SOURCES OF WATER There are two main sources of water in all oil fields: water confined in the oil-bearing strata and water occurring separate from the oil zones. In the first case water is sealed in the oil sand and normally it underlies the oil and gas. It may be sulphur or salt water, or, in fact, may contain many minerals. Originally it was meteoric or rain water that percolated through the earth until it reached the oil strata, where it was sealed or held in by the shaley or clayey beds that are gencially found above and below the oil sands. In some cases, however, this water is the sea water contained in the shales and sands before folding occurred. This water is generally under pressure, and as it is already in the oil zone, there is no way of eliminating it as a source of trouble. However, where there are two or more oil zones and but one is troubled with water, there may be great economy in shut ting off one of the troublesome zones.
On the whole, water confined to the oil zone is less dangerous to the industry than water occurring separate from the oil zone. The latter may occur above, below, or between the oil strata. Generally occurring under head and in large quantity, it becomes a source of great danger to an oil field when it enters the oil zones. Indeed it is the water most to be feared and most to be guarded against. Obviously, such water enters the oil zones only as the result of artificial or man-made causes, as the oil zones and water bearing strata are separate and distinct. Earthquakes or volcanic upheavals may cause faulting and shattering of the formation to such an extent that water enters the oil zones from the separate water sands, but for all practical purposes the latter causes will not be considered.
Two sources of water have been discussed. For the sake of clearness all those waters originally confined in the oil zone will be called primary waters, and all waters occurring in strata sepa rate from the oil zones and entering the oil zones from artificial causes will be called secondary waters. These definitions refer
to the original sources of the water. The terms "top water," "bottom water," "intermediate water," "surface water," and "edge water" are in constant use by oil men to express the occur rence of water in oil wells, especially those newly drilled. These terms do not take into account the sources of the water, whether or not it was originally confined in the oil zone by natural pro cesses or introduced by artificial means.
Primary Water water, as above mentioned, lies at the bottom of the oil sand under pressure. As the gas and oil are withdrawn from the sand, the water rises to replace them.
In time nearly all the oil is drawn from the sand and only the water is left. The field must then be abandoned. This is the result of natural exhaustion and is expected in most fields. Where there is but one oil zone, little avoidable danger or trouble occurs. Assuming, however, two strata (Fig. 74), as is commonly the case, water will rise from the lower stratum, B, into the upper stratum, A, unless proper precautions are taken to avoid this condition. Where the gas pressure is strong the water in B will be driven into A. Especially is this true when zone B be comes all or nearly all water (see Fig. 75). By properly confining the water to the bottom zone, water troubles would be averted for a time.
Again, there is an extreme con dition as shown in Fig. 76, page 179; in which the water takes possession of A and then enters B. This requires that the water in A be kept from B by methods differing from those used in the two previous cases.
These cuts illustrate the extremes of water flooding. Such conditions are theoretical, but are closely approximated in the oil fields. Generally all the oil is not driven from the well, but exists as an emulsion of finely divided oil and water which is very difficult to treat. However, the two cases given act as the limits within which all primary water troubles of the flooding type may be placed.