P A L E O N T O L O G I C A L R E S E A R C H I N S T I T U T I O N

T H E S C I E N C E B E N E A T H T H E S U R F A C E

M A R C E L L U S S H A L E • I S S U E N U M B E R 6 • J A N U A R Y 2 0 1 2

Understanding Drilling TechnologyA discussion of the technologies associated with unconventional natural gas drilling and how it compares to more familiar conventional drilling techniques.

IntroductionThe Marcellus Shale is an uncon-

ventional natural gas resource found beneath the surface of Pennsylvania, Ohio, West Virginia, and New York. The natural gas found in the Marcel-lus Shale is requires unconventional methods of extraction, in this case high-volume, horizontal hydraulic fracturing, because of the way the natural gas is trapped in the forma-tion. Until fairly recently, it was not economically feasible to extract the natural trapped in a tight shale like the Marcellus. In recent years, how-ever, a new combination of existing drilling technologies – horizontal drilling and hydraulic fracturing – has emerged that makes this possible.

Conventional Natural Gas Extraction

Unconventional natural gas extraction is new to New York State, but gas has been produced

by conventional drilling methods in New York since the 1800’s. A conventional gas resource is usu-ally one in which the gas forms in a specific rock unit, called a source rock (the Marcellus is a source rock), but migrates out of this rock unit toward areas of lower pressure. It is eventually trapped by an imperme-able rock unit, called a reservoir or reserve. Sometimes the reservoir is a void created when a layer of rock has broken and moved beneath the surface; sometimes the reservoir is created when gas collects in the pore spaces in rock layers that have formed subterranean ‘hills’ Conventional drilling techniques require geologists to find reservoirs of natural gas, and then drill down into them to extract the gas.

Sometimes it is difficult or impos-sible to set up drilling operations di-rectly above a natural gas reservoir. If the oil or gas deposit is located under

an urban area, for instance, surface conditions would not allow drilling there. To get around this problem, conventional oil and gas wells can be drilled at various angles underground in a process called directional drill-ing.

One version of directional drill-ing, horizontal drilling, starts with a vertical well bore (the hole the well makes under the surface) which is then angled until it is oriented hori-zontally. From there, the well can be drilled horizontally underground for

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thousands of feet. Horizontal drilling provides access to reservoirs that are too thin or compacted to be accessed with conventional vertical wells. Some directional wells have even been angled beyond horizontal, and have hooked back upward in order to access particularly difficult-to-reach oil or gas pockets. This is called fish hook drilling. Horizontal drilling is the type of directional drilling that is currently being used to access the natural gas in the Marcellus Shale in Pennsylvania, and that will be used to access the Marcellus Shale in New York if Marcellus wells are drilled.

If a source rock does not have abundant natural pore space or fractures, the oil or gas may not be able to migrate from its source rock into a reservoir rock from which it can be easily extracted. To get around this problem, such tight source rocks are artificially cracked or fractured, by any of a variety of techniques, in order to stimulate well produc-tion. Fracturing source rock in order to increase gas well production in conventional resources has a long history in New York State. Beginning in the 1860’s, the explosive nitroglyc-erin was used, sometimes illegally, to crack rocks in shallow wells. Since then, and throughout the United States, various fracturing fluids have been used to fracture rocks by hydrostatic pressure. These fractur-ing fluids, when under high pressure, either create or expand fractures in the rock, creating hydraulic fractures. The Marcellus Shale is one such tight source rock, so hydraulic fracturing is used to extract the natural gas from it.

Hydraulic fracturing not used solely to increase connectivity of pore spaces in the rock to the well bore. It is also used to decrease the hydraulic pressure in the areas nearest a verti-cal well in order to minimize drilling

complications. It can also be used to increase the gas flow rate of verti-cal wells damaged by the buildup of scale (the accumulation of minerals in pipes and other equipment) or other materials.1

Unconventional natural gas drill-ing is considered “unconventional” because the natural gas has not migrated from the source rock into a reservoir, but remains trapped within the source rock. Extracting this natu-ral gas requires

1) additional surface area con-tact between the well bore to the source rock, which is provided by drilling horizontal wells, and

2) the creation of pathways for the natural gas to flow out of the source rock and into the well, which is provided by hydraulic fracturing.

