Natural gas is a misnomer in the sense that it an umbrella term broadly applied to a commodity that may or may not derive from a natural geologic process. According to the prevailing definition, “natural” natural gas is a mixture of a variety of hydrocarbon gases, mostly methane (CH4) but with a substantial amount of ethane (C2H6), propane (C3H8), and butane (C4H10), that predominantly occurs in geologic formations similar to and frequently associated with those that result in petroleum deposits. Due to the simplicity of its elemental hydrocarbon chemical composition, natural gas is also produced concomitant to the accumulation of terrestrial land plant strata in coal beds and is also probably produced inorganically from carbonaceous cosmic debris to some extent. Regardless of its provenance, the gaseous material that comes out of the ground will likely also contain various heavier liquid hydrocarbons and other gases including hydrogen (H2), nitrogen (N2), carbon dioxide (CO2), water vapor (H2O) and hydrogen sulfide (H2S). All of these contaminants are removed from natural gas before it is sent through the pipelines. It is implicit that the term natural gas also refers to “commodity” natural gas that is used as a source of energy for commercial and domestic use that may or may not have had a “natural” origin.
“Natural” natural gas is for the most part the result of the same biologic and geologic processes that result in the formation of petroleum; gas is always found with petroleum deposits and is sometimes considered a type of petroleum. The petroleum-gas process starts with the deposition of organic matter of biological origin in sediments at the bottom of mostly marine but also inland fresh water seas. In the murky depths in the absence of oxygen, anaerobic microbes generate methane gas (along with CO2, N2 and NO – nitrogen oxide) as a waste product of the decomposition process; the “swamp gas” to which many seemingly supernatural apparitions are attributed is one example of this process. As geologic time passes, the sediments are inexorably covered over with subsequent layers until biological activity ceases and a complex material called kerogen is formed. When the depth of the sedimentation is such that the pressure and temperature are in the appropriate range (between 1,000 and 5,000 meters), liquid petroleum in the form of crude oil is formed, the accompanying natural gas is known as ‘wet gas.’ At deeper depths, the temperature and pressure are too high for petroleum, which breaks down in part forming natural gas which is known as ‘dry gas.’
The process of gas accumulation into reservoirs is also similar to that of crude oil. The highly compacted sedimentary strata that are the ultimate end result of the compressed biological gas-generating material are relatively impermeable. The gas thus migrates to more porous formations such as sand, sandstone and limestone (rock formed from the calcium carbonates of accumulated aquatic animal exoskeletons) and may accumulate in cavities. The formation of a reservoir requires that the migrating gas be trapped under an impervious layer known as a cap rock or in some other similar confining geologic trap lest it migrate to the surface where it would evaporate to the atmosphere. These reservoirs of gas, typically associated with petroleum reservoirs, are known as conventional gas. However, since gas is in the vapor state at atmospheric temperatures and pressures, it migrates much more readily than liquid crude oil and it is therefore much more ubiquitous in geologic formations. The gas deposits that are outside the petroleum datum are by definition unconventional, and are somewhat loosely designated as deep gas, tight gas, shale gas, coal-bed methane, and methane hydrate.
Deep gas is neither a creative nor a confusing name; it is appropriately descriptive. Deep gas comes from geologic strata that are below the conventional petroleum reservoirs where increasing temperature and pressure render petroleum unstable. This occurs below a depth of about 5,000 meters. Extraction of the so-called deep gas with drilled pipelines at depths in excess of this is a difficult, though manageable engineering challenge. In some cases, deep gas is highly pressurized as a result of the formation process, clay bed compaction having occurred at a rapid rate and imparting the concomitant high pressure to the gas – this type of deep gas is called geo-pressurized gas. Tight gas is equally descriptive, referring to a gas host formation of hard and impermeable rock, a result of the migration of gas from its source to adjacent strata. Tight gas is thought to be quite abundant, though difficult to extract without some rather difficult extraction techniques.
