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UNITS FOR NATURAL GAS COMPRESSION FROM COAL SEAMS, SHALE AND TIGHT SAND WILL BE DIFFERENT FROM THOSE USED IN CONVENTIONAL APPLICATIONS
Unconventional natural gases, such as those from coal seams, shale and tight sand have become an increasingly important source of energy in the world over the past decade. In 2010, unconventional gases provided over 20% of U.S. natural gas production. There are some predictions that in the coming decades, more than 50% of the U.S. gas supply will come from unconventional sources. The same is more or less true for Australia.
However, some recent studies suggest a high rate of decline at some unconventional gas wells. This may be an indication that unconventional gas production may ultimately be much lower than is currently projected. A key point could be proper unconventional gas compression units to compensate for pressure declines as the well diminishes while maintaining production at an acceptable level.
Coal-seam gas (CSG) is formed over millions of years when decomposing plant matter is heated and compressed deep underground. Over time, CSG becomes trapped in coal seams by water. Apart from methane, CSG contains small amounts of other hydrocarbon gases, such as ethane, propane, butane, as well as nitrogen and carbon dioxide.
Shale gas is trapped within shale formations. Shale, a fine-grained sedimentary rock, is easily breakable into thin, parallel layers. A soft rock, it does not disintegrate when wet. Shale can contain natural gas, usually when two thick, black shale deposits “sandwich” a thinner area of shale. The extraction of the shale gas from the shale formations is often difficult and expensive.
Tight-sand gas is stuck in a tight formation underground, trapped in unusually impermeable hard rock, or in a sandstone or limestone formation that is unusually impermeable and non-porous. A great deal of effort has to be put into extracting gas from a tight formation. Several techniques exist that allow natural gas to be extracted, including fracturing and acidizing. However, these techniques are costly and difficult to apply.
Current concepts for unconventional gases include standalone hub compression units, or field compression systems with minimal spare machines and equipment installed or retained, and minimal margins. The prevailing assumption is that this hub compression unit (or the field compression system) would be further expanded to accommodate future production.
Unconventional gas projects are marginally viable (commercially) and the proper principles for cost minimization should be identified and applied. Many conventional spares and auxiliaries should be eliminated or minimized. The right size should be selected for turbocompressors and accessories. In addition, standardization and modularization are key for success in the unconventional gas market, particularly in unconventional gas compressor packages.
Based on experience and extensive studies (with some exceptions), the unconventional gas compressor can be based on a fully electrified field with electricity supplied by a grid. Back-up electric power supply is assumed only for emergency shutdown to minimize cost.
A minimum number of piping sizes is always preferred for compressor packages. There have been compressor packages containing all pipe sizes from ½ inch to 16 inch, which is a poor design. Pipe sizes should be re-evaluated and proper standardization and modularization should be implemented.
The battery limits (the tie-in) of the vendor compressor skids should be terminated at the skid edge. The vendor should design and install supports for all piping in the vendor scope, and the piping termination points should be stand-alone flanges near the package base-plate limit. There have been many discussions and issues (and sometimes complicated problems) associated with the offskid package piping.
Turbocompressor packages should also be designed for easy relocation. This is a different concept compared to permanent (conventional) turbocompressor packages. A fully modularized design to allow a simple plug-and-play installation and relocation is always preferred due to the nature of unconventional sources. Shale gas, for example, often involves relocation of equipment to move to a more productive well site.
CSG has usually been vented to the atmosphere as part of the coal mining process in parts of the world such as the U.S. and Australia. With increasing energy demands and stricter environmental protection legislation, this widely abundant gas is attracting sharper interest. CSG can be extracted through wells drilled into coal seams (typically 600 meters). When water is pumped out, CSG is released from the coal seams.
Considering the compressor design for CSG compression, it should be noted that CSG is a fully saturated gas. Proper liquid separation and suitable compressor packages are required.
As an indication, the following examples are noted for various CSG compressor packages: For a capacity of around 6,000 m3/h and a discharge pressure of about 18 Barg, the estimated power should be about 3 MW. For a capacity of around 4,200 m3/h and a discharge pressure of about 16 Barg, the estimated power would need to be approximately 2 MW. And for a capacity of around 2,100 m3/h, and a discharge pressure of about 17 Barg, the estimated power would be around 1 MW.
Considering commonly used compressor sizes and application ranges, variablespeed operation and VFD systems are discouraged. Constant-speed electric-motor drivers are preferred. Soft-start (VFD for start-up) should not be specified if at all possible. However, if soft-start is required, a single soft-start unit for each bank of the compressors could be used.
Otherwise, small, high-speed, direct-driven centrifugal compressors and small integrally geared centrifugal compressors are the main turbocompressor candidates for unconventional gas compressors (competing with the oil-flooded screw compressors).
Amin Almasi is a Chartered Professional Engineer in Australia and U.K. (M.Sc. and B.Sc. in mechanical engineering). He is a senior consultant specializing in rotating equipment, condition monitoring and reliability.