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Natural gas is fossil fuel in in its purest form. It contains just two elements – carbon and hydrogen, and is a gas in its raw state. This means it requires minimal processing and creates fewer emissions in its production and use than other fossil fuels.  That makes natural gas an important fuel for reducing carbon dioxide and other atmospheric emissions.

Like all fossil fuels, natural gas was created over millions of years from the breakdown of organic materials below the earth’s surface.  Conventional natural gas fields consist of large free-flowing pockets of trapped gas that can be tapped from a single well. In tight gas and shale fields, the gas accumulation occurs within smaller and tighter pore spaces in the rock. 

What is shale gas?

Shale gas is a description for a field in which natural gas accumulation is locked in tiny bubble-like pockets within layered sedimentary rock such as shale.  Think of it as similar to the way tiny air pockets are trapped in a loaf of bread as it bakes.

While geologists have known for decades that shale gas existed deep beneath many areas of the North American continent, traditional vertical oil and gas drilling methods were able to access only a small fraction of the gas within these formations. But recently, operational efficiencies and proven technology have come together to make shale gas both accessible and economically competitive. 

To extract the gas from shale formations, Shell uses thoroughly tested technology in a responsible way.

What is tight gas?

While shale gas is trapped in rock, tight gas describes natural gas that is dispersed within low-porosity silt or sand areas that create a tight-fitting environment for the gas. How tight? Tight gas is defined (in the U.S.) as having less than 10 percent porosity and less than 0.1 millidarcy permeability.

  • Porosity is the proportion of void space to the total volume of rock. For example, fresh beach sand has around 50 percent porosity.  Tight gas is held in pores up to 20,000 times narrower than a human hair.
  • Permeability is the ability of fluid to move through the pores. A person can blow air through a rock sample having about 1000 millidarcies permeability.

In general, the same drilling and completion technology that is effective with shale gas can also be used to access and extract tight gas. Shell uses proven technology in responsible ways to access this needed resource.

What is sour gas?

In some areas, including portions of the Rocky Mountain range, natural gas occurs mixed with higher levels of sulfur, creating hydrogen sulfide (H2S), a corrosive gas. This “sour gas” requires additional processing to purify it. We also put in place specific safety and environmental protections to prevent the hydrogen sulfide from harming people, animals, air or water, or corroding and damaging well materials and equipment.  We believe our standards exceed many industry practices.

The ways shale gas and tight gas are trapped within rock and sand formations require advanced technology to access these resources. We use horizontal or S-shaped drilling techniques to reach a large underground area from a single wellpad, and use hydraulic fracturing technology to release the encapsulated gas and stimulate it to flow into the wellbore.

Drilling for tight and shale gas

Because the rock in which the gas is contained has such low permeability, special techniques are required to produce enough gas to make the well economically practical.

To access trapped gas, multiple wells reach in different directions deep below the surface.

To access trapped gas, multiple wells reach in different directions deep below the surface.

  • Horizontal drilling. This technology makes it possible for a well to be drilled vertically several thousand feet or meters, then curved to extend at an angle parallel to the earth’s surface, threading the well through the horizontal gas formation to capture more pockets of gas. From a central location Shell can drill multiple wells in different directions that penetrate the reservoir vertically or horizontally. This limits the number of drilling locations – known as well pads – on the surface.
  • S-shaped drilling. In some geological settings, it is more appropriate to directionally drill s-shaped wells from a single pad to minimize surface disturbance.  S-shaped wells are drilled vertically several thousand feet or meters, then extend in arcs beneath the earth’s surface. 

During drilling, mobile drilling units are moved between wells on a single pad. This avoids dismantling and reassembling drilling equipment for each well, making the process shorter  and saving resources.

Hydraulic fracturing

To stimulate the gas flow from sand or shale formations, where gas is trapped in tiny pores in the rock (rather than accumulated in large pools or more porous rock), drillers use a technique called hydraulic fracturing. 

  • Horizontal drilling. This technology makes it possible for a well to be drilled vertically several thousand feet or meters, then curved to extend at an angle parallel to the earth’s surface, threading the well through the horizontal gas formation to capture more pockets of gas. From a central location Shell can drill multiple wells in different directions that penetrate the reservoir vertically or horizontally. This limits the number of drilling locations – known as well pads – on the surface.
  • S-shaped drilling. In some geological settings, it is more appropriate to directionally drill s-shaped wells from a single pad to minimize surface disturbance.  S-shaped wells are drilled vertically several thousand feet or meters, then extend in arcs beneath the earth’s surface. 

During drilling, mobile drilling units are moved between wells on a single pad. This avoids dismantling and reassembling drilling equipment for each well, making the process shorter  and saving resources.

Hydraulic fracturing

To stimulate the gas flow from sand or shale formations, where gas is trapped in tiny pores in the rock (rather than accumulated in large pools or more porous rock), drillers use a technique called hydraulic fracturing.  

How does hydraulic fracturing work?

Hydraulic fracturing is a proven technique that has been used for decades in many kinds of oil and gas wells, but is particularly valuable in tight gas and shale gas formations.

A tool called a perforating gun is lowered into a newly drilled well and lined up precisely within the target formation using seismic images, well logs, global positioning systems and other indicators to target the spots from which tight gas appears most likely to flow. When fired, the gun punches small holes in the well casing, cement and rock.

Then fracturing fluid is pushed out the perforations under high pressure, creating small cracks in the formation that allow the natural gas to flow from the rock. 