Shale gas is rather obviously natural gas that is contained in shale deposits. Shale is a very fine-grained (< 0.02mm – sand is about 1-2 mm) sedimentary rock that results from the deposition of broken fragments of preexisting rock; clastic is the geological term for rocks that were formed in this manner. It is also characterized by the geologic term fissility, which means it can be split into thin layers. This distinguishes it from other sedimentary rocks that form from small particles of clay and silt which are called siltstone, claystone or mudstone according to the relative abundance of the constituents. The fissility of shale is attributed to the alignment of minerals that tend to form in plates, notably mica, into stratified layers. Shales in general are mostly comprised of silicon dioxide (SiO2) and alumina (Al2O3), about 60 percent of the former and 15 percent of the latter, the balance consisting of varying amounts of iron, sodium, potassium and magnesium. The constituent fine grains of silt that form shales are lighter, and are therefore carried the furthest distance from the eroding source material, the larger and heavier clastic fragments having already precipitated. Thus, shales are considered of marine origin that settled in quiet and deep waters, the natural gas that they contain the result of the decomposition of entrained organic material under conditions of elevated temperatures and pressures. The resultant gas is trapped in pores and in any other areas of discontinuity in the shale in addition to gas molecules being adsorbed onto the surface of the mineral grains. The subsequent extraction of shale gas for commercial use is increasingly possible with advances in drilling technology, with some attendant hazards as will be discussed below.
Coalbed methane has historically been considered a bane, as it is the causative factor in many (frequently fatal) coal mine explosions. It is not called coal gas because of the precedent use of this term to refer to a gas produced by the destructive distillation of bituminous coal that has historically been used for heating and lighting. Coal seams are the end result of the deposition and gradual decomposition of plants in swampy areas that were prevalent in the Mississippian and Pennsylvanian Periods (collectively called the Carboniferous Period), an 80 million year long time span that occurred about 350 million years ago. The burial of the plant detritus in anaerobic shallow waters prevented the normal decay by oxidation resulting in the formation of coal. These conditions also promoted the formation of natural gas and many if not all coal beds contain some natural gas either directly or in surrounding rock. When the coal is extracted by mining operations, the coal-bed methane leaks out, resulting in a potential safety hazard. While this gas has historically been ventilated to the atmosphere, increasing natural gas demand coupled with decreasing supply has engendered an interest in capturing it for injection into the natural gas pipeline networks.
The historical use of natural gas is not as prevalent as that of petroleum and coal due in large measure to its ephemerality ; it evaporates. It is thought that the first use of natural gas was from the petroleum seeps near the site of modern Baku, Azerbaijan that were probably first ignited by lightening and used by the Ancient Persians for the “eternal fires” of their religious ceremonies. The first natural gas well was established in central China near the present day city of Chongqing in 211 BCE using bamboo tubing to extract gas from a depth of 500 feet. Natural gas would not discovered in Europe until 1659, about two millennia later. However, the discovery that a ‘natural’ gas extracted could be extracted from coal in 1670 led to the first practical use. Coal gas or “town gas” spread through Europe in the early19th Century; the town of Fredonia, New York became the first municipality in North America to use coal gas for lighting in 1821. However, widespread use of natural gas was limited to local jurisdictions within 100 miles of the source by the absence of a long-range distribution system; gases are much harder to consolidate and transport than liquid petroleum, solid coal, or wired electricity. The development of improved containment pipe materials to replace the legacy, mostly lead pipes in the late 19th Century promoted a steady expansion of natural gas distribution systems that began in the 1920’s and accelerated after World War II.
