b'Plant & Process | Engineer InnovationThe coal seam gas story Gas volumes released inside water Coal seam gas (CSG) has become agathering pipelines are relatively small valuable new energy resource in(typically less than two percent of total Australia. In Queensland alone some volume) and therefore insufficient to 60 billion US dollars have been investedcreate an intense two-phase flow regime in this industry in recent years. Moresuch as slug flow. Instead, free gas is than 10,000 gas extraction wells haveswept along and accumulates at pipeline been drilled to date producing some high points where it forms local gas 20 million tons of unconventional bubbles.gas per annum. Gas and waterpipeline networks of a total length ofConsidering this build-up of gas bubbles 12,800 kilometers (km) have beenat pipeline high points is of major installed so far.importance in order to understand the hydraulic mechanisms at work. This CSG project engineering includes theeffect is further amplified by the hydraulic modeling of both gas andcommon practice of oversizing water water gathering systems. This is togathering pipelines for the purpose of identify the optimum pipe networkaccommodating new future wellheads. configuration required to ensure a safeThis practice causes relatively low system at the best economic return forpipeline velocities and hence reduces the investment.any gas sweeping effect at high points. The challenge In general, the volumes of gas Hydraulic modeling of gas gatheringaccumulated at these high points are systems has been very well understoodcontrolled by two principal HPV venting in recent years. However, waterscenarios: (A) HPVs working correctly gathering systems create some additionand (B) HPVs out of service. challenges, especially when located in hilly terrain. In particular, downhillDownhill water flow hydraulicsrunning water gathering systems oftenTo demonstrate venting scenario A include gravity flow sections where HPVs working correctly, steep downhill water column separation occurs. In suchsections are shown in the pipeline model an event the laws of internal fluid flow(figure 2). The vertical elevation (high no longer apply.point to high point) is well in excess of 10 meters (m). Water column separation As pressure and temperate decline in thewill occur in the event of the nodal high water gathering pipes, gas drops out ofpoint pressure being below 0 kilopascals solution and forms small bubbles. These(kPa) gauge as HPVs will start admitting gas bubbles are swept along by theair into line. However, some HPVs may water flow and collect at high points incontain a non-return function, causing the water line where the gas is eitherthe water to vaporise in the downhill vented to atmosphere by high pointsection as it cannot be depressurized vents (HPV) or transferred into thebelow -95 kPa at ambient temperature. nearby gas line.Alternatively, the downhill section may fill with gas until it reaches HPV trigger The process of CSG extraction ispressure of about 20 kPa gauge. Any generally achieved in three phases asfurther gas coming in will be vented at shown in figure 1. Sometimes athis pressure. significant volume of water has to be removed from the coal seam before theVenting scenario BHPVs out of service gas starts flowing at the intendedis illustrated in figure 3. In this scenario commercial rate. Also, further wellsthe incoming gas is not vented but may need to be added at a later date incontinuously accumulates thus pushing order to make up for the declining ratedown the water column in the downhill of gas production within a particularleg until the gas bubble reaches the gathering zone.bottom of the bend. Any further gas 49'