The coal seam gas (CSG) developments of the Surat Basin in Australia are a world-first in CSG-to-LNG.Methane is extracted from thinly-bedded, shallow Jurassic coals by de-watering the coal fracture network to promote desorption of gas from the coal matrix. Drill stem tests (DSTs) or wireline conveyed tests (MDTs/FRTs) are conducted to collect permeability and pressure dataas part of specialised data acquisition in newly drilled wells. Reservoir depletion in the various coal layers is currently measured bydedicated pressure monitor wells located offset to producing development wells; however, the number of these wells is limited. This paper presents a new, inexpensive approach tomeasure seam-level reservoir pressure depletion using Packer Inflation Bleed Off Tests (PIBOTs).

Using an existing dual packer drill stem test (DST) tool, formation intervals as small as 1.5 metres can be isolated, allowing individual coal seam measurements to be made. The packer inflation bleed off test (PIBOT) uses the overbalance of mudweight to drive the equilibration and measurement of reservoir pressure between the packers. Supercharging has not been observed in these high permeability coals and no fluid is produced or injected during a PIBOT, which significantly reduces the rig time required. The high permeability in much of the Surat coal allowstypical pressure stabilisation times of 30-60 minutes, meaning that up to 14 intervals in a given well can be tested in a single rig crew shift.

As part of an infill drilling program in areas offset to existing production, QGC has conducted several PIBOT surveys to date. The significant advantage of PIBOTs over DSTs is the reduced cost and rig time. In the first campaign, 81 PIBOTs (9 wells) were performed at the same cost as 12 DSTs (4 wells). The opportunity to collect this increased number of reservoir pressure data points for characterisation brings significant value. The results have confirmed, with high confidence,that significant differential depletion has occurred between the upper and lower seam layers in producing areas of the Walloons and the consistency of these depletion patterns across very large areas. The results of these novel formation testshave implications for gas recovery estimates, and thelocation and design of future wells. The reservoir model can also be validated at a more granular level by data that captures the non-virgin conditions at a distance from producing areas.

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