Page 55 - Handbook Of Multiphase Flow Assurance

50 3. PVT and rheology investigation

Gasmolar volume = 25 2./ (0 874 1 225. ∗ . ) = 23 5. ( Lgasmolgas)
/
Oil molar volume = 194 9./ (141 5./ (285. +131 5 )∗1000 ) = 0 220. ( Loil//moloil)
.

3
3
∗
.
.
/
.
Volumetricgasratio = 1 445 23 50 220 = 154 3 ( mgas moil)
/.
∗
∗
GOR = 0 159 35 3 154 3 = 866 ( scf STB).
.
/
.
.
The value of GOR estimated from the material balance is in fair agreement with the
reported value of 876 scf/STB. This also confirms quality of the sample.
Water sample quality checks
The quality of the water sample is important for various production chemistry analyses.
The following initial quality checks may be done by a specialist to verify sample data.
1. Ionic balance
Ionic balance is the similarity of the combined weight of positive ions and the combined
weight of negative ions. Ionic balance indicates that sampling and lab analysis of the water
was performed correctly.
2. Drilling mud contamination
Most modern deepwater wells are drilled using an oil-based drilling mud, which keeps
water emulsified. This limits the exposure of water-sensitive rock such as water-swelling
clays to water, and also limits the exposure of the sample water to the mud water.
Most non-deepwater wells are drilled using a simpler water-based drilling mud.
If a water based drilling mud was used, brines used to weigh the mud could contact and
contaminate the downhole water sample with ions such as sulfate which originates from sea-
water carrier for salt, and with barite from salts which do make the mud heavy.
2−
2−
Typical reservoir water would have sulfate SO 4 under 200 mg/L because sulfate SO 4
2−
ions convert to sulfide S and precipitate as iron sulfide FeS, zinc sulfide ZnS or other miner-
2−
als in reservoir over geologic times. Seawater would typically have around 2600 mg/L SO 4 .
2−
Any reservoir water with SO 4 greater than 200 mg/L may be contaminated with water from
2−
the drilling mud. The reservoir water samples with the lowest SO 4 concentration should be
selected for further analysis.
3. Other salts
The following salts may be used to weigh the mud:
calcium chloride CaCl 2
calcium bromide CaBr 2
zinc bromide ZnBr
potassium formate KHCO 2
cesium formate CsHCO 2 for HPHT wells and reservoirs
Scale saturation index may be used to check water sample quality. Downhole samples with
scale saturation index below zero or saturation ratio below one calculated at reservoir condi-
tions should be used as this indicates that water sample is not oversaturated with minerals.
The saturation index is calculated using any of the available scale prediction methods.
Bicarbonate HCO 3 concentration will decrease after sampling due to HCO 3 conversion to
carbonic acid H 2 CO 3 and to CO 2 and evolution of CO 2 .