It is the combination of tight source rock with the application of uncon-ventional drilling technology (just hydraulic fracturing, just horizontal drilling, or a combination of both) that makes a resource unconvention-al. Either the gas resource itself or the technologies used to extract the gas can be referred to as unconventional. The remainder of this discussion will consider the unconventional natural gas drilling being used in the Marcel-lus Shale, which requires both hori-zontal drilling and hydraulic fractur-ing of a geologically unique reservoir rock to be economically viable. Unconventional and Conventional Well Drilling – A Matter of Scale

Vertical well operations differ from horizontal well drilling opera-tions in scale. A vertical well pad (the area at the surface, surrounding the well bore that is used to conduct the drilling and hydraulic fracture activities) is smaller than a horizontal well pad – a fraction of an acre to about 3 acres for a vertical pad and 3

to 6 acres for a horizontal well pad. The size of the well pad in both hori-zontal and vertical drilling operations increases as on-site storage needs increase for equipment and fluid. The amount of water and chemicals used in a horizontal well that is hydrauli-cally fractured is greatly increased (by up to 100-fold) over vertical wells that are hydraulically fractured. Vertical wells typically use 20,000 – 80,000 gallons of water for hydraulic fracturing, and New York State regu-lations state that any well fractured with more than 300,000 gallons of water is considered a “high-volume” well.2 Horizontal wells, however, use between 2 and 9 million gallons of water; 4.3 million gallons of water is the average in a Marcellus well in the Susquehanna River Basin of Pennsyl-vania.3

Hydraulic fracturing operations in both vertical and horizontal wells incorporate a variety of chemicals to enhance recovery of oil or gas. Just like the drilling process itself, the chemicals used have changed over time, largely through trial and error experimentation, but also through regulation. In the past, they have included acid, water and acid com-bined, squeeze-cement, napalm and gasoline, and a variety of gelled fossil fuels.4

Modern methods of hydraulic fracturing evolved in the 1960’s with the advent of water-based gels that were used to carry proppant into fractures. Proppants are natural or manufactured (e.g., ceramic beads) sand grains that are used to hold open the fractures created during hydraulic fracturing. Adding prop-pants to the fracture fluid led to further experimentation with fluid viscosity. Surfactants and crosslinkers (chemical additives) were added to adjust the viscosity of the fluid in an attempt to optimize the amount of

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proppant carried into the fractures and the speed at which they were transported. For information on the chemicals used in Marcellus Shale hydraulic fracturing operations, please see Marcellus Shale Issue 7: Water – Into the Wells. Eventually, the combination of these different meth-ods created what is called high vol-ume, slickwater, horizontal hydraulic fracturing, which is currently used in the Marcellus Shale and other tight shale gas plays (the area in under de-velopment by natural gas operators). High-volume, horizontal hydraulic fracturing is so named because it differs from other drilling methods in a few important ways. In this technology, a vertical well is drilled with a horizontal leg (or lateral) that runs parallel to the rock being drilled. Wells that travel horizontally through a gas-bearing layer require a higher volume of water to fracture, or crack, the rock than wells that only bore vertically; about 4.5 million versus 80 thousand gallons, respec-tively. The term slickwater refers to the combination of chemicals added to the fracturing fluid to modify its

viscosity, thus increasing the speed at which it travels in the well.

Drilling a WellDrilling a well to extract natu-

ral gas from the ground may sound simple but it is not. Vertical or horizontal, hydraulically fractured or not, all gas well drilling requires a long series of usually complex steps. Before any drilling takes place, the well pad must be established, and be large enough to support all of the equipment needed to drill and hy-draulically fracture a well. In the case of horizontal wells, a special drill bit is used to turn the drill at an angle at a predetermined depth, referred to as the “kickoff point.” To turn a well fully horizontal takes around 1000 vertical feet of drilling.

As the well is being drilled, it is lined with steel casing in order to prevent collapse of the hole. Casings also prevent drilling fluids and gas from escaping through the sides of the well. Steel casing is inserted into the drilled hole, cemented in place, and then the well is drilled deeper with a slightly smaller drill bit. Then

another, slimmer casing is hung in the deeper well hole and cemented in place.4 Guiding shoes on the ends of the casing help the lengths of cas-ing move down the well safely, and a spring-like centralizer positions the casing in the center of the well hole.