Natural gas currently comprises about a quarter of all energy production in the United States, where it is consumed at the rate of about 22 Trillion cubic feet (Tcf) per year, which is about one fifth of the world consumption rate of 108 Tcf per year. Based on the proven natural reserves of 237 Tcf in the United States, there is enough natural gas for about 10 years (worldwide proven reserves are estimated at 6,399 Tcf, enough to last 60 years at current consumption rates). The paltry longevity estimate of one decade is skewed by the rules governing the designation of proven reserves, which consists of those quantities of natural gas which can be estimated with a high degree of confidence to be affordably recoverable from known reservoirs under current economic conditions. This designation relies on an analysis that includes an estimate of the physical size of the geological formation and engineering data such as pressure and gas composition to arrive at a quantitative result. Estimates of reserves from previous gas fields that have been depleted provide empirical results for comparison verification. The extent of unproven reserves, which, depending on the methodology employed, can produce a result of ten times the proven reserves, are much more subjective. There are no universal estimating standards and different entities accordingly apply their own methods which are likely biased by the desired result, which almost certainly would favor overestimation.
The Energy Information Agency (EIA) of the U. S. Department of Energy provides a unbiased source of data; there is no economic advantage associated with their estimating methods. The EIA recognizes two additional categories beyond proven reserves. Inferred resources are those natural gas reserves that can be predicted with analysis methods based on extrapolation from existing proven reserves. In the case of the United States, this amounts to just over 1,000 Tcf, almost half of which comes from shale gas and coalbed methane. The other category, called ‘undiscovered technically recoverable reserves’ is much more nebulous and is based on what is euphemistically called ‘integrated assessment and forecasting,’ essentially an educated guess. If one adds in the inferred and undiscovered natural gas resources, then the grand total rises to about 2,500 Tcf, which amounts to 100 years at current consumption rates. Like petroleum, the gradual depletion of readily recovered conventional natural gas reserves has led to increasingly circuitous extraction technologies. Just as the tar sands of Alberta, Canada are being excavated and heated to extract every last drop of oil, shale formations are being ‘fracked’ to extract every last molecule of methane.
A relatively recent renaissance in shale gas extraction resulted from the conjoining of slickwater hydraulic fracturing (‘fracking’ in the argot of the gas business) with horizontal directional drilling. Initially, all drilling was vertical, as the oil and gas reservoir was always underground, downward from the wellhead. Early in the petroleum revolution of the early 20th Century, it became manifestly clear that reservoirs were not really vertical, but were typically inclined at an angle. This led to the development of some limited directional drilling in the 1930’s using inclination and depth data from a gyroscope and azimuth (relative direction) data from changes in the magnetic field. The ability to turn the drill direction by a full 90 degrees to drill horizontally at a given depth became possible in the 1970’s with the development of hydraulic drilling motors that could rotate the bit at the bottom of the hole. Thus shale beds, which are typically not very thick but can be quite large geographically, became a viable configuration for natural gas extraction. The process of fracking, which has been in use for about 60 years, consists of injecting upwards of one million gallons of water, sand and a variety of chemicals into the shale formation at high pressure (about 6,000 psi) to break, or fracture, the layers along their stratified fault lines. With directional drilling, the fracking operation can now extend horizontally in roughly 1,000 feet increments from the injection point for about a mile, a total of five increments per bore. After the injection process is completed, the pressure is reduced, allowing about 40 percent the now fouled water to flow back out the pipe where it is stored in temporary catch basins, leaving the entrained sand in situ. The purpose of the sand is to remain in between the shale strata to prop them open (recall that shale particles are < 0.02mm and that sand is 1-2 mm) to open a conduit for the passage of natural gas, which can now be collected at the wellhead. The sand is called ‘proppant,’ a coined word meaning ‘that which props open.’
