What Is Shale Natural Gas and How Is It Different?
Natural gas mined from unconventional and conventional reservoirs are one in the same. Unconventional gas doesn’t refer to the type of gas itself, but instead the source rocks that create the gas found in these reservoirs. So what is the difference between shale natural gas, conventional natural gas, and unconventional gas?
Conventional Natural Gas
Conventional gas and oil refers to the crude oil, or petroleum, and raw natural gas that can be extracted from the ground through conventional methods and means.
Natural gas is a naturally occurring hydrocarbon gas that has formed through the burying of dead animals and plants over thousands of years. The intense pressure and heat caused through the decay of these materials under the Earth’s surface trigger a reaction that leads to the formation of natural gas – primarily CH4 (methane).
Conventional natural gas is obtained from porous sandstone reservoir formations that are capped by impermeable rock. Gas is typically trapped here by buoyancy. Without needing to pump, natural gas found in these reservoirs can rise to the surface through gas wells.
In the oil and gas industry, natural gas is known as a conventional gas. This gas is sourced through conventional resources. This means that it can be extracted from the Earth through pumping mechanisms (CAPP). This occurs simply by the naturally occurring well pressure and compression operations.
Following the depletion of maturing fields, the natural pressure found in the wells may become too low to produce significant natural gas quantities. In this case, various techniques can be used to boost production, such as depletion compression or water and gas injections. However, these natural gas and oil fields are still considered conventional resources in the face of these methods.
Beyond classical extraction methods, artificial lift and enhanced oil and gas recoveries are classified as unconventional. Unconventional sources are coal bed methane, tight gas, and shale natural gas, all of which require novel technologies in order to source.
Unconventional Natural Gas
In spite of decades of use and extraction, the overall size of US natural gas resources estimations have remained steadily on the rise since the early 90’s. This number has been largely encouraged by the increase in feasibly extracting gas discovered in unconventional deposits.
Unconventional gas is natural gas that is found in very dense rocks with low levels of permeability. This prevents gas from flowing through wells in commercial levels of volume. Unconventional natural gas typically requires the use of hydraulic fracturing. This fracturing extracts the gas resource and improves reservoir permeability in commercial quantities.
Unconventional gases were previously more costly and difficult to manipulate than conventional deposits. These unconventional methods includes methane hydrates, coal-bed methane, tight gas, and shale natural gas. That was until recently of course.
Such unconventional sources could aid in removing the growing gap between US consumption and domestic production, although they do pose greater challenges environmentally in their product methods.
The very first well in the US drilled specifically for the production of natural gas, entered a shale deposit located in New York in 1825. Due to the shale’s lower permeability however, convention extraction methods were not as cost-effected. Deposits that could be more easily exploited were sought elsewhere.
In this day in age, shale gas is the fasted growing natural gas resource throughout the entire world. Recent developments in horizontal drilling technology have allowed single wells to pass shale natural gas reservoir in larger volumes. This allows for production of larger amounts of gas.
The advancement of fracking, also known as hydrofracking and hydrofracturing, or hydraulic fracturing technology, has vastly improved the accessibility of shale gas deposits.
Gathering Shale Gas
The process itself requires the injection of larger volumes of water mixed together with fluid chemicals and sand, into drilling wells at higher pressures. This fractures the rock, increasing overall permeability and production rates.
The first step to extracting natural shale gas involves the drilling of a production well vertically down into the ground. It drills until it reaches it’s designated shale formation. At this point, the wellbore will turn in order to pursue the shale at a horizontal angle. Casing, or steel tubing, is then inserted into the well. This protects the wellbore’s integrity and keeps it open. Then, cement is pumped down into the well forcing the outside steel to move up and seal the well. This works to prevent produced water, chemical, fracking fluids and natural gas from leaking into surrounding groundwater supplies.
Once casing and drilling is complete, small explosives are detonated into the well’s horizontal portion. This creates periodic holes within the casing where hydrofracking will occur. During the fracking operation, fluid is pumped in at a pressure that is carefully controlled. This effort is to fracture rock located several hundred feet away from the well.
Sand that has been mixed with these fracturing fluids work to prop cracks open while fluids are pumped out. Following fracturing efforts, gas flows into the well bore and lifts to the surface where it is then collected.
Natural Gas Today
Following these technical advances in the unconventional natural gas sector, higher natural gas prices between the years 2001 and 2008 have provided a greater incentive for engineers to develop a reliable shale natural gas resource.
However, recent increases in shale gas production paired with the economic recession have lent a hand to a dramatic decline of gas prices since 2008. According to the World Shale Resource Assessment 2015 report, it is estimated that the US holds nearly 623 trillion cubic feet of verifiable shale reserves that are recoverable within current operations and economic conditions. These have been mostly sourced in Oklahoma, Pennsylvania, Arkansas, Louisiana, and Texas. These natural deposits are expected to be located throughout the United States, generally close to where conventional gas resources have been found.
Conclusion: Does shale natural gas have a greater environmental impact?
While fracking, or unconventional shale gas extraction methods can sound pretty scary in comparison to conventional production methods we used for hundreds of years, are their objections that much different than its conventional counterpart? The integrity of wellbore is as much as a concern in conventional methods as it is for shale. The dangers of shale natural gas are primarily associated with incremental increases in well numbers. However, they do not have an affect to related natural gas infrastructure – processing facilities, pipelines, etc.
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