Eight types of cement are clas-sified by the American Petroleum Institute (API) for use in well cas-ings. This classification system was designed to address quality concerns such as poor raw ingredients, and the cement chosen is commonly mixed with cement additives that modify its setting time and density. The cement is pumped down the well inside the casing. When it reaches the bottom of the hole, it flows out and back up the space between the casing and the drilled well, the area called the annulus. To ensure that the cement in the casing is pushed fully into the annulus, a wiper plug is inserted behind the wet cement to force the cement out of the well bore, clean the inside walls of the casing, and separate the cement from additional drilling muds. After the wiper plug is inserted, drilling muds (substances that lubricate the drill bit and make drilling easier and faster)are pumped into the well to continue to force the cement into lining the outside of the casing wall.5

Marcellus wells are drilled and cased in multiple stages. This process is called a casing program. The first piece of casing, called the conduc-tor pipe, has the largest diameter. Its primary job is to prevent the collapse of the top of the well. It also pre-vents exchange of fluids between the well and nearby shallow water and gas reserves and provides a path for drilling muds. Blowout preventers (valves that help regulate erratic pres-sure changes that can be found while drilling) are attached to this level of casing.5

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Conventional Drilling Unconventional Drilling

Well type vertical horizontalWell pad footprint >1 acre to 3 acres 3 to 6 acresRoad Construction Footprint

Similar to unconven-tional drilling

5.7 acres

Water required 20,000 to 80,000 gallons

2 to 9 million gallons; average 4 million gallons

Chemicals requiredTime to drill well ~ 1 month ~ 3 monthsHydraulic fracturing required

sometimes almost always

Source rock large pocket of re-source; easy to extract

resource scattered throughout rock, hard to extract

Table 1: A Comparison between Conventional and Unconventional Drilling

Surface casing is the next level of casing, and is commonly around 13 ¾ inches in diameter. Its job is similar to the conductor pipe; it pre-vents contamination of groundwater by drilling muds and keeps sediment from caving into the well. Interme-diate casing, which is typically 8 5/8 inches in diameter, is added to mitigate potential problems at greater depths, like areas of unusually high pressure (due to shallow gas pockets). The last casing to be inserted into the well is thinnest in diameter (between 2-4 inches) and is called the produc-tion casing. It is run through the length of the well that will be pro-ducing natural gas.

Because casing lines the en-tirety of the well, it must have holes punctured into it in order for natural gas to have a pathway into the well. These holes are created by perforation guns (perf guns) that are positioned in the lateral part of the well with a wire line or drill pipe and guiding wheels. There are many varieties of perf guns, but they all function in a similar way. Once in position, the perf gun shoots small projectiles (called shaped charges), which are essentially armor-piercing bullets, directly into the casing. These charges punch through the steel cas-ing and cement-filled annulus.

After the well has been complet-ed, the gas company runs a series of pressure tests, commonly called “shut ins,” on the well. These tests, which usually last 72 hours, assess whether the newly drilled well was drilled and cased correctly. After these tests, a device known as a Christmas Tree is placed on the top of the well at the surface. This device allows gas to be pumped into production pipelines. It also monitors production and holds the regulating blowout valve that is designed to control erratic pressures in the well and to seal the well in an

emergency.

Unconventional Natural Gas Extraction

Fossil fuel extraction by conven-tional means is very costly. Fewer than 50% of oil and gas wells ever drilled in the U.S. have become com-mercially successful. This is because many turn out to have little or no oil or gas that can be extracted economi-cally. Some of them are truly ‘dry holes’ while others have oil or gas, but the source rock was damaged during drilling, lowering permeabil-ity near the well, or were otherwise less prolific than originally thought.

New drilling techniques were originally developed in an effort to extract more resources from those conventional wells in a second or third drilling attempt. These tech-niques, known as secondary or tertiary recovery, received the first hydraulic fracturing treatments. Later, the technology, combined with horizontal drilling, became useful for recovering fossil fuels in imperme-able, unconventional reservoirs. The Horizontal Drilling Process

