased temperature performance for the downhole compressor application.
Currently research teams are able to produce SiC diodes that are performing at 400ºC. However, there are very significant trade-offs between voltage and current at these elevated temperatures. Newcastle University has been researching this semiconductor material for some time and has a significant group looking at various doping requirements and the thermal behaviour of such devices under various metallisation configurations. It is generally viewed that, if the devices can be operated at face switching temperature, then it is possible through various packaging to enable IGBTs to operate in the environment enabling main gas streams to be in excess of 150ºC. This, in turn, would considerably increase the number of wells suitable for the application.
Equally, there are many other applications if the switching temperatures can be achieved. Sheffield University will be looking much more closely at the switching structure to enable the circuit to be packaged to the constraints of the application. Nottingham University will be much more concerned on the packaging techniques to achieve good conductivity from the devices to the gas stream. Then all parties will be involved in validating the switching and converter geometry to achieve manufacturable, reliable systems.