Natural Gas Drilling Basics

Author: Tyler Umstead
Last Edited: 3/25/2015

Natural Gas Boom

Fossil fuel resources such as coal, petroleum, and natural gas provide for most of the energy requirements in the U.S., but recent advances in technology have allowed a recent boom in natural gas extraction by a process known as high-volume hydraulic fracturing (HVHF). The importance of shale oil and gas to the global energy demand can be seen in the U.S. Energy Information Administration’s (USEIA) 2013 oil and gas assessment, which estimated about 7299 trillion cubic feet (TCF) of gas and 345 billion barrels of oil, could technically be recovered from shale deposits in the U.S. and 40 other countries.1 There are more than 20 locations that account for a majority of all shale gas deposits in the continental U.S. (Figure 1).
Figure 1: Extent of natural gas exploration in the United States.12

While the shale gas deposits in the U.S. are projected to provide 38% of the entire U.S. hydrocarbon use by 2040, most of this energy (33%) will come from the Marcellus and Utica formations in the Appalachian region (Figure 1).2 Thanks to gas extraction in shale deposits (Marcellus, Bakken, Utica, Barnett, etc.) the portion of electricity generated has increased from ~20% (2000) to ~30% (2012) and is projected to increase to ~50% by 2040.3 Horizontal drilling and hydraulic fracturing have been used in many forms for decades, but it is the combination of HVHF and the technology of horizontal drilling that has brought about hydrocarbon extraction from resources that were previously uneconomically attainable.1 Since the year 2000, the production of natural gas from underground reservoirs has greatly expanded. Between 1/1/2000 and 3/3/2015, a total of 46,969 wells were drilled (38,034 conventional and 8,935 unconventional) and 18,964 unconventional permits were issued in Pennsylvania (Figure 2).4,5

Figure 2: Extent of conventional and unconventional oil and gas drilling in Pennsylvania (PASDA, USGS).

Unlike other methods of fossil fuel extraction, hydraulic fracturing is poorly regulated by the federal government. Fracturing wells are not regulated by the Safe Drinking Water Act (SDWA), wastewaters from hydraulic fracturing are not regulated by the Resource Conservation and Recovery Act (RCRA), and only the recent Emergency Planning and Community Right-to-Known Act has allowed the U.S. Environmental Protection Agency (USEPA) to request drilling firms to voluntarily report some of the chemical constituents in their fracturing fluids.6,7 For the sake of human health and environmental safety, it is critical that coordinated, long-term sampling and water monitoring is conducted near these shale gas operations to promote an increase in knowledge and stewardship of natural gas extraction.7

Natural Gas and Marcellus Shale

Figure 3. Black shale core sample
Formed from fossilized organic matter and millions of years of heat and pressure, natural gas is a mixture of methane (dry gas) and heavier hydrocarbons such as ethane, propane, and butane (wet gas).8 Wet gases are a vital resource because they can be converted into other fuels and materials. For example, the use of petrochemical plants, or "cracker plants," would enable the conversion of extracted ethane into ethylene, which is used extensively in the production of plastics.9

In terms of obtaining oil and gas from underground rock formations, the two types of drilling include vertical (conventional) and horizontal (unconventional) drilling. Conventional drilling, which includes drilling straight into the target formation, is relatively easy and the trapped gas can flow to the surface. With fine-grain rock features, shale has a low permeability to water and gas. Gas molecules trapped within the shale either occupy natural fractures in the shale or they are tightly bound to clay surfaces.8 Because of this, the more complex unconventional drilling process is required. It utilizes both horizontal drilling and high volume hydraulic fracturing (HVHF) to break apart shale formations so that enough gas can escape to the surface. Unconventional drilling is used to extract resources like methane hydrates, shale gas, deep gas, tight gas, and coal-bed methane.8