Horizontal drilling in and of itself is not highly controversial. Because horizontal wells are so much longer, however, they are drilled by larger pieces of equipment, take more time to drill, and require more hydraulic fracturing fluid to be stimulated successfully. During this process, a far greater amount of rock must be drilled through, which generates more cuttings (a mixture of coarse chips and finer particles of rock that are produced as the well is drilled) that must be disposed of from a horizontal well compared to a verti-cal well. And the length of time a horizontal well is in the development phase – and therefore the total local impact of drilling – can be much

greater than for vertical wells. The direction and length of the

lateral (horizontal part of the well) depends on the local geology, the amount of land available to the driller, and the available technol-ogy. Laterals in shale gas wells have commonly been around 4,000 to 5,000 feet in length, but companies are expected to increase the length of laterals in the Marcellus as the limits of the technology are more widely tested. The directions in which the laterals extend from the vertical part of the well bore are determined as a result of analysis of the current underground stresses and the pre-dicted effects of existing and stimu-lated fractures. In the Marcellus, gas companies are directionally drilling laterals in the north-northwest and south-southeast directions.6 For more information on why laterals are drilled in this direction, please refer to our pamphlet Marcellus Shale Issue 5: Jointing and Fracturing in the Marcellus Shale.

The Hydraulic Fracturing ProcessHydraulic fracturing, specifically

the high volume, slickwater, horizon-tal fracturing used in the Marcellus Shale, is the process of forcing liquid into the gas-bearing rock unit under high pressure to fracture the rock adjacent to the well. These fractures form pathways for the natural gas trapped in the rock to escape via the well bore. Hydraulically fracturing a horizontal well in the Marcellus Shale requires about 4.5 million gallons of water, which is mixed with a variety of chemicals and sand. The chemicals the fracture process by doing things like keeping the fractures free of bacteria and transporting the sand grains. Various-sized sand grains, called proppant hold open the frac-tures created by this process. To learn more about the chemicals required

4 / Marcellus Shale • Issue Number 6 • January 2012

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Figure 1: Drilling and Casing a Well

The process of drilling and casing vertical and horizontal wells is the same in the initial drilling depths. 1) The drill initially bores into the ground. At this point the drill is lubricated with...and the drill hole is empty. 2) After the initial hole is drilled, a steel casing is inserted into the hole, guided by a springlike ‘guiding shoe’ that positions the casing in the center of the hole. The space between the casing and the rock on the outside of the hole is called the ‘annulus.’ 3) Cement is poured into the casing from the surface. It flows to the bottom of the hole and begins to infill the annulus. 4) When enough cement has been poured into the well to line the casing, wiper plugs are inserted into the casing to wipe the inside clean of wet cement. The plugs are pushed down by drilling muds, which are later used to lubricate the drill bit. The cement is allowed to dry. 5 & 6)The new well hole is now the only void space in the well. A drill is re-inserted into the well where it drills through a layer of cement and into deeper rock. 7) The process of drilling, casing, and cementing the casing is repeated with thinner and thinner casings that are intended to prevent any exchange between fluid flowing through the well and surrounding groundwater sources. The production casing lines the length of the well and is intended to be porous in the region where natural gas is to be extracted (in this diagram, the horizontal portion will undergo small blasts to puncture the steel and allow fracture fluid out and natural gas in).

Drill

DrillHole

1

Casing

Centralizer

GuideShoe

2

Casing

Cement

Annulus

3

DrillingMuds

WiperPlug

Cement

4

EmptyWellHole

Cement

5

New DrillHole

6

7

FirstCasing

SecondCasing

ThirdCasing Production Casing

Cemented Casings

in high volume, slickwater hydraulic fracturing, see our pamphlet Marcel-lus Shale Issue 7: Water: Into the Wells.

Because the horizontal portion of a Marcellus well is typically 4,000 to 5,000 feet in length, the well is hydraulically fractured section by section, a process called multi-stage hydraulic fracturing. The section farthest away from the vertical part of the well is fractured first, and then closed off from the remainder of the well. Then the process is repeated with the next section, and so on until the full lateral length of the well has been fractured. It has been common for a multi-stage fracture to have three or four sections of fracturing, but as lateral lengths of the wells increase, so could the number of stages in the full hydraulic fracture of a well.

The actual process of fracturing occurs in three phases. First fractur-ing fluid, consisting of water and chemicals, is injected into the well, creating fractures near the well bore, although sometimes acidic or basic fluid precedes the fracturing fluid to break down natural cements in the rock or to mitigate buildup of miner-al deposits.7 Second, more fracturing fluid is mixed with proppants and injected into the well. During this phase, the initial fractures are elon-gated and the proppants are forced into the fractures to hold them open after the pressure from hydraulic fracturing is released. Finally, the well is flushed out to remove the excess fracturing fluid from the well.5 In the Marcellus Shale, only around 9-35% of the fracturing fluids return to the surface.2

