Assessing the Magnitude and Consequences of Reservoir Souring Figure 4: Example of the Effect of Increasing Water Cut on Well Souring
Constant Gross Production (1000 bbls/d) @ 1000 scf/bbl, 100% IWBT Microbiological Souring (40 mg/kg H2
200 400 600 800 1000
900 700 500 300 100
0 Jan-15 H2 Jan-16 Jan-17 S in Gas (Biological Souring) Jan-18 Jan-19 Water cut bbl = barrel; d = day; IWBT = injection water breakthrough; ppmv = parts per million by volume; scf = standard cubic feet.
Table 1: Example of Fluid Phase Partitioning on Hydrogen Sulphide Concentration in Gas Phase Oil
Water Date
9/1/2015 1/1/2016 1/1/2017 1/1/2018 1/1/2019 1/1/2020 1/1/2021 1/1/2022 2/1/2022 1/1/2023 3/1/2023 5/1/2023 7/1/2023 9/1/2023 11/1/2023 12/1/2023
• In situ pH = 6.8.
• Temperature = 60˚C. • Absolute pressure = 10 bar. •
• Oil density = 0.824 kg/l. • •
The gas:oil ratio is held constant at 178 m3 /m3 (1,000 scf/bbl). Average molecular mass of the separated gas = 26.7 g/mole bopd = barrels of oil per day; bwpd = barrels of water per day; mmscf = million standard cubic feet; ppmv = parts per million by volume.
sulphide generation in the reservoir because the two main nutrient substrates that SRB require, sulphate and readily utilisable organic carbon (most commonly acetate), are combined together and injected into the cool zone around injection wells.
Given no other constraints, the maximum H2S generation would occur when sulphate and acetate are equal in molar concentration.
However, in many cases, the salinity of the formation water is too high to support SRB growth; therefore, the maximum sulphide generation will occur close to the highest salinity mixing ratio of seawater to re-injected formation waters which allows SRB growth.
In many reservoirs the main mechanism of sulphide retardation is one of scavenging by reaction of sulphide with iron-containing minerals, specifically iron carbonate and iron oxides. This tends to have the effect of disguising the souring process until such time as the mineral has exhausted its scavenging capacity, at which
94
time H2S production may commence suddenly and then accelerate. The scavenging minerals are siderite (FeCO3), haematite (Fe2O3) and magnetite (Fe3O4). On coming into contact with H2S, they will form pyrrhotite (FeS) and pyrite (FeS2). The scavenging properties of the minerals depend on their stability: unstable iron minerals concentration in the water, thereby
will create a high Fe2+
promoting direct, rapid precipitation of FeS and lowering the H2S content of the water.
The end result of this process at the producing well is a delay in
production of H2S from when the first injection water breakthrough occurs. In cases of reservoirs with sufficiently high mineral iron or dissolved Fe2+ ion content, souring may never occur during field life. The risk that souring will occur is also lower for reservoirs with extremely high salinity formation water, very high temperatures and/or very high absolute pressures (>600 bar) than for lower temperature, low salinity reservoirs.
EXPLORATION & PRODUCTION – VOLUME 9 ISSUE 2
1,000 960 840 720 600 480 360 240 230 120 100 80 60 40 20 10
(bwpd) 0
40
160 280 400 520 640 760 770 880 900 920 940 960 980 990
Gas
Production Production Production (bopd)
0.960 0.840 0.720 0.600 0.480 0.360 0.240 0.230 0.120 0.100 0.080 0.060 0.040 0.020 0.010
(mmscf/day) 1.000
Mass of Oil
(kg/day) 131,016
125,775 110,053 94,332 78,610 62,888 47,166 31,444 30,134 15,722 13,102 10,481 7,861 5,241 2,620 1,310
Mass of Water
(kg/day) 0
6,436
25,745 45,054 64,363 83,672
102,981 122,290 123,899 141,599 144,817 148,035 151,254 154,472 157,690 159,299
Jan-20 Jan-21 Jan-22 Jan-23 Jan-24 S Generation in Injection Water)
100 90
20 40 60 80
70 50 30 10
0
Mass of Gas
(kg/day) 33,756
32,406 28,355 24,305 20,254 16,203 12,152 8,102 7,764 4,051 3,376 2,701 2,025 1,350 675 338
Total Production (kg)
164,772 164,618 164,154 163,690 163,227 162,763 162,299 161,835 161,797 161,372 161,294 161,217 161,140 161,063 160,985 160,947
H2S in Gas
(ppmv) 0.00
5.47
24.50 48.75 80.71
124.74 189.29 293.05 304.62 487.33 537.47 596.15 665.74 749.59 852.60 913.46
H2
S in Glas (ppmv)
Water cut (%)
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