|Fig. 1: Shale and Oil gas plays for the Lower 48 States.  (Courtesy of the U.S. Department of Energy)|
Natural gas is changing the US energy market drastically. The increasing technological advancements in the field of shale gas recovery have meant that this fossil fuel is now the future of power generation in the United States. The US is eager to seek energy independence in the short and long run, and the recovery of shale gas will be critical to reaching this goal.
Shale gas is a form of natural gas, a fossil fuel, which is trapped within shale formation, making it much more difficult to reach than conventional natural gas. This type of natural gas is often referred to as 'unconventional' natural gas and according to the EIA," 'Unconventional' natural gas does not exist in...conventional reservoirs - rather, this natural gas takes another form, or is present in a peculiar formation that makes its extraction quite different from conventional resources." 
Shale gas exists in large quantities in the United States, approximately 97,000 BCF, according to the U.S energy information agency (EIA), and this number continues to grow as new reservoirs are discovered. It is significant to note that "Implications of increased use of shale gas will not be the same in all regions. In the short to medium term, however, no other region is likely to be able to emulate the success of shale gas in North America supported by its large and well-developed gas transport infra-structure and gas market structures." Shale gas plays a key role in North America particularly due to the infrastructure and capabilities in place to transport it effectively. This readiness is the reason why shale gas is much more relevant in North America and why "A widely anticipated global gas market revolution has not happened outside of the United States."  Fig. 1 shows the distribution of shale gas and oil reserves across the United States. 
The discovery and exploration of shale gas reserves in North America has led to a fall in natural gas prices "In mid-2011, natural gas prices in North America hovered around US $3.70/Mbtu, which is about 72% less than at the heights of 2008."  The US produced about 84% of all its natural gas usage in recent years, hence imports of natural gas have been scarce into the US, sub sequentially gas prices in general have fallen across the world. Looking ahead, shale gas is expected to make up about 46% of the U.S natural gas supply by 2035; this is a significant jump from the 14% make-up in 2010. 
In the past, the costs associated with fracking, which is the technique required to reach and extract shale gas, had been expensive, and unfeasible. With the development of multi-stage fracturing and horizontal drilling the feasibility of the extraction process and the cost effectiveness have been made attainable. This advancement in research and technology is the main reason that shale gas has become a viable energy source for the future.
As the focus on alternative fuel continues, and unconventional resources are thrust into the limelight of the energy world, many obstacles are still faced as we continue to learn about the properties of gas in tight locations and under high pressure.
Several challenges are currently faced to better understand the physics and flow behavior of shale gas. The tight nature of shale gas governed by an extremely low permeability in the range of nanodarcy and a low porosity poses a difficulty in conducting lab measurements and replicating the setting in a controlled enviroment. In order to be able to identify reserves of shale gas and make accurate estimations of these variables it is essential to understand the behavior of the gas. Measuring porosity and permeability are key variables required to estimate gas reserves and extraction requirements.
The conventional way for measuring permeability across a core using Darcy's law is only appropriate for rocks with permeabilities that are higher than 1 millidarcy.  In the case of shale gas and tight rocks, the pulse decay technique developed by Brace is 1968 represents a good estimation of permeability. The physics of this experiment is governed by analyzing the pressure behavior across the core by emitting a pulse from a known upstream volume to a downstream volume with the core connected in between. One way of designing the experiment is to have an upstream volume with a constant pressure while the downstream volume is building up. The pressure will travel across the core and this way we will analyze the pressure build up across the sample. 
The concern is that the Brace mathematical solution only solves the one-dimensional diffusivity equation assuming a steady state liner pressure gradient across the sample.  Also, sorption effects are not captured through this method which could account for an increased permeability due to the storage capacity in the minerals and organic matter within the shale. Therefore, an accurate analytical solution accounting for the heterogeneity and storage capacity of shale rocks is necessary to better understand the flow behavior and estimate the permeability of shale.
Porosity is also an important variable when considering the availability and extraction needs of shale gas, especially when it comes to estimating the original gas in place. An experiment can be conducted utilizing Boyle's law with P1V1 = P2V2 to estimate the pore volume in the rock. Typically, helium is the gas used to measure porosity. However, it is worth noting that helium has a smaller molecular size compared to methane and hence could access more pore throats within the shale rock. This could lead to an overestimation in porosity value, which directly increases the reserves estimation by a significant number.
The difficulty in accurately measuring permeability and porosity of gas in shale formations create barriers to estimating the true amount of reserves available in North America, and around the world. As the technology and experimental techniques continue to improve, we can expect that more shale gas and other unconventional energy sources can be discovered, and with the understanding of how the gas behaves under high pressure, better fracking and horizontal techniques can be developed to reach the reservoirs.
Shale gas will continue to be a vital source of energy not only in the US but across the globe. With the drop of prices seen in recent years, the fuel is becoming more important for different industries looking to reduce their operating costs. However, there are still a lot of challenges currently being faced in the industry and within the academic world to better understand the physics and flow behavior of shale gas. Investments in researching and better understanding the physics of shale gas will bring us closer to accessing the worldwide reserves. Proper understanding of the petrophysical properties of shale gas will be essential for future field development, production forecasts, and reserve estimations. The US is likely to be at the forefront of this growth, due to its developed gas transportation infrastructure and storage readiness, but eventually the recovery of shale gas will be a priority for Europe and the rest of the world, where potential reserves are still untapped.
© Hamza Aljamaan. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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