Atypical Reservoirs: Tight Gas, Shale Gas, and Coal bed Methane
A Devonian age formation, the Marcellus shale has common components of sedimentary rock formations that include black, organic-rich shale.6,10 This black shale (Figure 3) is a mudrock that contains an accumulated combination of silt, organic matter, and clay-sized mineral grains.11 A majority of black shales are of marine origin and can cover areas exceeding thousands of square kilometers. Such formations can contain increased concentrations of metals like Mo, Ag, Zn, Cu, Cr, Ni, V, and in some black shales Co, Se, and U.11 The clay-sized grains typically lie flat during black shale accumulation, and a thin layered formation of shale rock is formed after pressurized compaction.6 As organic materials in the deposits undergo anaerobic degradation, natural gas is formed. Most of the Marcellus shale gas is thermogenic and dry natural gas is primarily produced thanks to high heat and pressure.6

Shale gas is a fine-grained rock that was formed from the mud or clay at the ocean's bottom. The gas trapped within shale is contained in both naturally occurring fractures and attached to clay surfaces in the shale itself. In December, 2008, the Potential Gas Committee projected the total natural gas resources in the United States at approximately 1,836 trillion cubic feet. In the North East, the Marcellus and Utica shale formations underlay areas of New York, Pennsylvania, Ohio, West Virginia, and portions of Virginia and Maryland. While shale gases have been known to be located 500 to 11,000 ft beneath the surface, the gas trapped in the Marcellus formation is closer to 6,000 - 9,000 ft deep. Some of the other large shale gas reserves in the U.S. include the Barnette, Haynesville, and Fayetteville formations. Because of the geologic nature of shale, the gas is scattered in the rock formations and has very poor ability to flow. This requires hydraulic fracturing methods, which utilize horizontal drilling, water, sand, and chemicals to break apart the shale enough to allow the gas to escape to the surface.8

Hydraulic Fracturing

Extracting oil and gas from organic-rich low-permeability shale layers has become possible with high volume hydraulic fracturing (HVHF) and unconventional drilling, which involves vertical drilling into the oil or gas containing rock layer and subsequent horizontal drilling into the target formation.2,3 On a well pad, six or more horizontal wells can be drilled exceeding 2000 m laterally, with a network of fractures exceeding 500 m or more into the target rock layer.2 This multistep process includes site identification, construction of the well pad and infrastructure, drilling, HVHF, and further production that can include additional HVHF.1

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thumper trucks

Figure 4. "Thumper trucks"

Gas Exploration and Drilling

After leasing the mineral rights and determining the best location for a well pad, the drilling company must obtain a drilling permit from the Pennsylvania Department of Environmental Protection (PADEP), Bureau of Oil and Gas Management.12 This step is a major challenge for the well pad location has the potential to impact habitat or sensitive ecosystems.13 Because of this, well pad locations can be adjusted to account for environmentally sensitive regions, such as wetlands, streams, or protected and endangered wildlife.12 Well pad locations are also based on the distance from other producing wells and the
placement of entrance roads and gas pipelines. Careful planning is used to minimize impacts on citizens and the land. Plans to minimize natural erosion and sedimentation processes must also be developed. Before drilling can begin, geologists need to understand the underground rock formations and potential gas reservoirs. Studies are made on surface rocks as well as rock cutting samples 

Figure 5. Drilling rig in 
Mercer County, PA.
acquired from other nearby drilling operations.12 In addition, seismic surveys are made to create 3D images of the subsurface and natural gas reservoirs.13 This process, known as seismic reflection, uses surface sensors and either "thumper trucks" (Figure 4) or in-ground explosives to produce sound waves in all directions.12 These sound waves are reflected off the varying rock formations and return to the surface to be detected by the sensors. With this information, geologists can map out the types of formations and their depths. Using these tools, geologists find the most prospective target for oil or gas, which may be where the rock reservoir is most porous, permeable, or thickest.

