Page 14 - Geochemistry of Oil Field Waters

INTRODUCTION 3

tions proved that interstitial water was generally present in the oil sands.
Publications by Garrison (1935) and Schilthuis (1938) give detailed informa-
tion concerning the distribution of water and oil in porous rocks, and of the
origin and occurrence of “connate” water with information concerning the
relationship of water saturation to formation permeability.
The word “connate” was first used by Lane and Gordon (1908) to mean
interstitial water that was deposited with the sediments. The processes of
rock compaction and mineral diagenesis result in the expulsion of large
amounts of water from sediments and movement out of the deposit through
the more permeable rocks. It is therefore highly unlikely that the water
now in any pore is the same as that which was there when the particles that
surround it were deposited. White (1957) redefined connate water as “fossil”
water; it has been out of contact with the atmosphere for an appreciable part
of a geologic time period. Connate water is thus distinguished from meteoric
water which has entered the rocks in geologically recent times, and from
juvenile water which has come from deep in the earth’s crust and has never
been in contact with the atmosphere.
Meanwhile petroleum engineers and geologists had learned that waters
associated with petroleum could be identified with regard to the reservoir in
which they occurred by a knowledge of their chemical characteristics. Com-
monly the waters from different strata differ considerably in their dissolved
chemical constituents, making the identification of a water from a particular
strata easy. However, in some areas the concentrations of dissolved con-
stituents in waters from different strata do not differ significantly, and the
identification of such waters is difficult or impossible.
The amount of water produced with the oil often increases as the amount
of oil produced decreases. If this is edge water, nothing can be done about it.
If it is bottom water, the well can be plugged back. However, it often is
intrusive water from a shallow sand gaining access to the well from a leaky
casing or faulty completion and this can be repaired.
Enormous quantities of water are produced with the oil in some fields,
and it is necessary to separate the oil from the water. Most of the oil can be
removed by settling. Often, however, an oil-in-water emulsion forms which is
very difficult to break. In such cases, the oil is heated and various surface-
active chemicals are added to induce separation.
In the early days, the water was dumped on the ground where it seeped
below the land surface. Until about 1930, the oilfield waters were disposed
into local drainage, frequently killing fish and even surface vegetation. After
1930, it became common practice to evaporate the water in earthen pits or
to inject it into the producing sand or another deep aquifer. The primary
concern in such disposal practice is to remove all oil and basic sediment from
the waters before pumping them into injection wells, to prevent clogging of
the pore spaces in the formation receiving the waste water. Chemical com-
patibility of waste water and host aquifer water must also be assured.
Waters produced with petroleum are growing in importance. In years past,