OCCURRENCE OF WATER IN OIL FIELDS. 
15 
DRY SANDS. 
In most well logs an attempt is made to differentiate water sands 
and “dry” sands, the “dry” sands being typically those which 
absorb water instead of yielding it. Under ordinary conditions of 
drilling it is generally necessary to maintain a certain depth of water 
in the drill hole, ranging from a few hundred feet when drilling 
shallow wells by the standard method to the total depth of the hole 
when the rotary method is used. The driller’s characterization of a 
stratum as water sand or dry sand must then depend largely on the 
fluctuation of the water level in the hole. If the water in the sand 
is under high artesian head the water will rise in the drill hole and 
may flow at the surface (fig. 1, C). However, a column of drilling 
water a thousand feet high exerts a pressure of 434 pounds per square 
inch at the bottom of the hole, and this pressure must be counter¬ 
balanced by the hydrostatic pressure in the sand before the water 
level in the drill hole will rise. If the hydrostatic pressure is equal 
to the weight of the column of drilling water no change in level will 
occur, but if the pressure is lower the water level will sink and most 
of the drilling water may be absorbed. Thus sands which rapidly 
absorb most of the drilling water might instead yield a short column 
of water if the drill hole were dry, and if they were pumped might 
prove steady producers. Many such sands have doubtless been 
reported to be dry. This distinction is by no means an academic 
one. Water under low hydrostatic head may be unimportant in the 
early history of an oil field, but at a later period, after the gas pressure 
has diminished and a considerable portion of the oil has been removed, 
the oil sands constitute a convenient reservoir, and such water on 
entering them is capable of doing considerable damage. 
The factors causing artesian flows are well known and need not be 
discussed here, but some of the causes of unusually low hydrostatic 
head, leading to the conditions described above, may be mentioned. 
If the sand is a sealed lens completely filled with water it can neither 
receive nor yield water when it is penetrated by the drill. If, on the 
other hand, it is open at both ends so that there is free circulation 
the head may be either moderate or low, depending chiefly on the 
inclination or dip of the bed and the amount of resistance encoun¬ 
tered by the water in its passage between the sand grains (fig. 1, B). 
If the resistance is extremely low the head will be very low, for the 
water will follow the path of least resistance. In many wells of this 
kind the head is probably less than that of the water in the drill hole, 
and as circulation is comparatively free, part of this water will be 
absorbed. 
However, in some drill holes, in which the drilling water has dis¬ 
appeared into a sand with great rapidity, the conditions just described 