While the fate of the fracturing fluid that remains underground after the Marcellus Shale is fractured is not yet fully understood, geologists and engineers hypothesize that the remaining 70-90% of fracturing fluid

is trapped in the multiple, tiny frac-tures due to a combination of capil-lary action and the swelling of clays. In very small spaces the molecular at-traction between water molecules and the molecules in the shale fractures can be stronger than other pressures acting on the water. This holds the fracturing fluid in the fractures and is referred to as capillary action. Clay can also “trap” the fracturing fluids because clay grains can absorb water, swell, and decrease permeability. Because many of the inorganic sedi-ments that make up the Marcellus Shale are clay grains, geologists think that some of the fracturing fluid is absorbed by clay at depth. For these reasons, only a small portion of the fracturing fluid is returned to the sur-face during when the well is flushed out to remove excess fluid. 7,8,9

The characteristics of the target formation (the rock from which gas is being extracted) determine what specific techniques will be used to extract gas. Data must be collected for every new region and formation that is developed for gas drilling. The stresses on the rock at the depth of gas extraction and the rock perme-ability help determine what fluid and propping agent characteristics are needed to fracture wells most ef-fectively. More, larger fractures yield more natural gas produced from the unit, but are costlier to create. It takes time to develop the informa-tion on each region, but eventually the engineers in charge of each well use the data to establish the optimum fracture treatment for each well.1

The effectiveness of a hydraulic fracture treatment can be measured using a combination of microseismic mapping in the field and measure-ments taken in the well (i.e. tempera-ture, production, and video image logging) to supply information on fractures immediately near the well

bore, and modeling. These tech-niques can be very expensive, and are usually used in research wells to initially characterize a region being drilled in order to design the fractur-ing process for that region.1

Hydraulically fracturing horizon-tal wells requires a large quantity of water mixed with a variety of chemi-cals ranging from benign to toxic, and a portion of this fracturing fluid is returned to the surface. In addition to the chemicals used by the drilling industry, this flowback water can also contain a variety of heavy metals, salts, and naturally-occurring radio-active material found in the gas-bear-ing unit. Because of this, handling, treating, and disposing of contami-nated flowback fluids has become an important issue in the Marcellus Shale. Options for wastewater cur-rently include: deep fluid injection wells, where contaminated water is injected into a deep, impermeable formation and stored permanently; disposing in a wastewater treatment facility designed for flowback fluids; or onsite water treatment and recy-cling for additional hydraulic frac-turing jobs by the industry. To learn more about wastewater treatment options, see our pamphlet Marcellus Shale Issue 8: Water: Out of the Wells.

It is currently unclear how many times a well will be hydraulically fractured in the Marcellus Shale. A multi-staged hydraulic fracture of a well is typically considered one hydraulic fracturing event. Should a portion of the well become dam-aged or if the well performs beneath the expectations of the company, a portion of the well or the whole well may be hydraulically fractured again. To what extent this will occur is not yet known, as most wells in the Marcellus region are less than a few years old.

6 / Marcellus Shale • Issue Number 6 • January 2012

Natural Gas Extraction and Transport: What happens when the gas leaves the well

Once a Marcellus Shale natu-ral gas well has been successfully drilled, natural gas is extracted, processed, and sold for profit. It is estimated that Marcellus Shale wells will produce economically valuable quantities of natural gas for at least 30 years. It is hard to predict, at this stage in Marcellus Shale gas devel-opment, the accuracy of that time frame. All gas wells produce the high-est flow rates of natural gas in the first weeks of production, and this rate declines over time as the pres-sure in the gas-bearing unit declines. When a well is producing at rates so

low it is barely profitable, it is called a stripper well. Because no wells in the Marcellus Shale have been producing for an extended period of time, it is hard to estimate when they will no longer be economically viable. How-ever, with each month analysts gather more data, and the predicted rate of decline, frequently called the decline curve, of Marcellus Shale gas wells can be more accurately estimated.5 Some of the first Marcellus wells that have been producing since 2004 in West Virginia currently show decline curves that suggest a given well will be profitable for around 20-25 years.7

If the gas being extracted from the Marcellus Shale is nearly free of impurities, it can be immediately

sold by the gas industry to the pipe-line industry, transported to a final gas processing plant and placed on the market. If there is a high level of water or hydrocarbon liquids in the gas, or if the gas contains corrosive gases (like carbon dioxide or hydro-gen sulfide), the gas must be pro-cessed in the field before being sold to the pipeline industry.5

Once the gas is of a high enough quality to be sold to the pipeline for final processing and sale on the mar-ket, it is collected by gathering lines which feed into compressor stations and metering sites. These stations connect to larger pipelines owned by pipeline companies who distrib-ute and sell natural gas to utilities.