The Marcellus Shale formation is at the center of the debate about fracking and energy independence. It is in the Allegheny Plateau region of the Appalachian Basin underlies areas of western New York and Pennsylvania, eastern Ohio and most of West Virginia, comprising an area of 65 million acres. It is estimated to contain about 350 Tcf of recoverable natural gas in close proximity to the high energy demands of the Northeastern metropolitan areas. Citing concerns for the watershed of New York City, the New York Department of Environmental Conservation established onerous and expensive review requirements that eliminated the economic viability of shale gas fracking in that state. The governor recently issued an executive order banning horizontal fracking pending the development of achievable, though stringent, new regulations. Pennsylvania, on the other hand is in the midst of a natural gas rush. Starting in 2005, when two wells were drilled, the drilling rate has risen asymptotically to 210 in 2008, 768 in 2009 with expectations for 5,000 applications in 2010. Between July, 2009 and June, 2010, Pennsylvania’s Marcellus wells yielded 180 billion cubic feet of natural gas.
Environmental concerns about fracking have been raised due to the collection and disposal of the concomitant fouled water and due to the potential and postulated effects of the chemicals on groundwater. The treatment and disposal of the millions of gallons of fouled water generated by the fracking process raises the same legitimate concerns ass those raised against mining tailings and factory farm excrement; what do you do with the waste? The current method is to treat it and dispose of it, though there have been some allegations of improper disposal leading to watershed contamination. The chemicals used for the fracking process, generally referred to with the whimsically pejorative name fracking chemicals, are a proprietary mix that varies according to the company using them and to the specific geological conditions at the well site. The specific chemicals and quantities used are exempt from the underground injection control provisions of the Safe Drinking Water Act due to the 2005 Energy Policy Act, which is sometimes called the Halliburton loophole to reflect its association with the former Halliburton CEO, Dick Cheney, Vice President at the time. In general, each fracking operation requires about ten chemicals that include corrosion inhibitors to minimize rusting, biocides to kill inimical microorganisms, tribological additives to reduce friction, and acids to keep the spray head nozzle holes clean. Among the fracking chemicals listed by the major gas extraction companies are methanol, hydrochloric acid, ethylene glycol and petroleum distillate blend. The non-profit Endocrine Disruption Exchange has compiled a list of 344 different chemicals used in fracking, many having known adverse effects on the sensory organs, the lungs and the liver, among other things. Even though these chemicals are added in parts per million (ppm) concentrations and comprise less that one percent of the overall fracking liquid volume, the enormity of the fluid requirements result in the injections of hundreds of thousands of gallons of chemicals in the area surrounding each wellhead.
The gas industry and many geologists argue that the chemicals are injected at a depth of more than a mile (average 7500 feet) and are separated from the shallow (100-200 feet) groundwater by a layer of impermeable rock . Although there have been no confirmed instances of contamination of groundwater wells due to the fracking process, there have been numerous allegations that have yet to be substantiated. Rancher Louis Meeks of Pavillion, Wyoming reported that his wells were contaminated by nearby fracking and subsequent testing by the Environmental Protection Agency (EPA) confirmed the presence of the toxic chemical 2-Butoxyethanol (or 2-BE). There are currently two lawsuits in Texas filed by residents who claim that their wells were contaminated by fracking. Six wells have been shutdown due to contamination in Bradford County, Pennsylvania since 2008 when large scale fracking of the Marcellus Shale formation began; Residents of Susquehanna County, Pennsylvania have alleged that their water supply is contaminated with methane since the water was pure before the advent of fracking. It suffices to say that there is enough concern for the apparent syllogism of fracking and groundwater contamination that further study is warranted. The EPA has started a two year study due to complete in 2012 to evaluate groundwater contamination and fracking wastewater disposal to address the legitimate concerns of the public. The problem is that it is very difficult to prove the cause and effect, as groundwater contamination by natural gas is not all that uncommon.
