Tag Archives: natural gas
Tight gas and shale gas difference
Explanation of each of one concepts related to the exploitation and as a source in the reservoir, where lies the tight gas and shale gas difference.
TIGHT GAS. Tight gas is unconventional natural gas which is difficult to access because of the nature of the rock and sand surrounding the deposit. Because this gas is so much more difficult to extract than natural gas from other sources, hydraulic fracturing and directional drilling is necessary to produce the well. This is also found trapped in impermeable rock and non-porous sandstone or limestone formations, typically at depths greater than 10,000 feet below the surface. The viability of sandstone reservoirs is determined by their porosity, or the open space between grains, and permeability, or how easily fluid or gas moves through the rock. In some cases, the gas can be found in small, isolated zones within 20 feet of each other, but due to the density of the rock formation, are inaccessible via the same vertical well.
The United States has been producing tight gas for more than four decades, and it now accounts for approximately 40 percent of the nation’s unconventional gas output.
Shale gas different from tight gas
Shale gas refers to natural gas that is trapped within shale formations. Shales are fine-grained sedimentary rocks that can be rich sources of petroleum and natural gas. Shale is one of the Earth’s most common sedimentary rocks. It is a fine-grain rock composed mainly of clay flakes and tiny fragments of other minerals. Shale can be a gas reservoir, but only formations with certain characteristics are viable for development.
Over the past decade, the combination of horizontal drilling
and hydraulic fracturing has allowed access to large volumes of shale gas that were previously uneconomical to produce. The production of natural gas from shale formations has rejuvenated the natural gas industry in the United States.
Thermogenic (from the Greek word meaning ‘formed by heat’) gas forms when organic matter in shale is broken down at high temperatures, often a result of burial deep underground. The gas is then reabsorbed by organic material to trap the gas within the shale. Shale gas is the most commonly known unconventional gas.
HISTORICAL GLOBAL ENERGY CONSUMPTION SINCE 1940 TO PRESENT DAY
Figures 1 and 2 are based on data presented by Schollnberger and show the dominance of fossil fuels in the energy mix at the end of the 20th century.
Figure 1.
Figure 1 shows historical energy consumption in units of Quads, a unit of energy that is often used in discussions of global energy because it is comparable in magnitude to global energy values. One Quad equals one quadrillion BTU or 10^15 BTU. In SI units, one Quad is approximately 10^18 J. The source of energy is presented in the legend in the same order as it appears in Figure 1.
Beginning at the bottom of Figure 1, we see that fire wood, coal, oil, natural gas, water and nuclear energy were the major contributors to energy in the latter half of the 20th century. The dominance of fossil fuels in the energy mix at the end of the 20th century is illustrated as a percent of total energy consumed in Figure 2. Each percentage distribution shown in Figure 2 applies to the associated point in time.
Figure 2.
NATURAL GAS HYDRATES FORMATION
Gas hydrates are chemical complexes that are formed when one type of molecule completely encloses another type of molecule in a lattice. In the case of gas hydrates, hydrogen-bonded water molecules form a cagelike structure in which mobile molecules of gas are absorbed or bound. Although gas hydrates can be found throughout the world, difficulties in cost-effective production have hampered development of the resource. Gas hydrates are generally considered troublesome for oil and gas field operations, but their potential commercial value as a clean energy resource is changing the industry perception. The potential as a gas resource is due to the relatively large volume of gas contained in the gas hydrate complex.
COALBED METHANE GAS
Coalbeds are an abundant source of methane. The presence of methane gas in coal has been well known to coal miners as a safety hazard, but is now being viewed as a source of natural gas. Coalbed methane exists as a monomolecular layer on the internal surface of the coal matrix. Its composition is predominately methane, but can also include other constituents, such as ethane, carbon dioxide, nitrogen and hydrogen. The gas, which is bound in the micropore structure of the coalbed, is able to diffuse into the natural fracture network when a pressure gradient exists between the matrix and the fracture network. The fracture network in coalbeds consists of microfractures called cleats. Gas flows through the microfractures to the production well.
