Case History: Reservoir Analysis of the Fruitland Coals Results in Optimizing Coalbed Methane Completions in the Tiffany Area of the San Juan Basin

Abstract

TX 7508 3-3836, U.S.A., fax 01-972-952-9435. AbstractHistorically, Fruitland coal wells in the Tiffany area of the San Juan basin have to be hydraulically fractured to be economical. The need for artificial stimulation is that the Tiffany area coals are characterized by lower permeability than the coals in the San Juan basin "fairway" to the south. The post-stimulation gas and water production rates in the Tiffany area vary approximately from 50 to 250 Mcf/D and 40 to 500 BWPD respectively, but not all stimulated wells are economically successful. A study was initiated to develop a better understanding of the reservoir characteristics and completion effectiveness for the purpose of optimizing production from the Fruitland coals in the Tiffany area.Public domain 1,2 production data was used to determine the average gas and water production rates for Fruitland coal wells in the project area. This data was combined with a detailed working geologic model for use in reservoir simulation analysis. History-matching the a verage well production resulted in the identification of several critical well performance issues: 1) the top Fruitland coal is not receiving treatment with the single-stage completion, 2) the basal Fruitland coal is not being effectively completed, 3) the basal Fruitland coal and the underlying Pictured Cliff sandstone is receiving most of the fracturing treatment, and 4) water production that does not show any significant decline is probably a result of contribution from the water-wet, lowerpermeability Pictured Cliff sandstone underlying the basal Fruitland coal.Based on these significant findings, stimulation treatments have been completely revised. The top and basal Fruitland coals are now being treated separately at lower rates so that only the coals and not the water-wet, Pictured Cliff sandstone is stimulated. In addition, the perforations are now being carefully placed in the middle of the coal interval leaving one to two ft. at the top and bottom boundaries. In the wells that were orig inally stimu lated with a single-stage treatment, the top Fruitland coal is being restimulated. The results obtained from these modifications have been substantial. In the four wells that were restimulated, a minimum of 100%, to a maximum of approximately 400% increase in gas production rates were observed. Water production has decreased in most of the wells to less than 50 BWPD since only the coals are being dewatered and not the water-wet Pictured Cliff sandstone. In some of the new wells, even though there is a high initial water rate, it clearly shows a declining trend unlike the old wells completed with high rate, one-stage stimulation treatments. In addition, these new wells produce substantially higher gas rates when compared to the old wells. Specific examples f rom the project area are presented to show the effectiveness of the new stimulation procedures. Although the results of this study are specific to the Tiffany area, the methodology and techniques used can be applied effectively in other coalbed m...