Page 51 - Handbook Of Multiphase Flow Assurance

46 3. PVT and rheology investigation

If the low GOR sample shows methane content relatively lower than in other samples, this
may indicate gas loss during sample recovery to surface. Usually the low quality samples are
not used. However, if very few samples are available, it may be possible to offset the loss of
gas which could occur during or after sampling and see if GOR may be matched by adding
methane back to the composition.
Conversely, if GOR of a downhole sample is significantly higher than the rest of the sam-
ples, it may indicate gas coning and intake at the sampling depth. This may happen in sat-
urated reservoirs where a gas cap is present or pressure is at or below the bubble point, and
should not be the case in reservoirs with undersaturated fluid or dense phase. If GOR of one
of the samples is higher than the average, the sample may have been collected near a gas cap
if reservoir is saturated and free gas exists.
An example of a GOR check is shown below.
Fluid A-1 from 3050 m depth in well A has a GOR of 800 scf/stb, fluid B-2 from 3060 m in
well B has a GOR of 810 scf/stb. Fluid B-1 from 3055 m in well B has a GOR of 600 scf/stb.
It is possible that the sample container B-1 lost some gas during sample transfer from well
bottomhole to the lab. Laboratories record and report the pressure which the beneficiated
sample exhibited at opening of the container to verify whether pressure is similar to the other
containers and if any part of the sample may have been lost.
2. Check for drilling mud contamination
Oil based drilling muds typically are formulated with diesel fuel, kerosene or another
available fraction of hydrocarbons. This hydrocarbon base of the mud would usually have
increased content of hydrocarbons in the range between C 16 and C 20 . Some synthetic muds
can be formulated to have only even-numbered paraffins such as C 16 and C 18 dominate the
composition. The presence of these paraffins can be noticed in gas chromatography analy-
sis as an increased content relative to the composition of all paraffins on a logarithmic plot.
These components are not expected to be present at the increased content in a reservoir
fluid and can affect its properties. Additional steps are required to de-contaminate the
sample or to find the sample with the least drilling mud contamination. Contamination
level with a drilling mud below 1–2 wt% is considered acceptable for a PVT analysis.
An example of a mud contamination check is shown below.
Log10 plot of a GC analysis of sample A-1 composition in Fig. 3.2 shows a linear trend of
C 10 + component mass percent versus carbon number.

Log10 of A-1 component mass %
0.6
0.5
0.4

0.3
0.2
0.1
0
C 10 C 11 C 12 C 13 C 14 C 15 C 16 C 17 C 18 C 19 C 20 C 21 C 22 C 23 C 24 C 25 C 26 C 27 C 28 C 29 C 30 C 31 C 32 C 33 C 34 C 35
FIG. 3.2 Example of a fluid with little to no contamination with oil-based drilling mud.