This paper presents the development of a coalbed methane reservoir description from the analysis of production and pressure transient data for a producing open hole cavity well and two observation wells completed in the Fruitland Formation in the San Juan Basin, Colorado. Data collected and evaluated include open hole drill stem tests and a post-fracture pressure buildup test for the observation wells and multi-well interference tests and pre- and post-cavitation pressure buildup tests and production data for the cavity completed well. The main objective of the work presented in this paper was to gain a better understanding of the mechanisms controlling gas production from open hole cavity completion wells.
The recent emphasis on evaluating the productivity of open hole cavity completion techniques relative to the productivity of cased hole hydraulically fractured completion techniques makes this reservoir characterization important. A good reservoir description may help quantify an increase in the effective permeability, if any, of the coal natural fracture system and help us to understand the reservoir conditions where cavitation may be a suitable stimulation technique for coalbed methane reservoirs. Suitable reservoir conditions may include high permeability, high pressure or a combination of the two. On the basis of the data analyzed, we found that the open hole cavity completion appears to be characterized best with a reservoir description where an altered zone of higher permeability is present around the radius of the cavity to account for the increase in gas production. We will demonstrate that our reservoir description is reasonable by showing a history match of pressure transient and production (gas and water) data both prior to and after the creation of the cavity. We determined the permeability anisotropy of the reservoir natural fracture or cleat system through the analysis of the pre-cavitation interference tests. The geometry and magnitude of the altered zone were determined by matching the postcavitation pressure profiles of the observation wells. We verified the reservoir description by comparing a production forecast based on this description with observed long term data. This study provides a reservoir description that illustrates how a cavity completion can dramatically improve the productivity of a coalbead methane well.