Natural gas is lauded as a non-polluting energy source and is considered benign by most people; the media message of ‘clean natural gas’ is intended to reinforce this perception. It is on all accounts cleaner that coal, as it produces no smoggy soot and it is superior to petroleum in releasing fewer unburned constituents. Natural gas, once refined to nearly pure methane that is the commercial standard, produces energy when oxidized according to the basic combustion :
CH4(gas) + 2O2(gas) CO2(gas) + 2H2O(liquid) + 891 kilojoules
The energy produced by this reaction is 891 kilojoules for every molecule, which, in terms more familiar, is about 21 kilocalories (this is the unit that one sees on food packaging as calories – the calorie is really 1 kilocalorie or 1,000 calories), which is the amount of energy required to raise the temperature of 21 kilograms of water by 1 degree centigrade (or Celsius if you prefer). Oil and coal contain many other constituents and the energy conversion is not as efficient. What this means in practical terms is that natural gas produces approximately 40 percent less carbon dioxide than coal and 25 percent less carbon dioxide than gasoline for the same energy output. While this is certainly an improvement, it does not address the fundamental problem – which is that increasing carbon dioxide in the atmosphere is the primary forcing function in the empirical and now apparent elevation of temperatures that is a manifestation of the greenhouse effect on climate. Using more natural gas to supplant other fossil fuels is not a panacea but only a temporal mitigation that must ultimately be addressed with alternative renewable energy sources.
There is a secondary issue with natural gas utilization in particular and with methane in general that must be considered in the debate about energy alternatives. A CH4 molecule is more than 20 times more effective in trapping heat in the atmosphere than is CO2, and is accordingly one of the 6 gases considered by the Intergovernmental Panel on Climate Change (IPCC). It is estimated that the amount of methane in the atmosphere has increased by a factor of 1.5 over the last 250 years and that it contributes about 10 percent of the total greenhouse effect. There are three primary sources of methane release to the atmosphere: landfills, enteric fermentation and natural gas emissions incident to its transport and use. Taken together, these three sources make up about three quarters of the total methane greenhouse effect Landfill methane is the result of the decomposition of trash when acted upon by anaerobic bacteria. Although the number and size of landfills has increased over the last several decades, the amount of methane has declined due to improved collection methods. Enteric fermentation is the result of the digestive process in ruminant animals (including cattle, sheep, goats and camels) as microbes resident in the rumen stomach ferment the ingested plant material and produce methane as a by-product, which is subsequently expelled by eructation (belching). While non-ruminant animals (notably pigs, horses and humans) do expel methane it is in much smaller magnitudes than the ruminants. Cattle are by far the largest contributor, with beef cattle contributing 70 percent and dairy cattle 25 percent of the total enteric methane contribution. Some other significant sources of atmospheric methane are coalbed methane and manure management. Coal mining was at one time a major source but improved coalbed methane capture has reduced this contribution by almost 50 percent in the last ten years. On the other hand, manure management methane has increased due to the increasing concentrations of factory farms in the pork and dairy industries. It suffices to say that natural gas is a problem not only through its conversion to carbon dioxide during combustion, but also as an inherent property of its molecular constituency.
There is one other form of methane that must be considered in any discussion of energy and the atmosphere – methane hydrate. In the future, it may well prove to be the most important form of methane just as it is currently the most esoteric. A hydrate is a generic name for any stoichiometric (having a fixed ratio of elements) compound formed in which water (H2O) is an integral part. Methane hydrate is therefore methane and water in a compound. The name is a misnomer in this application, however, since methane hydrate is non-stoichiometric, belonging to a group of compounds called clathrates. First discovered as a chlorine compound in the 19th Century by the scientists Sir Humphrey Davy and his laboratory assistant Michael Faraday (better known for his pioneering work with electro-magnetic induction), clathrates were later determined to be mechanically rather than a chemically associated. Fundamentally, clathrates consist of a rigid lattice structure of water molecules with a large central cavity that is occupied by another molecule without any chemical bonding. In the case of the clathrate methane hydrate, the central molecule is methane, one of several molecules that have the right size and shape to provide some stability to the crystal lattice structure of the water.