After all permits are acquired, the suitable area of land determined for the well pad is cleared and access roads and pipelines are constructed. Wellpads are constructed to accommodate multiple wells at once, which can be as little as 15 feet apart.12 A drilling rig (Figure 5) is then used to drill vertically to
approximately 1000 feet above the target gas reservoir. Modern drilling equipment usually entails an rotary bit or an air hammer (pneumatic) bit. Next, the specialized drill starts to angle the well hole to direct it horizontally into the shale formation of interest (e.g. Marcellus Shale). The well is then drilled several thousand feet into the target formation. Air and water- and synthetic-based fluids are pumped down the well hole to ensure a faster and easier drilling process.12 Air returning to the surface is vented, and the drilling mud is pumped into large containers or a waste pit. Drill cuttings, or any rock fragments or soil excavated by the drill bit to the surface, are stored on site until being transported to disposal sites.10,14 Drill pipe is then added to the well until the "casing point" is reached. The casing point depth indicates when the drill is removed and a steel and cement casing is installed. Several layers of steel and cement are used to prevent well cave-ins and to protect underground aquifers (Figure 6). The cement is circulated down the bottom of the hollow casing and back up the outside casing of the well. Once the cement is dry the casing layers are complete, further vertical and subsequent horizontal drilling into the target formation is completed.12
well casing
Figure 6. Wellbore casings utilizing steel and cement.12

Logging the Well

logging the well
Figure 7. "Logging the well" - Gamma ray and neutron logging tools used to determine rock type and depth.12

Once the well is drilled and casing is established, geologists will "log the well."12 This is a technique used to record depths and characteristics of the rock formations penetrated by the drill. Tools specially shaped for the well are slowly lowered on a cable down the well borehole. These tools contain sensors that constantly record characteristics such as rock type, porosity, electrical resistivity, hole diameter, and temperature. Data obtained from the sensors are transmitted to the surface and are used by geologists to determine drilling accuracy and whether the formation permeability is great enough for oil or gas extraction.

For rock type and depth, gamma-ray and neutron logging tools are used to generate vertical graphs (Figure 7).12 Every rock type emits a unique signature of natural gamma radiation. For example, limestone and sandstone emit a very small amount of radiation (Figure 7, right on scale). Black shale can contain concentrated amounts of naturally occurring radioactive materials (NORMs). With these elements, black shale emits much more radiation, which is projected far to the right on the vertical graph in Figure 7. Neutron logging measures rock porosity in response to the amount of hydrogen present. In the neutron logging vertical graph, values are opposite of gamma-ray analysis.12 Higher values are on the left and lower values are on the right. Both petroleum hydrocarbons and water contain hydrogen and can be present in the pores between rocks. An increase in hydrogen from neutron logging indicates higher pore space.

High Volume Hydraulic Fracturing (HVHF)

Before hydraulic fracturing and hydrocarbon capture, a final casing is extended along the well to the farthest extent of the drilling.12 This seals of the entire extent of the well before HVHF. Explosive charges are inserted into the well and placed in locations along the horizontal portion of the well where fracturing is set to occur. The explosives are detonated and this creates perforations, or holes, in the casing along the horizontal portion of the well. The perforations will allow sand (proppant) and fracturing fluids to fracture the target formation. The next step includes the highly pressurized injection of 8-40 million liters of fracturing fluids into the wellbore to ensure the target resource formation is fractured and remains open for oil and gas hydrocarbons to escape to the surface.1–3,15 The fracturing fluids entail a mixture of water, proppant, and other chemical additives (accounting for 1% total volume) that include polymers, acids, alcohols, biocides, organic solvents, friction reducers, and lubricants.2,15,16 These additives are used to protect the well from corrosion and fouling, increase shale porosity, and transport proppant to the fractures.2 Proppant keeps the rock fractures open for hydrocarbon escape (Figure 8). HVHF starts with the farthest end of the horizontal portion of the wellbore. Each stage of perforations made by the previous detonations is fractured with fluids and proppant. A plug is inserted into the well to isolate each stage from the rest during HVHF. When each perforation stage is fractured, the plug is removed. Once the well is opened, fluids, debris, and hydrocarbons are allowed to flow back to the surface.

