Active Vocabulary

Date: 2015-10-07; view: 548.

Unit 4. Unconventional Gas Reservoirs

С. Discussion

Topics:

1. Oil and gas migration.

2. Accumulation of oil and gas.

3. Reservoir rocks.

Substantial amounts of gas have accumulated in geologic environments that differ from conventional petroleum traps. This gas is termed unconventional gas and occurs in "tight" (i.e. relatively impermeable) sandstones, in joints and fractures or absorbed into the matrix of shales (often of the Devonian Period about 360,000,000 to 408,000,000 years old), dissolved or entrained in hot geopressured formation waters, and in coal seams. Unconventional gas sources are much more expensive to exploit and have to be produced at much slower rates than conventional gas fields. Moreover, recoveries are low. In all likelihood, unconventional gas will continue to complement conventional gas production but will not supplant it.

Deep Natural Gas

Deep natural gas is exactly what it sounds like - natural gas that exists in deposits very far underground, beyond 'conventional' drilling depths. This gas is typically 15,000 feet or deeper underground, quite a bit deeper than conventional gas deposits, which are traditionally only a few thousand feet deep, at most.

Deep gas has, in recent years, become more conventional. Deep drilling, exploration, and extraction techniques have substantially improved, making drilling for deep gas economical. However, deep gas is still more expensive to produce than conventional natural gas, and as such, economic conditions have to be such that it is profitable for the industry to extract from these sources.

Tight gas

Tight gas occurs in either blanket or lenticular sandstones that have an effective permeability of less than 1 millidarcy (or 0.001 darcy, which is the standard unit of permeability of a substance to fluid flow). These relatively impermeable sandstones are reservoirs for considerable amounts of gas that are mostly uneconomical to produce because of low natural flow rates. The outlook for increased production of gas from tight sandstones has been enhanced by the use of massive hydraulic fracturing techniques that create large collection areas in low-permeability formations through which gas can flow to a producing well. A fractured well in a tight gas formation usually produces at a lower rate than a conventional gas well but for a longer time. About 2 percent of the gas production in the United States comes from tight sandstones.

Devonian shale gas

Devonian shale gas was generated from organic mud deposited during the Devonian Period. Subsequent sedimentation and the resultant heat and pressure transformed the mud into shale and also produced natural gas from the organic matter contained therein. Some of the gas migrated to adjacent sandstones and was trapped in them, forming conventional gas accumulations. The rest of the gas remained locked in the nonporous shale. The production history of Devonian shale gas indicates that the recovered gas occurs in well-connected fracture porosity. Production is generally at low flow rates but is long-lasting. The factor of greatest importance in commercial production is the presence of natural fractures, but wells can be stimulated by explosives or by hydraulic fracturing, which sometimes enhances gas production. About 1 percent of the gas produced in the United States comes from Devonian shales.

Coal-bed gas

Considerable quantities of methane are trapped within coal seams. Although much of the gas that formed during the initial coalification process is lost to the atmosphere, a significant portion remains as free gas in the joints and fractures of the coal seam and as adsorbed gas on the internal surfaces of the micropores within the coal itself. Since coal is relatively impermeable, any methane recovered usually must flow through existing fracture systems. Therefore, coal seams that are highly fractured appear to be the best sources of coal-bed methane.

Geopressurized Zones

Geopressurized zones are natural underground formations that are under unusually high pressure for their depth. These areas are formed by layers of clay that are deposited and compacted very quickly on top of more porous, absorbent material such as sand or silt. Water and natural gas that is present in this clay is squeezed out by the rapid compression of the clay, and enters the more porous sand or silt deposits. This natural gas, due to the compression of the clay, is deposited in this sand or silt under very high pressure (hence the term 'geopressure'). In addition to having these properties, geopressurized zones are typically located at great depths, usually 10,000-25,000 feet below the surface of the earth. The combination of all of these factors makes the extraction of natural gas in geopressurized zones quite complicated. However, of all of the unconventional sources of natural gas, geopressurized zones are estimated to hold the greatest amount of gas. Most of the geopressurized natural gas in the U.S. is located in the Gulf Coast region. The amount of natural gas in these geopressurized zones is uncertain. However, experts estimate that anywhere from 5,000 to 49,000 Tcf of natural gas may exist in these areas! Given the current technically recoverable resources are around 1,100 Tcf, geopressurized zones offer an incredible opportunity for increasing the nation's natural gas supply.

Methane Hydrates

Methane hydrates were only discovered in the late twentieth century, and thus constitute the last unconventional gas resource to appear. They are also, very probably, the largest resource, and, along with geopressurized zone gas, the best means of prolonging the carbohydrate age of energy. Methane hydrates, also known as clathrates or gas hydrates, are potentially a huge resource, which by the most generous estimates stores thermal energy equivalent to that of 100 trillion barrels of oil.

Methane hydrates are unstable compounds formed from water and methane in which the water molecules form a sort of cage or lattice around the methane molecules and the two establish weak chemical bonds with one another. Clathrates only appear under conditions of intense pressure and extreme cold and are found in arctic permafrost and in the depths of the sea. Interestingly, submarine clathrates are not confined to regions of cold surface waters, but are quite abundant in the Gulf of Mexico, and presumably elsewhere in warm regions of the globe. This may be explained by the fact that the temperature of abyssal waters is normally just above freezing at all latitudes.

Methane hydrates look like chunks of ice, although frequently they are shot through with bursts of iridescent colors representing minor impurities. Upon being exposed to normal atmospheric pressures they revert very rapidly to their constituents, methane and water.

A number of techniques have been proposed for harvesting methane hydrates, and at least a couple has been attempted on an experimental basis. Whatever technique is utilized, the gas has to be released in situ, at or immediately adjacent to the area where the methane hydrate deposit occurs. If one attempts to bring the clathrates up from the sea floor or from an excavation in the permafrost, most of the methane will be lost in transit.