Methane hydrates were considered to be only an interesting curiosity until the 1930’s, when it was discovered that they were responsible for flow stoppages in natural gas pipelines. This led to some early research in understanding the low temperature and high pressure conditions under which they formed do that the costly hiatus in gas distribution could be eliminated. The formation of solid methane hydrate is still the bane of deep-water oil and natural gas extraction; the first attempt to cap the BP Deep Horizon blowout in the Gulf of Mexico was stymied by the formation of a methane hydrate plug. Interest in methane hydrate was piqued in the 1960’s when it was discovered in the Messoyahka gas field in Siberia and in the sediments of the North Slope in Alaska. Based on these discoveries, the hypothesis that methane hydrate was a globally ubiquitous and pervasive constituent of deep ocean bed stratigraphy was proffered by scientists from the Soviet Union. Their exploration led to the first discovery of methane hydrate nodules in the Black Sea in 1974. Subsequent to that initial discovery, the U. S. research vessel Glomar Explorer (the same Howard Hughes vessel that notoriously raised part of a sunken soviet submarine) began drilling ocean cores, many of which were found to contain traces of methane hydrate. Based on this empirical result, the U. S. Department of Energy and the U. S. Geological Survey initiated a decade-long research and development effort that culminated in 1995 with the initial estimate that domestic methane hydrate deposits contained on the order of 200,000 Tcf of methane (recall from above that the world proven reserves are only 6,399 Tcf). This would mean that methane hydrate contained more carbon than all known petroleum, coal and conventional natural gas combined. Beginning in 1998, international interest and effort resulted in the drilling of two wells to specifically evaluate methane hydrate concentrations in deep wells, one in the McKenzie River delta in Canada’s Northwest Territories, and the other off the southeastern coast of Japan adjacent to the Nankai Trench. The feasibility of natural gas extraction for commercial purposes is currently under evaluation.
The apparent existence of a heretofore unknown gargantuan global repository of methane at the deepest ocean depths raises some potentially troubling and difficult international public policy questions. Of prime importance is the need to understand the provenance of the deep ocean methane hydrate. It is quite possible and even probable that it plays a key role in the carbon cycle that is literally at the very heart and soul of life on earth.. It is hypothesized that methane hydrates are a key storage medium in the carbon recycling process, the death and decay of plant, animal and perhaps more importantly bacterial life forms that remove carbon during their life and growth restore it in expiation as they fall to the ocean floor or are subducted in the recycling of the tectonic plates. The relative instability of methane hydrate also mandates a thorough understanding before any attempts are made to commercially exploit it as a potential energy source. It rapidly decomposes when exposed to atmospheric conditions of temperature and pressure, which is one of the reasons that it has taken so long to detect; deep ocean drilling was a prerequisite technology. The rapid decomposition of methane hydrate and the resultant release of massive amounts of methane into the oceans and then into the atmosphere is among the more relevant and plausible doomsday scenarios that are suggested by climate change extrapolations. The environmental Armageddon scenario is that increasing temperatures which are already causing the inexorable melting of the polar icecaps and glaciers will result in a warmer ocean which will disrupt the stability of the deep ocean habitat. The concomitant release of methane would be autocatalytic, the greenhouse effect of methane evaporation trapping more heat to release yet more methane. It would be wise to proceed with extreme caution.
The Methane Hydrate Research and Development Act was legislated in 2000 and amended by the Energy Policy Act of 2005 to establish a program to address this issue;
The long-term goals are to assess: 1) the role of gas hydrate in global environmental systems such as carbon cycling, global climate, and sea-floor stability; and 2) the potential for, and impacts of, degassing resulting from either ongoing “conventional” oil and gas exploration and production activities or future gas hydrate production activities. Among the key objectives is to evaluate the impact of the release of gaseous methane to the atmosphere. In 2008, the Department of Energy entered into a joint international agreement with Japan, India and South Korea to coordinate and share methane hydrate research. This would appear to be a rational attempt to understand the issue and slowly move forward on the basis of sound science; to an optimist the end of the beginning and to a pessimist the beginning of the end. Time will tell.