STORAGE OF OIL When the oil is above ground, it is usually carried direct from the well to the refiner or other consumer by the pipe-line com pany. However, the pipe-line company may store the oil for future use; or the producer may store his own oil for a future market.
The refiner who uses large quantities of oil must keep a suffi cient reserve in storage to take care of his needs.
Oil storage is of two main types: 1. Earthern reservoirs, either lined or unlined with concrete.
2. Tanks, steel or wooden.
The most natural reservoirs are the sump holes or temporary earthen reservoirs that have served to store oil around a flowing well. They are, however, makeshifts, serving only to hold the oil until better storage facilities can be furnished. Such reser voirs are generally constructed hastily. A few hours' work with a scraper creates an embankment across a gully behind which the oil collects.
In California, in Mexico, in Texas, and in fact in all the fields where the oils are largely of fuel grades, say from 14 to 2213e., sump holes have played an important part.
The losses by evaporation and seepage in reservoirs are large. A loss of 15 to 25 per cent is not unusual where the oil stands for 5 to 6 months. The loss in gravity alone ranges from 4 to 6°Be. However, such losses must be expected where flowing wells of 25,000 to 60,000 bbl. per day break loose and ade quate storage is not available.
Losses.—The most natural improvement in earthen reservoirs was putting roofs over them, which largely prevented evaporation losses. The next step in progress in reservoirs was the cement lined reservoir. Large earthen reservoirs capable of holding 500,000 to 1,000,000 bbl. are constructed and these are care fully lined with cement. The losses in such reservoirs are very low, being in some cases less than one-half of 1 per cent per year.
When the Lake View gusher was brought in, several narrow canyons were converted into reservoirs across which concrete dams were built and the oil pumped into those reservoirs. These canyons were deeper and much narrower than the ordinary sump hole and evaporation losses were necessarily smaller since evapor ation losses depend directly on the surface of the oil exposed to the atmosphere.
Tanks.—Steel and wooden-tank storage has been used almost entirely in the Eastern, the Mid-Continent, and the Wyoming oil fields. The light oils of those fields are so susceptible to loss that wooden and steel-tank storage was accepted at once.
Steel storage is by far the most efficient. Some wood tanks are used as receiving tanks on the leases, but oil is not stored in them for more than a few weeks at most.
Tanks vary in size anywhere from 50 to 55,000 bbl. Fifty to two-hundred-barrel tanks are used on leases for storage of oil.
The big storage tanks range from 37,500 to 55,000 bbl. in capacity. A tank of 37,500 bbl. capacity has a diameter of 94 ft. and a height of 30 ft. A 55,000-bbl. tank has a diameter of 1142 ft. and a height of 30 ft. (See Fig. 68, page 160.) Some tank dimensions are given in the Table 11 after Bacon and Hamor, page 69 of American Petroleum Industry: Such tanks are made of six rings of thin steel plates. The lowest ring of plates is the heaviest-23 lbs.; the top the lightest —8 lbs. in weight.
Wooden tanks are in general use on leases for water and also for storing oil. They are shaped like a conical frustrum and have steel hoops or straps binding them.
Tanks are gauged or " strapped." Allowance is made for all beams and supports inside the tanks. The value of each Y in. of height of the tank is carefully determined and tables con structed for tanks of given diameters. When gauges are taken it is a simple matter to consult these tables and determine from them the amount of oil in a tank.
Steel tanks have lives varying from 15 to 35 years. Twenty years may be considered a fair average life. Tankage costs vary greatly. A 250-bbl. tank costs $300, or $1.20 per barrel. A 55,000-bbl. tank would range from 30 to 350 a barrel. Wooden tanks cost from 35 to 40˘ a barrel.
Concrete-lined reservoir costs vary greatly. A large 750,000 bbl. reservoir should cost in the neighborhood of 150 per bbl.