We fracture the well in stages and set a plug between each stage.  After we fracture all of the stages in the well, we drill out the plugs, which allows the gas to flow up through the well to begin production.

This fluid is typically 99 percent or more water and sand, with the remainder made of chemicals.   The sand helps prop the cracks open and the chemical additives help reduce friction and prevent bacteria growth and scale from forming and blocking the flow of gas.

Hydraulic fracturing is an often-misunderstood technique. Here are answers to some common questions.

Hydraulic fracturing

Can the fractures allow natural gas to seep up into the water table?

Typically, North American gas formations that require fracturing are located a mile (1.6 kilometers) or more below the water table, trapped below many layers of impermeable rock. These thousands of feet of rock overlying the tight gas formations, combined with the low permeability of the tight gas formations themselves, keep the natural gas and other hydrocarbons contained within the target formation, and also help prevent migration of any hydraulic fracturing fluids that may be pumped into such formations.

Can fracturing fluid seep from the gas formation into the groundwater supply?

Shell’s drilling, casing, and cementing procedures, which meet or exceed regulatory requirements, are designed to protect groundwater by isolating the well from any groundwater supplies. 

We pump the fracturing fluid through the well into the rock or shale zone containing the gas, which has very low porosity and is typically trapped far below the deepest source of potable water and underneath many layers of impermeable rock.  Consequently, the fracturing fluid should either stay within the target formation or be forced back out through the well, but in either case it  remains isolated from groundwater zones.

Can fracturing fluid get into the water supply from the well?

We design and test the integrity of our wells to meet strict specifications based on the local environment.  The upper portions of the well, where the wellbore passes through the water table, are extensively reinforced to prevent either gas or fluids from escaping into the surrounding ground. Wells are made of steel pipes and sealed in place with cement from the surface to below the level of drinking water supplies, typically to a depth of 1,000 feet (about 300 meters) or more.

These barriers help to contain the fracturing fluid and, along with the depth at which fracturing takes place, prevent the fluid from mingling with drinking water close to the surface.

During and after hydraulic fracturing, wells are monitored with pressure sensors to check that they are firmly sealed. Shell also periodically monitors the fractures and the fluids using micro-seismic technology to map the formation, which helps to make production as efficient as possible and protects the environment.

Chemicals

Are the chemicals used in hydraulic fracturing dangerous?

Most of the fluid used in hydraulic fracturing is water. We add chemicals, typically 1 percent of fracturing fluids, to keep the pipes cool by reducing friction and to prevent scale build-up and bacterial growth. Many of these additives are compounds that are used in other applications you encounter in your daily life, from citric acid and guar gum, commonly used as food additives, to ethylene glycol, commonly used in household cleansers and automotive antifreeze.

Some of the chemical additives can be hazardous if not handled carefully. We take great care when using all the compounds added to fracturing fluid, and meet or exceed all regulatory requirements related to handling hazardous materials.

The formulas for fracturing fluids vary, partly depending on the composition of the gas field and partly on the expert opinion of the operator or fluid supplier as to what works best. These formulas are owned by the supplier and some are considered proprietary. Shell is transparent about our operational methods. This includes releasing information about the chemicals we use in our hydraulic fracturing operations to the extent we are permitted by suppliers to release their proprietary information. We support  regulatory efforts that require our suppliers to release such information. 

Find data at fracfocus.org for any of our wells in the U.S. that have been hydraulically fractured since January 1, 2011. 

Can the chemicals stored on the site affect groundwater?

Safe handling of all water and fluids on site, including chemicals used for hydraulic fracturing, is a high priority for Shell. We comply with all regulations regarding containment, transport and spill handling. To protect groundwater, we keep chemical containers and all fluid handling equipment located on site within secondary containment barriers that could capture a spill in the unlikely event that one would occur. The containers must pass regular integrity checks and certification. 

We check water storage tanks for integrity before we use them and check them visually each day. Single-use containers that are used to transport small volumes of chemicals are returned and disposed of offsite.  Any spill that were to occur on a well pad would be immediately cleaned up, reported and documented according to both our own and regulatory requirements.

Disposal

What happens to the fracturing fluid after it is used?

We take multiple steps to properly handle hydraulic fracturing fluid. First, when possible, we reuse it for additional wells in a single field. This both reduces our overall use of fresh water and reduces the amount of recovered water and chemicals that must be disposed of. Second, we keep the recovered water in storage tanks or lined storage pits until it is disposed of in a permitted saltwater injection disposal well or taken to a treatment plant for processing. 

All treatment and disposal facilities used by Shell undergo extensive on-site environmental assessments to ensure they meet regulatory standards and Company-established specifications prior to use.

Injection Wells

All of our Shell-operated injection wells are designed to meet federal regulations set by the Environmental Protection Agency to protect groundwater.  Our geologists and engineers work together to design, drill and test injection wells for integrity and compliance. We isolate the wells in three ways:

  • We choose zones that have multiple confining layers above the zone to keep the injected fluids within the target formation.  
  • We use multiple layers of well casing and cement (similar to our production wells) and periodically run mechanical integrity tests to verify that the casing and cement are holding the liquids.  
  • We control how much fluid we inject and at what pressure (specified in each well permit) to help keep the fluids in the target zone, and we monitor the pressure in the injection well and the spaces between the casing layers (also called the annuluses) to check and verify the integrity of the injection well. 

We also screen any non-Shell-operated injection wells that we use to verify that they comply with all regulatory requirements.

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