Optimal Placement of Swellable Packers
Coal seam gas wells typically intercept multiple coal formations. Within each geological formation (ie Walloon Coal Measures in the Surat Basin) there are multiple ‘seams’ of coal. Between each seam is other material, often sand or clay, which is undesirable to be exposed to the production casing because of solids production. Solids production results in damage to artificial lift systems as well as clogging the coal face reducing the well performance.
Ideally, only the coal seams should be exposed to the production casing and all non-coal regions isolated. One completion type used to achieve this are open-hole slotted liner wells. In this completion, prefabricated casing (steel pipes) are run into the well. Pre-perforated casing is aligned to the coals which should be exposed, and these regions are isolated from the interburden regions using swellable packers. A swellable packer is essentially a sleeve that is slid onto the casing and inflated after the well has been completed which creates a seal against the well wall.
While it is ideal to isolate all interburden from the coal this is not practical. Instead competing constraints must be balanced when determining which regions to expose and which to isolate. The key considerations are:
- How many isolation zones to use, and in turn how many swellable packers are required? Fewer zones will also typically reduce the casing part count by increasing the mean casing length.
- What is the optimal balance between coal exposure and interburden isolated? Interburden isolated and coal exposed are competing constraints which must be balanced depending on the value placed on each.
- What is the optimal placement of each isolation zone (which coals to group together)?
These design choices are important because they can have a big impact on the performance of the well from solids production perspective as well as the cost to complete the well (using more parts increases rig time and the material cost).
For example, with the plot below there is a big improvement in isolation by reducing coal exposure from 100% to 95% and a reduction in the benefit of a further 5% decrease in coal exposure from 95% to 90%. Based on this, 95% coal exposure is selected (understanding of the effects of coal exposure and interburden isolation on long term well performance come from reservoir modelling studies as well as experience from existing wells – consequently it may be the case anything below 100% coal exposure is unacceptable in some regions while in others it can be ok to not expose all coals). There is reducing marginal gain in isolation by increasing the zone count beyond 8, and accordingly select 8 coal exposure zones. This balances the significant long-term benefit of reduced solids production from improved isolation with the short term cost incentive to reduce the well completion cost (once again this trade-off can be driven by economic analysis when costs are attached to the each piece of casing run and solids production). This set of isolation intervals can then be used to specify the casing tally (order of casing parts to be run into the well).
Endla helps coal seam gas operators assess this trade-off by generating this analysis from the geologist’s coal picks with an easy to use tool included as part of AlphaTally. The output allows for a quick and accurate assessment as well as consistent performance well to well, another step closer to the well production line model.