The domestic United States has more than 30,000 coalbed-methane (CBM) wells drilled and producing. The majority of these are vertical, producing wells that have been completed with hydraulic fracturing to access coal-seam gas. Mature CBM fields can still provide an opportunity to deliver more gas production. While incomplete primary-stimulation and additional-pay intervals can be easily identified as candidates, wells with scaling issues, fines invasion, or proppant production can be more difficult to ascertain. Determining the best recompletion solution requires an integrated review of the well history from its construction through completion as well as each well's production profile.
This paper describes the methodology used for determining which wells to recomplete in a 30-well program in 2007. Four types of recompletions were established with at least three wells in each category, allowing comparison of different recompletions methods. A diagnostic approach is described that determined the proper course of action as field work confirmed the well problem. After at least 12 months of post-treatment production, this project was deemed a success with an uplift greater than 10 Mcf/day on 63% of the wells and 403 MMcf in additional gas sold to market.
The operating company has approximately 2,100 (CNX Public Website 2009) producing CBM wells in the Oakwood field of Virginia (Fig. 1), with the majority having been hydraulically fractured using multiple-stage techniques. With the earliest well completed in October 1984 (Thakur 2009), well histories are available for review to determine decline rates, workover events, and production issues. New well drilling has continued to add 176, 253, and 294 wells in 2005, 2006, and 2007, respectively, as the company expanded their drilling program to exploit an increased asset position in Virginia.
In January 2007, an intercompany team was tasked with finding 30 recompletion candidates that would be funded by capital expenditure to be completed and put back on production by July 31, 2007. This task included identifying wells that would be completed with four different procedures to compare production results and economic impact. After review, additional recompletion candidates would be implemented using the most cost-effective approach determined from the 30-well studies.
Methane production in the Oakwood field is obtained by fracture stimulating 15 to 25 coal seams with four to six stage treatments. Fig. 2 illustrates the stratigraphic column for the Buchanan County, Virginia area (Rodvelt et al. 2001). The Pocahontas No. 3 is typically the lowermost coal, four to six ft thick, and makes up the first frac stage. It is of the gassiest coal in Appalachia, with gas content between 400 to 600 cubic feet per ton. Where it is mined, either steel production casing is set to the top with a packer shoe, or fiberglass casing is used across the mine interval. Additional frac stages are made up of three to seven thin seams in 200 to 250-ft intervals, until all the coal seams have been accessed through the Greasy Creek. Completions are comprised of four or more frac stages using two ball-and-baffle pairs to isolate the first two stages and composite frac plugs (Guoynes et al. 1998) for additional stages above. Initial completions were performed with water and sand fracs. The predominant frac fluid currently used is 70%-quality nitrogen foam. Sand volumes vary from 3,000 lbm/net foot of coal up to 10,000 lbm/net foot, with an average of 5,000 lbm/net foot using 20/40-mesh brown sand for the majority of the treatment, followed by 5,000 lbm of 12/20-mesh brown sand tail-in on each frac stage.