【正文】 e effort to convert heavyduty trucks to run on natural gas, in part to minimize . dependence on foreign oil. Recent studies conclude that, beyond their ability to reduce local air pollution, natural gas vehicles also lower greenhouse gas emissions by roughly 25 percent pared with oil, far less than the reductions possible in power generation but significant noheless. The big question now facing energy planners is whether sufficient natural gas will be available at a petitive price to allow for significant displacement of oil in transportation and coal in power generation. The answer to that question will likely be determined in large measure by efforts to develop new sources of natural gas, which has already had profound effects on the . energy industry in recent years. II. The Unconventional Gas Revolution A newfound abundance of natural gas promises to tip the fossil fuel balance further in its favor. 山東建筑大學(xué)畢業(yè)設(shè)計(jì) 外文文獻(xiàn)及譯文 4 Gas production in the United States peaked in the early 1970s, along with oil, but in recent years technology advances have dramatically reversed the decline. Advances in horizontal drilling and hydraulic fracturing have unlocked gas resources in ―unconventional reservoirs, such as tight sands, coal bed methane, and shale rock rich in organic materials. As a result, resource estimates have increased sharply, and as lessons learned in the United States are applied to exploration and production of unconventional resources internationally, natural gas has the potential to shed the supply, price volatility, and energy security concerns that have surrounded it during the last few decades. Unconventional gas is found in lowporosity sedimentary rock formations that act as both sources and reservoirs for hydrocarbon deposits. Because of their low porosity, gas is more difficult to extract from unconventional formations than from conventional gas reservoirs, which generally contain stores of hydrocarbons that originated in other formations. But as conventional resources have been exhausted, the industry has turned its attention to new sources of gas that were previously dismissed as too difficult and expensive to extract. In the 1970s, gas producers began to develop tight sands that they had discovered in the course of exploration for conventional gas. Using hydraulic fracturing and horizontal drilling, they were able to recover gas from these resources economically, largely in the Rocky Mountain states. Since then, tight sands have grown to account for more than 30 percent of all gas production in the United States. Natural gas is also found in coal seams, where it can pose serious health and safety risks to coal miners and can, if leaked to the atmosphere, contribute to climate change. Methane is adsorbed onto the pores of the coal, which has very low porosity. This methane, which would otherwise leak into the atmosphere over time, can be extracted economically by drilling into the coal seam. Coal bed methane development, most of it at relatively shallow depths, has been expanding since 1989, starting in Alabama, New Mexico, and Colorado, and later in Utah, Virginia, and Wyoming. Total . production of coal bed methane reached almost 2 trillion cubic feet in 2020—10 percent of total . gas production. In the past few years, the focus of the gas industry has turned to a third unconventional source: deep shale formations, or nonporous sedimentary rock that mostly lies thousands of feet underground. Starting in the 1990s, independent gas producers began to develop a technique, known as hydraulic fracturing, for injecting highpressure water into these deep formations, allowing the gas to be released and brought to the surface. First deployed on a large scale in Texas’s Bart Shale, the technique has subsequently been adapted 山東建筑大學(xué)畢業(yè)設(shè)計(jì) 外文文獻(xiàn)及譯文 5 to a range of shales in other parts of the country, each of which has its own geological distinctions. When natural gas prices shot upward after 2020, the shale ―gold rush was on. The largest gasbearing shale formation, the Marcellus Shale, extends across five states from West Virginia to New York, and is attracting great attention in the northeastern region where energy prices are high and most gas is imported from over 1,000 miles away. While shale rock does not give up its methane easily, this is more than balanced by its abundance. The Potential Gas Committee, an independent authority on gas supplies based at the Colorado School of Mines, estimated potential . natural gas resources in 2020 to be 1,836 tcf, up 39 percent from 2020—with the difference due mainly to a steep increase in estimates of recoverable shale gas. Proven reserves have increased 13 percent to 238 tcf, bringing total gas resources to 2,074 tcf. Assessments by ICF International, the . Energy Information Administration (EIA), and Navigant Consulting Inc. confirm the magnitude of this resource. These figures suggest that . supplies of natural gas could last for 90 years at current rates of consumption. And some experts expect the resource estimates to continue rising as exploration proceeds and as extraction techniques are further developed. Since 1990, unconventional gas production has already increased fourfold, with an even steeper rise in the past few years contributing to a sharp decline in gas prices and a collapse in the North American market for imported liquefied natural gas. Surprisingly, the boom has only slowed marginally in the face of a steep recession and a sharp decline in the price of natural gas since 2020, suggesting that unconventional gas may be cheaper to produce than conventional gas. The breakeven price for shale gas in various . basins is reported to range from just under $3 to $ per million BTU. Notably, some of the most recent basins to be tapped, including the Marcellus, are among the least e