This paper discusses remedial hydraulic stimulations used to increase production rates from two coalbed methane wells at GRI's Rock Creek project. The poorly performing stimulation was plugged with cement and the well restimulated using a non-damaging gel, nitrogen foam, and sand stimulation. These treatments suggest:
cement can be used to abandon undesirable hydraulic fractures prior to restimulation,
the use of an alternative gel polymer, identified by core testing for a specific coal sample, minimized permeability damage during stimulation, and
a special foam and sand stimulation created a more effective fracture than the original water and sand treatment in the same well.
Two remedial hydraulic stimulations significantly increased production rates from coalbed methane wells at GRI's Rock Creek project site near Birmingham, Alabama. The first treatment was performed on a well that was under performing compared to the other wells, even though reservoir properties indicated it had the potential to produce more. The second well was an average-to-good well, but was restimulated with the intent of increasing fracture length and production, and to further investigate the effectiveness of a novel restimulation technique. Incremental production rate increases ranged from over 300 MCFD (8495 m3/D) in the first well to sustained incremental rates greater than 90 MCFD (2550 m3/D) in the second well.
Because both wells had already produced gas for more than two years, R was felt that a foamed fluid should be used to minimize the addition of water to the reservoir. Because potential damage to the formation with gelled fluids was identified in the literature, particular attention was paid to designing a system that would not damage the coal. Prior to implementing a recompletion strategy, it was imperative that the potential problem causing poor production be diagnosed and a strategy developed for logically increasing the rate.
The relative location of the wells at the project site is shown in Figure 1. Details of the project are described elsewhere. Figure 2 compares production from well P3 to surrounding wells. After 3.5 years of production following the original Mary Lee and Black Creek coal group stimulations in October 1987, well P3 still only produced at one-third the gas rate and two-thirds the water rate of four offset wells. All zones were stimulated with various types of cross-linked hydroxypropyl guar (HPG) gel systems designed to get long fracture length and contact multiple production horizons.
A notable difference between P3 and adjacent wells is the configuration of perforated seams (Figure 3). The 8 Black Creek seams in P3, depths of 1180 to 1420 feet (360 to 432 m), were stimulated with 3 separate treatments by the baffle frac method, followed by a Mary Lee group stimulation at 1028 ft (313 m). The Black Creek group stimulations in other wells had only the bottom Black Creek H seam perforated, called restricted access. Production rates from both coal groups were higher in the wells without perforations in upper Black Creek seams.