Iraqi Kurdistan All in the Timing

Reservoir Management
oil company that reservoir souring due to
microbial metabolism was occurring. This
worrying conclusion, however, was based on
two mistakes.
When the data was received it was
attributed to separator gas from a specific
well, but the individual who collected the
sample did not appreciate that knowing
the precise location that the sample was
collected from was important to the
interpretation of results.
The second mistake was not realising
that sulphur isotope abundance in various
materials in nature can vary considerably, so
that merely obtaining data for the sulphur
isotopic ratio in a given sample will generally
not allow conclusions to be made as to
whether a given material originated from
chemical or from biochemical reactions. For
the microbiological conversion of sulphate
to hydrogen sulphide, depletion of 34 S in
the sulphide has been reported to typically
range from -15‰ to -25‰ and this can only
be determined by obtaining sulphur isotopic
data from the sulphate in the produced
water and in the H 2 S in the gas: both
samples should be from the same well.
Moreover, the precise location of any samples collected
is important, as was shown in subsequent testing. The data
in the table below illustrate that subsequent, more thorough
testing documented that there was essentially no change
in the sulphur isotopic ratio when H 2 S samples obtained
from primary separator gas samples were compared with
values from sulphate in the produced water from these
same wells. However, when H 2 S samples were obtained
from topside water-handling system locations downstream
from the primary separators, then significant depletion of
34
S in the sulphide was observed, which is consistent with
microbiological sulphate reduction. Additional testing (data
not shown) also demonstrated that organic acids such as
acetic acid and propionic acid were present in topside water
samples but not in separator water samples. Organic acids
are a byproduct of microbial metabolism. Furthermore,
microbial growth tests and genetic tests (qPCR or quantitative
Sulphur isotopic data from offshore gas and sulphate samples
Well
Sample Location
Biological cause: desulfovibrio vulgaris is the best-studied sulphate-reducing bacteria species; the
bar in the upper right is 0.5 micrometre long.
δ 34 S ‰
#1, sample set A separator 33.9 32.9 1.0
#1, sample set B topside water system 8.4 32.9 -24.5
#2, sample set A separator 30.8 32.7 -1.9
#2, sample set B topside water system 8.6 32.4 -23.8
Gas
polymerase chain reaction) documented the presence of acidproducing
bacteria, sulphate-reducing bacteria, and other
microorganisms in topside water samples but not in separator
water samples.
Biological Processes Not Responsible
When all of these data are considered, there is no evidence
for microbiological activity in the subsurface creating
reservoir souring; however, once the produced water is
brought topside it is vulnerable to microbial contamination
and the subsequent production of H 2 S. The source of the
thermogenic (chemically created) H 2 S recently observed in
the gas from some wells is unknown, but is presumably due
to the presence of a H 2 S-containing gas/oil/water source that
is in communication with the oil-producing zones tapped
by these oil wells. Knowing that biological processes are
not responsible for the observed H 2 S in reservoir gases will
allow the oil company to take
appropriate steps in producing
oil from existing wells, and
‰ Differential it will influence the future
Sulphate
exploration and production
activities at this oil field.
As a result of this study
biocide has been added to
the topside water-handling
system at this oil platform, and
a programme for the routine
monitoring of corrosionassociated
microorganisms has
been implemented.
62 GEOExPro November 2015