Page 25 - Geochemistry of Oil Field Waters

14 SAMPLING SUBSURFACE OILFIELD WATERS

Sampling at the wellhead

It is common practice in the oil industry to obtain a sample of formation
water from a sampling valve at the wellhead. A plastic or rubber tube can be
used to transfer the sample from the sample valve into the container. The
source and sample container should be flushed to remove any foreign
material before a sample is taken. After flushing the system, the end of the
tube is placed in the bottom of the container, and several volumes of fluid
are displaced before the tube is slowly removed from the container and the
container is sealed. Fig. 2.4 illustrates a method of obtaining a sample at the
wellhead. An extension of this method is to place the sample container in a
larger container, insert the tube to the bottom of the sample container, allow
the brine to overflow both containers, withdraw the tube, and cap the
sample under the fluid.
At pumping wellheads the brine will surge out in heads and will be mixed
with oil. In such situations a larger container equipped with a valve at the
bottom can be used as a surge tank or an oil-water separator, or both. To
use this device, place the sample tube in the bottom of the large container,
open the wellhead valve, rinse the large container with the well fluid, allow
the large container to fill, and withdraw a sample through the valve at the
bottom of the large container. This method will serve to obtain samples that
are relatively oil-free.

We1 lhead

Oil and water

Fig. 2.4. Schematic of method of obtaining a sample at the wellhead.
Sample for determining unstable properties or species

The pH, Eh, and various species of elements are unstable and will change
with changes in pressure and temperature, and when the formation water is
exposed to the atmosphere. The pH of the sample will change because of the
oxidation of reduced species, because of release of dissolved gases, and be-
cause of hydrolysis reactions such as:

H+ H+
c03-* + HCO,-+ H,CO,