Deliverability of coal wells, like conventional gas wells, depends on bottomhole flowing pressure. Because coal wells often produce both gas and water, lowering bottomhole flowing pressure to increase gas rate also increases water rate. Thus, optimization of coal well profitability entails balancing gas revenues and water disposal costs. The present study was undertaken to determine if the relation between coal well bottomhole flowing pressure and gas and water production rates could be described by Vogel's Inflow Performance Relation (IPR). First, simulation studies were done to test the applicability of Vogel's IPR to coal wells. Secondly, productivity of actual coal wells was compared with Vogel's IPR curves.

Productivity of an oil well draining a solution-gas drive reservoir was investigated by Vogel1  using numerical simulation. A total of 21 simulations covering a wide range of oil, PVT properties, and relative permeabilities were made. By using dimensionless pressures and rates, Vogel found well productivity could be described by


where q is oil production rate in bpd, qmix is maximum oil production rate in bpd, pwf is bottomhole flowing pressure in psia, and pavg is average reservoir pressure in psia. Eq. (1), called Vogel's Inflow Performance Relation (IPR), was found to describe simulated well productivity with a typical accuracy of 10%. Errors as high as 20% were noted for simulations of viscous crudes and/or damaged wells with skin factors great than +5. Over the last quarter century, Vogel's IPR curve has been extensively used to predict oil well performance. Because of this success, the question arose as to whether Vogel's IPR could also describe gas and water production from a coal well.

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