Forecasting long-term gas production from shale.

Oil and natural gas from deep shale formations are transforming the United States economy and its energy outlook. Back in 2005, the US Energy Information Administration published projections of United States natural gas supply that stressed the need to develop an import infrastructure (1): by 2025, imports would account for almost one-third of United States consumption. When we compare those forecasts with the current ones to 2040 (Fig. 1) (2), it is inevitable to feel that a disruptive technology has emerged since. Natural gas consumption is expected to increase significantly over the next three decades, with strong demand growth from the electricity generation and industrial sectors. However, the United States will probably become a net exporter of gas before 2020, increasing domestic production by 44% over the projection period. The trigger for this revolution has been the large-scale deployment of a set of synergetic technologies that allows us to produce oil, and especially natural gas, from mudrock formations that were considered unproductive just a few years ago. Gas shales are tightly packed, fine-grained sedimentary rocks. Hydrocarbons form within these rocks and remain trapped in their pore space because of their ultralow permeability. Unlike conventional oil and gas reservoirs, which are highly concentrated geographically, shale formations are common around the world. The potential abundance of shale gas resources worldwide—and the fact that burning natural gas emits less CO2 and atmospheric pollutants than other fossil fuels—has created the expectation of a golden age of natural gas in the global energy system (3). Estimates of long-term production and technically recoverable resources are, however, highly uncertain (3–5). The fundamental mechanisms controlling shale gas extraction remain poorly understood, and the classic theories and simulation techniques used by the oil and gas industry have proven inadequate for shales (6, 7). In PNAS, Patzek et al. (8) make an important contribution toward reducing uncertainty and unraveling the physical mechanisms behind gas recovery for tight shale formations. Current shale gas production relies on two quickly evolving technologies: horizontal drilling and hydraulic fracturing. Horizontal drilling enhances the spatial access to the hydrocarbon resource by increasing the length of a single well within the gas-bearing shale. Hydraulic fracturing, or “fracking” (9, 10), provides reservoir stimulation via injection of fluids, granular suspensions (proppant), and chemicals at high pressure, sufficient to fracture the rock and enhance its permeability. Flow through shale poses a distinctive challenge that is not present in classic oil and gas applications: Pore throats in shale have typical widths in the order of a few nanometers, and are rich in organic material (kerogen) with adsorbed gas (11– 13). At these scales, the pore size is comparable to the mean free path of the gas molecules, and the Navier–Stokes equations with no-slip boundary condition do not properly represent the flow (11). Much experimental, theoretical, and computational work is still needed to understand the rock-fracturing process and the multiphase flow process that ensues. Shale gas projects have attracted vigorous opposition, and shale gas is rapidly emerging as a major social dilemma (14). Although the potential economic benefits are already fostering exploration outside the United States, it is unclear that these technologies will be deployed at a large scale worldwide unless uncertainty about the environmental impact of current recovery methods is reduced. There is a need for better understanding of the conditions under which shale operations lead to gas venting (15), to contamination of groundwater by methane or fracking fluids (16, 17), and their potential impact