Shallow coalbed methane (CBM) projects can have multiple pay zones available for completion. Cased-hole completions allow zonal isolation to direct treatments, but re-establishing a connection to a coalseam through drilling and cementing damage may require high breakdown pressures even with an acid spearhead. Isolating individual zones to help ensure completion effectiveness is paramount to a successful completion.
This paper details the use of composite epoxy-glass fracturing plugs to reduce time to complete, drill out, and clean up wells drilled in the Southern Illinois Basin (SIB) project. Over 80 wells have been completed in multiple stages using as many as three composite frac plugs to isolate four coals. Accurate placement of plugs by electric line to allow staging as close as 10 ft was accomplished without communication. These four-stage frac completions were accomplished in less than 8 hours before flowing back frac fluids from all zones. Less than a single day was required to drill out all the plugs, go to bottom, circulate clean, and prepare the wells to begin pumping. The authors compare this technique with the cost and time required to do completions with alternative isolation methods.
BPI Energy holds 100% of the CBM rights to 10,000 net acres in Saline County, Illinois. Through a Structured Resource Evaluation Plan (Rodvelt and Oestreich, 2005), pilot wells drilled in early 2004 led to a 30-well development followed by 56 additional development wells in 2005 and early 2006. During this development, completions expanded to include the Dekoven, Davis, Mt. Rorah, and Murphysboro coalseams. Each of these seams had prior testing for gas content and permeability indicating the capability to flow fair amounts of gas if adequately fracture stimulated. Early radioactive tracer work had shown the necessity to isolate the thicker seams into individual stages to successfully place proppant in them. If a coalseam was not accessed during the stimulation, production results reflected the lack of fracture stimulation first in water production and later in gas-production levels.
As project development moved from two stages in the Dekoven and Davis coals to multiple stages that included the Murphysboro and Mt. Rorah seams, the time on location of the perforating and fracturing equipment increased. The desire was to complete all fracturing operations in a single day, begin flowback of the treatment fluids, and prepare the well for rod-pump production in less than 3 days. Post-frac cleanup of the injected fluids was a cause of concern because water and borate-crosslinked fluids were the fluid systems being used to place the proppant. Previous authors, Eberhard et al., found that leaving completion fluids in an interval for an extended period of time reduced hydraulic fracture efficiency by 35 to 40% (Eberhard, et al., 2003). Given the shallow depth (400 to 900 ft) and multiple zones, the ability to drill out cast-iron equipment in a timely fashion appeared unlikely. Guoynes et al., documented the increased difficulty one operator had with multiple, cast-iron plug removal, which led to the development of the composite frac plug (COFP) for CBM multistage completions depicted in Fig. 1 (Guoynes, et al., 1998).
Fig. 2 displays a map of the BPI properties and location of the SIB project. Initially 43,000 acres, drilling commitments and a renegotiated lease resulted in a net 10,000-acre block with pipeline infrastructure to gather water and gas, and a compression site with commercial pipeline access. Commercial sales began in January 2006 with gas sales increasing to 88.8 MMcf in the last 6 months of the year (BPI Press Release, 2007). Nearly all this acreage is overlain by an underground mine void in the Springfield seam.
Fig. 3 (type log cropped) depicts the log tracks—Gamma, Density, and Resistivity—for a typical well in the SIB project. The first coal seen in this log is the Springfield, followed by the Houchin Creek, Survant, Colchester, Dekoven, Davis, Wise Ridge, Mt. Rorah, and Murphysboro. All the intervals show clean gamma sections that correspond to the low density of the coals and usually a higher resistivity except for the Colchester, which contains carbonaceous shale. Thickness measurements can be taken from a high-resolution density log and correlated to a cased-hole log for perforating via the gamma signatures. An operator can use the resistivity log to correlate if the hole is washed out and density value erroneous.