Simulation Assessment of N2/CO2 Contact Volume in Coal and Its Impact on Outcrop Seepage in N2/CO2 Injection for Enhanced Coalbed Methane Recovery

摘要

Methane (CH4) production from coal beds can be enhanced by injection of either carbon dioxide (CO2) to displace or nitrogen (N2) to strip methane from coal and accelerate methane production at sustained or increased pressures. Coal has the capacity to hold considerably more CO2 than either methane or nitrogen in the adsorbed state. However, the actual field performance of enhanced methane recovery processes is largely dictated by how effectively injected gases contact and interact with coalbeds over the active project lifetime. By history matching of the nitrogen breakthrough time and nitrogen cuts in BP’s Tiffany Unit, simulation revealed that the elevated pressure affected by N2 injection caused the coal fractures on the preferred permeability trends not only to expand but also to extend from injectors to producers. The early N2 breakthrough and high N2 cut indicated that the injected N2 may only contact a small portion of the total available pay. Methane seepage has already been observed from many locations along the north and west Fruitland outcrops in San Juan Basin. The concern is that injected N2/CO2 could likely follow the methane seepage paths and leak from the outcrops. Based on the geological setting of the Fruitland coal outcrop, a representative seepage model was used to simulate the effects of N2/CO2 contact volume (net pay interval) in coal and the injection distance from the outcrop on methane and N2/CO2 seepages. Under certain conditions, simulation predicted that a large volume of methane and N2/CO2 breakthrough could occur if the N2/CO2 injection wells are too close to the outcrop (within 2 miles).