Figure 8. High volume hydraulic fracturing in a targeted reservoir.12

Hydrocarbon Collection and Transportation

After unconventional drilling and HVHF, the well is ready to produce hydrocarbons for capture.12 There have been cases of wells producing both oil and natural gas but usually well will produce either oil or gas. Wet gases (ethane, propane, butane, etc.) are separated from dry gas (methane, CH4) after being chilled and subsequent dehydration.8,12 The wet gases can then be sold as separate products or used as chemical additives.9 Produced gas from the well is transported through gathering pipelines to processing plants, where dry and wet gases are separated.12 The dry gas is sent through transmission pipelines to meet energy demands. Compressor stations are constructed along the pipeline network to keep the gas moving and maintain pressure.

Plugging the Well

When production of oil or gas becomes no longer economically viable, the well operator must plug the well. The well is disconnected from all pipelines and casing inside the well is scrapped for value. Cement is then pumped down the borehole to seal off the well. A vent pipe is then installed on the surface to ensure a build-up in pressure does not occur.12

Other Types of Unconventional Gas

Tight Gas

Tight gas gets its name from the tight rock formations in which it is located. Extraction of natural gas from these tight rocks is very expensive and requires fracturing and acidizing. The Energy Information Administration estimates 310 trillion cubic feet of tight gas could possibly be extracted in the United States. The Montney formation in British Columbia, which is estimated to hold up to 800 trillion cubic feet of gas, is largest tight gas formation in North America.8

Geopressurized Zones

These zones of gas are very deep (10,000 - 25,000 ft) and under incredible pressure. A geoprocessurized zone is formed when clay containing natural gas is compressed over porous materials like silt or sand. Such a compression causes the gas to seep into the porous material beneath the clay. While these zones of highly pressurized gases are problematic for current drilling technologies, it is estimated that they could contain up to 5,000 - 49,000 trillion cubic feet of gas.8

Deep Gas

These sources of natural gas are exactly as the name implies, very deep and expensive to extract. Such gases can be at depths of 15,000 ft or more. Because of its depth, deep gas are not currently preferable in comparison to other much shallower conventional and unconventional gas formations.8

Methane Hydrates

Methane hydrates exist in the arctic permafrost areas and consist of methane pockets trapped in frozen water. According to the U.S. Geological Survey, it is estimated that these methane hydrate regions could contain more carbon than the entire world's source of oil, coal, and non-methane hydrates combined.8

Coal-Bed Methane

coal bed methane

Atypical Reservoirs: Tight Gas, Shale Gas, and Coal bed Methane -
Between 360 to 290 million years ago, dead organic matter from swamps was buried and the intense heat and pressure from moving tectonic plates caused the formation of coal deposits with trapped methane gases. Because coal is more porous than other natural gas deposits and has a larger internal surface area, it can hold up to 6 - 7 times more gas. Coal-bed methane (CBM) is extracted from coal seams that are usually too deep for mining. Depending on the depth, vertical or vertical drilling is applied to acquire methane. It is estimated that 160 - 700 trillion cubic feet of CBM is stored under the United States alone. Some of the largest CBM sites in the U.S. include the Powder River Basin located in Wyoming and Montana, the San Juan Basin in Colorado and New Mexico, the Uinta Basin in Utah, and several other sites in Kansas and Virginia. The methane is allowed to escape these underground formations after the water around the coal seam is pumped out. Unfortunately, this water is high in salinity and contains harmful elements like barium, iron, arsenic, and manganese. This produces a serious risk to the surrounding environment because the high salinity water can deplete soil nutrients, cause erosion, kill plants, cause sedimentation, temperature changes, and pose a risk to drinking water.8

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