Shale gas

Figure 1. Shale gas drilling tower that drills into the Marcellus Shale Formation.

Shale gas is a type of natural gas that is trapped within shale formations—fine-grained sedimentary rock that are considered possible sources for raw natural gas when they contain significant quantities of natural gas. This natural gas is stored within shale formations, both in the pore spaces of the rock and adsorbed onto organic matter. Shale is a common sedimentary rock composed primarily of clay-sized mineral particles and fine-grained sediments. Shale can serve as the source rock, reservoir rock, or seal for natural gas.

In Canada, shale gas resources are found in British Columbia, Alberta, Saskatchewan, Manitoba, Ontario, Quebec, New Brunswick and Nova Scotia. Most current development and drilling in Canada occurs in British Columbia.

Exploration

Exploring for shale gas uses the same basic methods as conventional oil and natural gas exploration, combined with specialized techniques to evaluate shale formations. Two-dimensional (2D) and three-dimensional (3D) seismic surveys, which use reflected acoustic waves to image subsurface rock formations, are used to determine the depth and characteristics of shale deposits. Exploratory drilling and rock sampling are then used to evaluate the physical and chemical properties of the shale and to estimate the quality and quantity of the natural gas resource.

Drilling

Generally speaking, shale formations have low permeability, meaning fluids do not flow easily through the rock and additional techniques are required to access the trapped oil or natural gas. Because of this, techniques such as hydraulic fracturing are commonly used to produce shale gas. Since shale gas requires specialized extraction techniques beyond those used for conventional gas reservoirs, it is classified as an unconventional natural gas resource.

Two major drilling techniques are commonly used to access shale gas. The first is horizontal drilling, in which a well is drilled vertically until it reaches the target rock formation before gradually curving into a horizontal borehole. Once the target shale formation is reached, the drill bit is directed horizontally, increasing the length of the well in the gas-bearing shale and exposing more of the formation to production. The horizontal section of the wellbore is typically 1 to 3 kilometres long.

The second method is known as hydraulic fracturing, also called hydrofracking simply fracking. In this technique, water, sand, and chemical additives are pumped into the shale gas well at high pressure. The high-pressure fluid creates or enlarges fractures in the rock, allowing natural gas to flow from the shale formation into the well. Without these techniques, natural gas flows too slowly to the well for most shale gas wells to be economically viable.

Environmental Concerns

A life cycle analysis of shale gas in Canada estimated that, on average, shale gas produces approximately 4% higher greenhouse gas emissions than conventional Canadian natural gas. However, these emissions are still lower than those from coal and oil but higher than those from most renewable energy sources.

Although natural gas generally produces fewer air pollutants and less carbon dioxide during combustion than coal or oil, environmental concerns remain associated with shale gas development and production. One concern is that drilling and hydraulically fracturing shale gas wells can require large volumes of water. In areas with limited water availability, large water withdrawals for shale gas development could reduce the water available for other users or affect aquatic ecosystems. In addition, drilling and hydraulic fracturing can generate wastewater containing chemical additives and naturally occurring substances brought to the surface from the rock formation. This wastewater must be appropriately treated, reused, stored, or disposed of in accordance with applicable regulations. Selecting an appropriate method for treating, reusing, and disposing of wastewater is complex because its composition and available management options vary among locations. Hydraulic fracking fluids can also affect soil, surface water, or groundwater if spills, leaks, or failures in well integrity occur. Some chemical additives used in hydraulic fracturing may be hazardous, and improperly contained releases could contaminate soil, surface water, or groundwater, potentially affecting drinking-water sources and wildlife habitat.

Concerns have also been raised about methane emissions associated with shale gas development, including drilling and hydraulic fracturing. Although large amounts of natural gas are produced, most of the gas produced is captured and either gathered for processing and sale or, in some cases, flared when it cannot be economically captured. Once a well is completed, the natural gas is processed using methods like those used methods like those used for conventional natural gas. Methane emissions can still occur during production and processing, but with appropriate controls they are generally comparable to those from conventional natural gas operations.

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