Page 343 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

310 Reservoir Engineering

(5-250)

from which residual oil is given by

(5251)

The long time needed for tracer to move between wells and the broad and
uninterpretable residence time distribution that would arise because of stream-
line and heterogeneity effects for normal well spacings are cited as reasons why
the well tracer test may be impractical as a between-well test, although there is
still some interest in applying this method to small pilot areas [319].

Single-Well Tracer Technique
The single-well or backflow tracer technique to determine residual oil satura-
tion is a recent innovation. Patents [337,338] assigned to Exxon Production
Research Company in 1971 described the injection of tracers to measure in-situ
oil saturation at distances of typically 10 to 30 ft away from the wellbore of a
producing well. The single-well tracer test overcomes many of the difficulties
that, in general, make the two-well test impractical.
Test Procedure. The method involves injection of a bank of water containing
an alkyl ester as the tracer. The selection of the ester will depend on tem-
perature of the reservoir. For example, ethyl acetate is used in higher tem-
perature reservoirs and n-propyl formate is used at lower temperatures [339].
These tracers are suitable for reservoirs with bottomhole temperatures ranging
from about 80°F to 2OOOF. The ester partially hydrolyzes within the formation
to form an acetate and an alcohol, the latter serving as a secondary tracer.
Methanol is also added as a material balance tracer to the injected bank and
also to the water which is used to push the bank into the formation [339-3421.
A desirable volume for the slug is 50-90 barrels per foot of formation.
Injection and production rates may limit the size of test that can be conducted
in a reasonable period of time. Primary tracer volumes have varied from
40-1,000 barrels while total water volumes have ranged from 175-2,000 barrels
[339-342] . Primary tracer concentration has normally been between 0.5-1
volume percent. In an example given by Deans [338], 500 barrels of 1% ethyl
acetate and 0.5% methanol in brine is followed by 1,500 barrels of brine
containing 0.5 % methanol. These quantities are determined by simple volumetric
balance based on the desired depth of invasion.
Depending on formation permeability, injection may normally require 1 to 3
days and shut-in time will be 3 to 12 days. After allowing a suitable time period
for hydrolysis of about 10% of the ester (usually about 3-12 days), the well is
put on production and tracer concentration in the produced water is monitored.
Because the alcohol formed by hydrolysis is much more soluble in the aqueous
phase than its parent ester, the alcohol is produced ahead of the ester. The
greater this separation between the two tracers, the higher the residual oil
saturation. The methanol tracer, which is soluble only in water, determines the
drift rate in the reservoir and also indicates the fraction of chemical slug that
is produced during the test [339-3421.