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An operating company exploring for and producing CBM from vertical wells in the eastern United States has increased its anticipated 5-year cumulative CBM production by 40%, and its estimated ultimate recovery (EUR) by 57% in three wells selected for a pilot study. The production improvement was observed in a pilot project of three CBM wells that were in the early development phase of the CBM well lifecycle. The five phases of CBM wells are (1) regional resources reconnaissance (2) local asset evaluation, (3) early development, (4) mature development and (5) declining production.
In the field trials, the service company employed a remedial stimulation service (RSS) that provides a backflush to help remove particulate damage while treating formation particulates (coal, shales, clays) to restrict their mobility. Chemicals included in the treating solution initially act as "clot-busters," breaking apart the internal particle bridges and agglomerates of coal fines and precipitates, then act as "clot-formers," imparting a tacky surface to the coal particle surfaces. Coal particles then adhere to each other, and the clots adhere to formation features and proppant grains away from the fluid flowpaths. This process, for which a patent is pending, results in a highly conductive flowpath from the coal matrix to the fracture and well bore, and significantly delays re-plugging.
In CBM reservoirs, attaining maximum differential pressure from the coal surrounding the well bore is key to effective drainage of methane through desorption.
Figures 1 through 3 show the production improvement results seen in the three trial wells. Because field trials showed the new potential of the Phase 3 field, the operator was able to upgrade its asset. Further, the operating company is expanding its acreage position to exploit the new-found production potential provided by the RSS.
All three production graphs in Figures 1 through 3, illustrating the July 2003 to June 2004 period, show significant upturn in methane production following treatments in the March-May period. The increase began after the wells were brought back to production after remedial stimulation.
How it works
Figure 4 is photomicrograph that illustrates blockage formed by clots of migrating coal fines within the propped fracture. The fines are carried toward the well bore during CBM production. To remediate the damage and prepare the well for a longer productive lifespan, the operator pumps the low-viscosity treatment fluid into the damaged fractures, breaking down the clots of coal fines and displacing the blockage away from the central flow paths within the fracture system. The well is shut-in to allow the chemical process to perform its job of locking the fines in place, preventing them from re-bridging and infiltrating the proppant flow paths. Some agglomerated fines will adhere to proppant and others to the formation surfaces.
Other key functions of the RSS chemistry are to (1) degrade residual organic polymers, and (2) dissolve in-situ geochemical precipitates or carbonate scales that may be contributing collectively to premature production declines.
Figure 4 illustrates the tendency of coal fines to collect in pore spaces; eventually, such plugging may result in damage to permeability and conductivity. The post-treatment view in Figure 5 shows fines segregated, stuck together in large groupings, and immobilized on proppant surfaces. Pore spaces are not plugged by the immobilized fines.
Since the late 1990s, the production company has operated a 125-well field where production rates range from near zero to about 350 Mcf/day. After reviewing the results of the RSS in western US CBM basins, the company investigated the potential of applying RSS techniques to its fields. A new on-the-fly delivery process, improved chemistry and zonal isolation techniques were designed to increase process efficiency. A key to success for this project was to match the new remedial solution to the challenging economics involved in boosting production without drilling new wells, re-fracturing, or applying other capital-intensive options.
After review of production response, field geology, well completions and placement options, the operator decided to try the RSS, which was designed to provide the option of on-the-fly or batch-mixing processes. The chemical formulation in the RSS is designed for treating CBM wells in either a remedial post-fracture mode or in conjunction with a primary well-stimulation treatment.
Due to the first trial focus of the technology in the region in addition to a trial of the new chemistry in these coals, the batch-mix option was selected for the initial proof of concept treatments.
One of the trial objectives was to test two different methods for fluid placement in the multi-seam completions where up to 25 coal seams had been perforated and hydraulically fractured. One method used was to apply treatments down the backside, i.e. down the tubing/casing annulus, and back up the tubing, with no seam isolation. This approach, although low in cost, was not expected to return a significant production increase. However, it was used in one pilot well to establish its capability.
Figure 6 illustrates the annular-application method. Note that most of the treatment fluid goes into the lower seams because the only pressure applied is the hydrostatic pressure from the fluid column; more of the pressure is applied against the lower coal seams.
Wells stimulated by this cheaper annular method yielded production increases of only 3% to 10%. Payout was 6 months of production. Although this was a positive outcome, results of using the isolation treatments were more successful from an economic-result viewpoint.
The second method evaluated included in the treatment package was one that required use of a workover rig. This technique was expected to produce more effective results, since seam packages were isolated to help ensure treatment fluid was placed where needed. This approach allowed the operator the opportunity to achieve optimal treatment performance while adapting the RSS process to the challenge of multi-seam completions.
Figure 7 shows schematically how the RSS is applied to a multi-seam CBM well. The method requires removal of the pump jack and use of a workover rig to remove production tubing and associated equipment. The many coal seams are grouped (three to five seams per group) for treatment, with the lower group treated first, the second group of seams treated second, and so on. In the illustration, the lower group of three seams has been treated and a bridge plug installed above the group to provide a new "bottom" to the well. A treating packer is set above the second group of seams, which are now being stimulated by the RSS. This process is continued from bottom up until all seams are treated.
Production improvement on the three-well pilot program resulted in payouts of about 3 months, despite the added expense of using a workover rig.