The cost of drilling wells increases with their depth and, there fore, where other productive conditions are equal, deeper fields will, in general, require wider spacing of wells.
Some idea of the proper distance between wells can no doubt be obtained in each field by trial and observation. Such a prac tice will be only approximate, but where careful observation shows that new wells are draining old ones the distance should be increased.
A study of maps of American oil fields shows that boundary lines have usually received first attention and that spacing of wells has, apparently, not been given much consideration. A distance of 440 ft. between wells, or 4.45 acres per well, seems to be very common in many American fields. Such spacing is probably due to the fact that legal subdivisions of land, by frac tional parts of sections, are conveniently covered by such distances.
A definite and arbitrary rule for spacing wells may be useful until observation has furnished a better one. Such information as is available suggests that depth shall be the controlling factor with wells 440 ft. apart in 1000 ft. territory (4.45 acres per well) and the distance increased up to 660 ft. in 3000 feet territory (10 acres per well). Such a rule is suggested for fields where geological conditions are comparatively simple; that is, stratified beds only slightly disturbed. Conditions in fields influenced by salt domes, igneous intrusions or pronounced folding may require other plans. However, even these special conditions would seem to warrant systematic study of the behavior of neighboring wells.
The first well completed in a locality frequently establishes drainage conditions which are most favorable for production, and these conditions are not easily overcome by later wells.
Wells are ordinarily located in rows running at right angles to each other. Such an arrangement allots a square of land to be drained by each well. A more even division of the area would stagger the wells so that each would be at the center of a hex agonal area. Such a refinement, however, would appear super fluous where it is not definitely known how far apart the indi vidual wells should be.i Investigation of Productive Formations to be Developed.—A given tract of land frequently contains several separate strata of oil or gas bearing formation. One of the early steps in a development program is to determine the position, extent and productiveness of the several strata. Such a determination not only serves the purpose of a mere inventory of assets but is also necessary in order that a particular stratum may be exploited without damage or waste to the others. Besides avoiding waste or damage to the underground deposits, a preliminary study of their relative positions makes possible a more economical plan for their development.
Underlying or lower strata of productive formations are fre quently sought for by deep prospect wells after an upper stratum has been partly or wholly developed. Such a procedure appears so simple and logical as to require no particular study or con sideration. However, in many instances, careful study has
shown that such development work failed to disclose the most easily accessible deposits.
In the Elk Hills field of California, a most productive gas zone, having a rock pressure of about 800 lbs. per sq. in., and yielding from 30,000,000 to 100,000,000 cu. ft. per well per day, was actually drilled through without recognition by the drilling crew who followed the ordinary observation practice. A subse quent geological study, based largely upon logs of wells in the locality, led to the actual discovery and development of the gas. When drilling was stopped at a designated depth and the well tested by being bailed dry, the gas flowed so forcibly that con siderable trouble was involved in capping the well.' In the Coyote Hills field of California, a very productive oil formation (one well yielding 10,000 bbl. daily) was drilled through by some 85 wells. Oil was noted in only 11 of the well logs. Systematic study of the logs and drilling conditions led to setting casing above the upper stratum, drilling short distances and test ing productivity by bailing the well. This procedure led to the discovery. Many similar instances have been noted.
In the Goose Creek field of Texas at least one instance was noted where a well became a very profitable producer through an accidental perforation of the casing about 1000 ft. above the supposed productive formation.
In the Somerset field of Texas early drilling generally pene trated the Taylor formation only a short distance and wells pro duced about 10 or 15 bbls. of oil daily. A systematic study of the situation led to a departure from previous practice and the formation was opened to a depth of about 200 ft. with the result that the daily production was increased to 50 or 75 bbls. per well.
Valuable natural gas reservoirs have in past years been waste fully depleted because the operators desired only to reach underlying oil formations. Systematic studies of geological and operating conditions have led to state legislation which aims to segregate and protect all valuable oil and gas bearing strata.
The work of the U. S. Bureau of Mines in the Oklahoma fields is an excellent example of the value of guiding underground development by preliminary scientific Rate of Development.—Some plan governing the rate of development is necessary when work starts. That the subject is of importance is shown by the fact that leases of oil land frequently specify that a certain number of wells shall be drilled each year.
The rate of development will frequently be entirely governed by such facts as the price of oil and the drilling activity on adjoin ing lands. There are, however, some general features which may also present themselves for consideration.
The promptness with which a tract of oil land is developed, by drilling wells, governs the speed with which the invested funds are returned.
The length of time consumed in drilling each well will, of course, have great influence on a plan of development. At a