Marcellus Shale • Issue Number 6 • January 2012 / 7

Figure 2: Highlighted Major Pipeline Infrastructure in the U.S.

Variously-colored grey lines represent pipeline systems that transport natural gas to recipients. The majority of natural gas is transported to the North-east region of the U.S. because of heating demands and high population densities. Note that pipelines can only transport natural gas in one direction, and is usually transported from Texas and the Gulf of Mexico in the south, and from Canada in the north, with some LNG being imported in the NE and the rest imported through the G

US LNG Terminals Existing Under Construction Permitted

Note: pipeline systems in certainregions omitted for clarity.

Interstate pipeline companies trans-porting natural gas are regulated by the Federal Energy Regulatory Com-mission (FERC) with help from the U.S. Department of Transportation’s Pipeline and Hazardous Materials Safety Administration (PHMSA). Gathering lines that connect wells to pipeline companies are not regulated under FERC and PHMSA. Instead, these are regulated locally by a state’s Public Service Commission. See Figure 2 fot existing US pipeline infrastructure.

Looking Ahead: More Technological Advances in Hydraulic Fracturing

Even as the technology associated with unconventional gas drilling in the Marcellus Shale is being refined, newer technologies are also being de-veloped and tested. A few companies are experimenting with the technol-ogy used to hydraulically fracture a formation with propane gas. Early testing suggests that the quantity of gas extractable from an unconven-tional unit using propane to hydrau-lically fracture a well is considerably more than with water-based hydrau-lic fracturing technology.

In principle, the idea behind hydraulically fracturing a well with propane is based on the physical properties of the propane, itself. Sur-face tension is defined as the ability of a liquid to resist an external force. Because propane has about ten times less surface tension than water, fric-tional forces associated with pushing propane through a well are far lower than when pushing water through a well.8,10 Propane is also eight times less viscous than water. Viscosity is a measure of a fluid’s internal friction (molasses has a high viscosity while water has a low viscosity). Finally, liq-uid propane is also a less dense liquid than water.

There are a number of predictable

advantages to hydraulic fracturing with propane, including less truck traffic than is currently seen hauling water, fewer chemical additives than is used in water hydraulic fractur-ing, and no hydraulic fracturing wastewater issues. However, there are also drawbacks to using liquid propane. Compressing large quanti-ties of propane on a regular basis increases the potential for explosions and fire. Propane, itself, is a fossil fuel that may need to be produced in larger quantities beyond existing production if it is to be directed to hydraulic fracturing. Finally, financial incentives may drive companies using propane to hydraulically fracture a well to drill more finely spaced verti-cal wells instead of horizontal wells, which could dramatically increase the amount of land altered by drilling in the region.

Currently, the process of hy-draulic fracturing with propane is not widespread, and the intellectual

property rights belong only to one company. The initial cost of propane is also much higher than water. As a result, it may take years for wide-spread industrial use of hydraulic fracturing with propane to occur. Certainly, as new technologies become available they bring with them the potential for more efficient natural gas extraction, but they also bring forth a new set of environmen-tal and other concerns that need to be considered.

SummaryThe geological context of the

Marcellus Shale plays an important role in the extraction of the uncon-ventional natural gas resource stored within. The extraction, processing, and transportation of that natural gas to its intended market is complex and actively evolving. The techniques required have environmental implica-tions that are discussed in other pa-pers in this series and socio-political

8 / Marcellus Shale • Issue Number 6 • January 2012

GlossaryAnnulus: The space between the casing and the drilled well.Blowout preventer: A valve that helps regulate erratic pressure changes that

can be found while drilling.Capillary action: In very small spaces the molecular attraction between water

molecules and the molecules in the shale fractures can be stronger than other pressures acting on the water. This holds the fracturing fluid in the fractures and is referred to as capillary action.

Centralizer: A spring-like centralizer positions the casing in the center of the well hole.

Casing Program: The multiple stages used to drill and case a well are col-lectively called a casing program.

Conductor Pipe: The first piece of casing inserted into the well with the largest diameter.

Conventional: A conventional gas resource is usually one in which the gas forms in a specific rock unit, called a source rock, but migrates out of this rock unit toward areas of lower pressure.

