Location and Spacing of Wells.
From the foregoing it is evident that, as far as is possible, the geological conditions should determine the locations of wells, es pecially in a new field where the first test, or 'wildcat,' well is to be drilled without positive knowledge of the presence of oil. When the structure is found to be anticlinal or that of a dome, and topog raphy, ownership, etc., permit, the well should be placed on the summit of the fold where the prospects are that the best showing will be obtained at the shallowest depth, thereby minimizing the expense. When the well is to be drilled to reach a measure that is exposed at the surface, its dips and surrounding strata should be learned. Faults, if any, should be determined, and, from the data thus obtained and the knowledge as to the approximate depth at which it is desired to penetrate the oil sand, a rough idea may be reached as to the distance from the outcrop the well should be placed.
Thus if the surface exposure dips 30° and it is believed from local evidence that the lessening in dip is such that the average dip to where the measure is 800 feet deep is 5° less, or 25°, then the determination of the horizontal distance to a point 800 ft. above the measure becomes a simple problem, in this case working out to be 1715 ft. If in this distance the elevation falls off, say 70 ft., below that of the outcrop, then the actual distance to be drilled becomes lessened to that extent. Such computations, due to the variable factor of the change in dip, are necessarily of indefinite value and can be used only in a very broad way. They do, how ever, bring out the point that where measures are steeply in clined it is to be expected that the field will be narrow and a pros pect well should be situated nearer the outcrop than where the dip is known to be more gently sloping. Such freedom as out lined above does not of course hold true where property lines pre scribe limits within which wells must be situated.
Since a well when once drilled derives its oil from a zone ex tending in all directions about it, the natural tendency is to place it as near to the neighbor's property as possible in order that a portion of his oil may be drawn on and contribute to the supply. For this reason mutual agreements are usually adopted by adjoin ing owners, to the effect that neither will drill within a certain dis tance of the line, say 100 or 150 feet. For this reason also the out
side locations, that is, the locations along the line at this stated distance, known as the 'line wells,' are usually drilled first and the inside locations later.
The spacing of wells is a matter that must depend entirely on local conditions, particularly those relating to the nature of the sand or other productive measure, and the gravity of the oil. If the oil is heavy and viscous or the source is tight, they may be situated much more closely together than where the oil is light and flows readily and the containing measure is open and porous. It is seldom advantageous to distance them less than 100 ft., while 300 ft. is more often good practice, and even 500 ft. or greater where gas pressures are high and the oil very mobile. The close crowding of wells that has resulted in some fields from the land being owned or leased in small parcels has meant a distinct eco nomic waste where half the number would have sufficed to pro duce an equivalent amount of oil.
When the outside wells have been finished, the inside locations are then drilled, usually according to some definite plan or system worked out by which it is designed to secure all the recoverable oil with a minimum number of wells and without interfering with surface improvements such as tanks, buildings, or sumps.
Logs. .
The log is a record of the well from the time of beginning until its completion and shows the depths and thicknesses of strata drilled, points at which water, gas, and oil are found as well as other data relative to its history. To this end the log should also contain not only the record of casing inserted but also any other features that may be of importance at some later time, such as unusual fishing jobs, tools, or casing left in the hole and side tracked. Such items, while apparently of little moment at the time as far as the log is concerned, may have an important bearing on work being carried on with the well possibly several years later when the knowledge as to just where different troubles had hap pened in the first drilling may prove of considerable value.