GASES.
GAS, Natural. By recognized usage the term "natural has been restricted to the inflammable gases found accumulated in reser voirs in the earth's crust — leaving the equally natural supplies of carbonic acid gas and of nitrogen to be given a more specific title. In composition, various samples of natural gas differ very considerably, but its principal com ponent is methane, or marsh gas (CH4), rang ing from 81 per cent for Pennsylvania and West Virginia gas to 98 per cent in some of the gas produced in Oklahoma. In the Glenn Pool (Oklahoma) gas, known as "wet" gas from its heavy content of petroleum vapors, the components are methane, 39 per cent, and ethane, 61 per cent. The gas at Dexter, Kan., has hut 15 per cent methane, and 83 per cent nitrogen. The combustion factor of the Pennsylvania and West Virginia gas is made up by 14 per cent of other hydrocarbons, chiefly ethane, and hydrogen. In addition, natural gas contains from 2 to 5 per cent nitro gen; up to 2 per cent of carbonic acid; up to IA of I per cent of carbonic oxide and up to 'A of 1 per cent of oxygen. Occasionally small quantities of helium are present; in one case 1.84 per cent. It has an average heating value of about 945 British thermal units per cubic foot at a pressure of four ounces per square foot, and a temperature of 60° F. At a greater pressure and at a higher temperature its heat ing value is increased by about one unit for each higher degree of temperature, and two units for each added pound of pressure.
The accompanying table presents a com parison of the principal types of natural as with the other economic artificial illuminating and fuel gases.
ever there are hollows in which the pres sure is less than its own, and only collects in considerable quantities where there is an im permeable stratum of clay or rock to prevent further movement. In these hollows it ac cumulates Linder varying pressures, depending upon their depth below the surface, the pres sure having a close relation to the weight per square inch of a column of water of the same height; from which it is argued that the pres sure found in natural gas reservoirs is derived from water forcing the gas out of a lower level. The maximum pressures reported have been 1,700 pounds per square inch in Green County, Pa., and 1,260 pounds more recently at Midway, Cal.
Wells and Conditions of Strata.— There is a great variation in the depth of natural gas wells, owing to the diversity of the strata in which the product exists and the changing po sition of the underlying rocks with reference to the general surface. Some natural vents
have produced natural gas in considerable quantities and have proved the incentive for drilling down to the original reservoirs, from which the gaseous fluid was escaping. Other gas wells have been discovered in drilling wells for oil or salt brine. Some of the most im portant gas fields have been located by expert geologists, who have traced out the summits of the anticlinals, or rock waves, for many miles from surface exposures of the strata.
Natural gas is regarded as a form of bitu men, closely related to petroleum, maltha and asphalt. It may be mentioned in passing that the asphalt which is continually rising on the lake at Trinidad is aerated by large bubbles of natural gas. Opinion is divided as to the origin of natural gas, hut the majority favors the theory that it is the product of vegetable matter slowly decaying at a low temperature. As it is found only in sedimentary rocks, the theory prevails that it is the product of car bonaceous matter deposited with those strata, although it is quite as likely to have been formed elsewhere, finding its way into those rocks because of their porous structure. Gas, because of its natural qualities, has a tend ency to make its way for long distances wher The depth of wells varies from 250 to 3,000 feet, while their diameter varies from two inches up to eight inches; their output, or open flow, varies from 500 cubic feet per day to 35,000,000 cubic feet per day; their shut in, or rock pressure, varies from I to 1,500 pounds to the square inch in extreme cases, while 300 to 400 pounds to the square inch and a volume of 1,000,000 cubic feet per day is considered a profitable commercial well. The greatest yield on record for any one well is 7,781,946,000 cubic feet in years, an average of 2,842,700 cubic feet per day for the entire period. This well is in Pennsylvania. It is down 3,000 feet tapping the Gordon sand. In 1915 it was still yielding 1,000,000 cubic feet per day. In many of the deeper wells, two or more reservoirs of natural gas are often found. The cost varies from a few hundred dollars in the shal low shale districts to $10,000 in the deep wells in West Virginia. All large wells are usually tubed and a packer set just above the gas sand; the top of the tubing is held by clamps attached to bolts that are anchored'and a heavy gate valve attached, so that the well can be shut in and the gas held in the rock when not in use.