Cuttings: A mixture of coarse chips and finer particles of rock that are pro-duced as the well is drilled.

Directional drilling: A gas well that is not drilled vertically is drilled direc-tionally.

Drilling muds: Drilling muds lubricate the drill bit and make drilling easier and faster

Fish hook drilling: Wells that have been angled beyond horizontal, and have hooked back upward in order to access particularly difficult-to-reach oil or gas

Partnering Organizations includeCornell Cooperative Extension (naturalgas.cce.cornell.edu)New York State Water Resources Institute (wri.cor-nell.edu), Cornell University Department of Earth and Atmospheric Sciences, and Cornell University Agricultural Experiment Station.

Funded by NSF GEO No.1016359.Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

Authored by Trisha A. Smrecak and the PRI Marcellus Shale Team Illustrations by J. Houghton unless otherwise attributed.

implications that are actively being studied by sociologists.

References1. Department of Energy, Hydraulic Fracturing Whitepaper, Appendix A, June 2004

2. Department of Environmental Conservation, Draft SGEIS on the Oil, Gas and Solution Mining Regulatory Program, September 2011

3. Susquehanna River Basin Com-mission, Frequently Asked Questions, http://www.srbc.net/programs/natu-ral_gas_development_faq.htm.

4. King, Robert F., Morehouse, David F. “Drilling Sideways-- A Review of Horizontal Well Technology and its Domestic Application.” U.S. Depart-ment of Energy- Energy Information Administration April 1993. 26 January 2011 Http://www.eia.doe.gov/pub/oil_gas/natural_gas/analysis_publications/drilling_sideways_well_technology/pdf/tr0565.pdf

5. Hyne, Norman J. Nontechnical guide to petroleum geology, exploration, drill-ing, and production. PennWell Books, 2001

6. Engelder, Terry, Gary G. Lash, and Redescal S. Uzcategui, 2009 Joint sets that enhance production from Middle and Upper Devonian gas shales of the Appalachian Basin, AAPG Bulletin; July 2009; v. 93; no. 7; p. 857-889

7. Conner O’Laughlin, Camp, Dresser, and McKee. Personal communication,

pocketsGas play: The area in under development by natural gas operators.Horizontal drilling: A gas well that starts with a vertical well bore which is

then angled until it is oriented horizontally.Intermediate Casing: This is typically 8 5/8 inches in diameter, and is

added to mitigate potential problems at greater depths.Lateral: The horizontal portion of the well bore.Multi-stage hydraulic fracturing: A process by which a well is hydraulically

fractured section by section.Production Casing: This is run through the length of the well that will be

producing natural gas.Proppant: Natural or manufactured (e.g., ceramic beads) sand grains that

are used to hold open the fractures created during hydraulic fracturing.Reservoir: An area of low pressure underground where gas has collected,

trapped by a layer of rock the gas cannot penetrate.Scale: The accumulation of minerals in pipes and other equipment.Shoe: Guiding shoes on the ends of the casing help the lengths of casing

move down the well safely.Source rock: A rock unit in which natural gas formed. Surface Casing: The level of casing after the conductor pipe, commonly

around 13 ¾ inches in diameter.Target formation: The rock from which gas is being extracted.Tight: Source rock that does not have abundant natural pore space or

fractures is considered tight. Unconventional: Refers to either the natural gas or the extraction process

to retrieve the gas when the gas has not migrated from its source rock and requires a combination of drilling technologies to extract.

Well bore: The hole the well makes under the surface. Well pad: The area at the surface, surrounding the well bore that is used to

conduct the drilling and hydraulic fracture activities.Wiper plug: The wiper plug is inserted behind the wet cement to force the

cement out of the well bore, clean the inside walls of the casing, and separate the cement from additional drilling muds.

July, 2011.

8. GasFrac Energy Services Inc., http://www.gasfrac.com/

9. Al-Mhaidib, A. I. (1997) “Influence of Loading Rate on Undrained Bear-ing Capacity of a Model Pile in Clay”

proceedings of the Fourth Regional Conference on Geotechnical Engineer-ing (GEOTROPIKA ‘97) , Johor Bahru, Malaysia, pp. 357-370.

10. Dictionary.com, 2011, http://dic-tionary.reference.com/ (accessed 2011)

Marcellus Shale • Issue Number 6 • January 2012 / 9