Although the above rule may be employed, it is by no means the best method. No two territories are alike and therefore a rule that will hold good for one field may be detrimental to another. The methods of spacing wells depend upon the porosity of the sand, the dip of the formation and the water in the sand. Consideration should also be given to the physical properties of the oil and the pressure of the gas.
Where the effective porosity of a sand is large, as in the case of the Texas salt domes, the Mexican water channeled limestones, the California and Russian unconsolidated forma tions, the wells are generally located very close together, the large amount of oil produced puts such wells on a paying basis, but from an economic standpoint such close spacing is unneces sary; one well will drain a much larger area under these con ditions than it would in a relatively less porous sand.
If the producing sand body has a pronounced dip, the wells should be closer together along the line of strike than along the dip. The water level must be watched, and in the case of large holdings it may become possible that the encroachment of water up the dip may be retarded by a vigorous pumping of the wells down the dip to check the upward movement of the water.
In a newly discovered pool the pressure of the gas is much larger and therefore, the original wells will produce a greater amount of oil during the initial stages than the wells that are drilled when the pool has been drained to a large extent.
There are various methods advocated whereby a certain number of wells may be off-set by a less number of wells, at the expense of some lost territory, but at a gain in the number of wells drilled. A method of computing the lost areas as stated
by Johnson and Huntley, in their " Principles of Oil and Gas Production," is as follows: Draw lines on the map midway between each line well and its two opposing line wells, if one is not exactly opposite. This is done by drawing circles with each well in question as a center and joining the points of inter section with a line. These lines make triangles with the lease boundary,showing areas lost or gained. (Fig. 112.) The area of the lost territory thus outlined must now be computed as well as any territory which may be gained from the neighbor. This may be done by making this construction on cross-section paper, counting the number of squares or fraction of squares included in the area. A more exact method is to compute the area of the triangle by the well-known formula of the base times one-half the altitude. In the event that the area is polygonal instead of triangular, it is divided into triangles the area of each computed and added together.
Discoveries of new pools generally create a great excitement and an inrush of oil men, and during such times it is advisable that the geologist should not be misled' by the various stories that are being told, but to go out in the field and obtain all the information that is reliable and base his deductions on unbiased data obtained from the evidence shown by the geological con ditions coupled with any other information available.