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ure A7 k e sure font is large enough to read at scale used. Fossil Fuel and Geothermal Energy Sources for Local Use in Alaska Figure A7. Comparison of organic contents of shales, coals, and tight sands (modified from Hartman, 2008). Chapter A, Introduction as coals, and typically contain less than 50 wt% (weight percent) of organic matter (fig. A7). Once gas is generated in a shale source rock, some of the gas is stored in the rock’s pore systems and some becomes attached to the surface of mineral particles comprising the shale in a process referred to as ‘adsorption.’ The latter gas is said to be adsorbed on the rock matrix. Refer to the sections describing the geologic requirements for exploitable conventional oil and gas, coal, and coalbed methane for more detailed explanations of source rocks, reservoir rocks, adsorption, and the conditions necessary to generate hydrocarbons. Controls on resource volume and productivity in shale gas systems are similar to those in coalbed methane systems, however, the shale gas reservoirs are typically thicker (30 to 300 feet) and have a much larger volume of free gas in pore space and much lower adsorbed gas content (Jenkins and Boyer, 2008). Whereas coalbed methane reservoirs rely on naturally-occurring orthogonal fracture sets called cleats, shale gas plays have much lower permeabilities than coalbed reservoirs (typically in the nano- to microdarcy range) and rely heavily on induced hydraulic fracturing (stimulation) to connect natural fractures to the wellbore to become gas producers. While both tight gas and shale gas reservoirs may require ‘fracking’ to maximize production, due to the extremely low natural permeabilities of shales, a special type of fracking suitable for shales is required. A technique called ‘slick-water frac’ results in maximizing the horizontal length of fractures and minimizes the vertical fracture height, allowing for much greater gas recovery from shales (Harper, 2008). As in most coalbed reservoirs, some shale-gas reservoirs are water-saturated, and require dewatering to initiate the flow of gas. As this water is produced from the natural and enhanced fracture system, the reservoir pressure declines, gas desorbs from the mineral matrix, and gas production increases. Shale gas production is similar to conventional gas reservoirs, with peak initial rates of production and slow decline thereafter as gas desorption replenishes the fracture system. As with coalbed methane, produced water must be disposed, and in Alaska’s high-latitude setting this poses significant challenges. Because shales ordinarily have insufficient permeability to allow significant fluid flow to a well bore, most shales are not commercial sources of natural gas in their natural state. Because of the low matrix permeability in shales, gas production in commercial quantities requires fractures to increase permeability. Shale gas has been produced for years from shales with natural fractures; the shale gas boom in recent years has resulted from modern technology in hydraulic fracturing to create extensive artificial fractures around well bores. In summary, shale gas reservoirs are geologically complex and because of their very low permeability (typically <0.1 md) these reservoirs require special techniques for evaluation and extraction. Thus, as in coalbed methane and tight gas reservoirs, detailed understanding of the geology of potential shale gas resource is essential. Gas Hydrate According to the U.S. Geological Survey, a gas hydrate is a naturally-occurring, ice-like solid in which water molecules trap gas molecules in a cage-like structure known as a ‘clathrate’ (fig. A8). A gas hydrate or clathrate is similar to ice, except that the crystalline structure is stabilized by the guest gas molecule in the cage of water molecules. Gas hydrates occur under a very limited range of temperature and pressure conditions, such as in the permafrost environments of the arctic, including northern Alaska. They also occur in deep marine environments at water depths greater than 400 or 500 meters (~1,300 to 1,640 feet), along most continental margins. In these environments (arctic and deep ocean) gas hydrates occur naturally where pressure, temperature, gas saturation, and local chemical conditions combine to make them stable. Before gas hydrates can form, there must be a source for gas molecules. Potential sources include sedimentary rocks that are rich in carbon, such as some black shales, limestones, and coal. Peat is a precursor to coal and can also generate gas under the right conditions. Refer to the summary of the geologic requirements for conventional oil and gas, coal, and coalbed methane for more detailed explanations on the origins of gas. Gas hydrates are currently considered to be a potentially vast, unconventional energy resource with the possibility Page 7 IntroductionPDF Image | FOSSIL FUEL AND GEOTHERMAL ENERGY SOURCES FOR LOCAL USE
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