Understanding the detailed geology of coal deposits is essential in evaluating prospective coalbed methane fields in Alaska. These geologic evaluations are important for determining the coal rank, quality, thickness, continuity, and pertinent characteristics of the surrounding rock layers, as well as of the basin’s hydrology. The ability for coal to produce methane as a resource is governed by the following critical controls: tectonic/structural setting, deposition environment, coal thickness and distribution, coal rank, gas content, permeability, and hydrogeology. Initial data collection activities to evaluate a prospective coal basin are detailed geologic mapping, collection of coal outcrop samples for coal quality and rank measurements, and measurement of fractures or cleats within the coals. The next step is drilling test holes to collect gas content data, measure permeability, and sample for water quality.
Few of the coal basins in Alaska have had this level of detailed study to determine whether they are viable candidates for coalbed methane production. Nearly all lack data on coal seam thickness, fracture spacing, and coalbed gas content. Coal quality data are sparse. As previously mentioned, coal rank is important because it directly influences the coal’s gas storage capacity. With the exception of the North Slope, Alaska Peninsula, northern Cook Inlet, and a few other areas, many of the coal basins that underlie or are adjacent to Alaska rural communities have not yet reached a depth of burial and a level of maturity to form thermogenic coalbed methane, nor have they developed adequate fracture systems. These basins with lignite coal are not viable candidates for CBM with today’s production technology.
Status of Coalbed Methane Exploration in Alaska
There is currently no coalbed methane production or developed coalbed gas infrastructure in Alaska. Attempts to explore and economically produce coalbed gas in the northern Cook Inlet region met with limited success. In 2004, a government-funded coalbed methane exploratory hole was drilled at the community of Fort Yukon. The exploratory hole reentered a previously drilled hole in order to sample two low-rank lignite seams and collect samples to measure gas content. This followed a shallow seismic study conducted in 2001. It had determined that the lignite was laterally continuous beneath the community.
The results of the gas content measurements were disappointing, as the upper coal seam averaged 13% cubic feet per ton of gas; the lower lignite 19% cubic feet per ton. Testing indicated that the permeability of the lignite was extremely low with few fractures present within the lignitic coal seam. This project, including a preliminary seismic study and a single core test hole, cost in excess of $1.7 million. The importance of this study was that it demonstrated that low-rank lignite coals that are prevalent in many of Alaska’s coal basins are not candidates for economically viable coalbed methane production.
In 2007 the Department of the Interior (USGS and BLM) conducted exploration for coalbed methane at Wainwright, Alaska on the western North Slope. Their initial results were promising, and in 2008 they drilled a delineation well to test the lateral extent of the coal beds, as well as an array of wells for a production test. Coals in the subsurface at Wainwright are bituminous in rank and appear to have generated sufficient methane to merit continued testing in 2009. Based on the known characteristics of the western North Slope coal (optimum rank, thick and laterally continuous seams, and sufficient gas content and burial depth), this region contains the best potential for coalbed gas production in all of Alaska.
In addition to finding coal of sufficient rank and gas content, there are other, unique challenges to coalbed methane production in Alaska. Given that coal bed methane production often involves significant water production, there must be some way to dispose of the fluid, especially if it does not meet strict EPA quality standards. The biggest challenges to production and disposal of this water are cold temperatures and permafrost. Usually, produced water is either surface disposed in large evaporation ponds, surface discharged into existing bodies of water, or re-injected into deep disposal wells. Evaporation ponds are common in many Lower 48 production facilities, but they are not a plausible option for Alaska because of the long freezing-cold winters. Surface discharge of even high quality water into rivers or lakes is unlikely to be viable because these streams are frozen about 70 % of the time, and such a practice is likely to be restricted due to possible impacts on fish habitat. Downhole re-injection of produced water is also problematic because the effects of disposal in permafrost are unknown. A well bore can freeze up during pump failure and cause significant problems that include gas field shutdown. Additionally, re-injection has a danger of fluid communication between aquifers if the subsurface geology is poorly understood and the rock layers are connected, resulting in contamination of a local source of drinking water. These hurdles can be overcome, but in Alaska it is important for developers to make special considerations which may result in significant additional costs.