Disposal of produced water from oil fields is a major concern to oil companies for environmental and economic reasons. One way to dispose off this water is to mix it with injection water. A carbonate reservoir in Saudi Arabia produces wet oil where the salinity of the produced water is high (TDS up to 230,0 mg/L). The produced water contained dissolved gasses (H2S and CO2), and suspended solids (oil, corrosion products). GOSP disposal water has CaCO3 scaling potential. It contains 750 mg/L of H2S. The aquifer water contains 2 mg/L of iron. Iron sulfide scale forms once the disposal water is mixed with the aquifer water. The objective of this study is to assess potential formation damage that can result when the two waters are mixed and injected into the tight carbonate reservoir.
The current study included detailed analysis of field waters, determination of scaling potential of various waters, and extensive coreflood testing using reservoir cores. A unique feature of this study is that the cores were examined after the injection of the mixed waters by CT and SEM techniques.
Injection of mixed water into reservoir cores created wormholes which increased core permeability. This new finding indicates that disposal water is not always damaging. The effect of iron sulfide particles was found to be a function of the initial core permeability. Iron sulfide particles (0.25 micron) caused damage to cores with permeability < 20 mD after injecting 1,000 PV of the mixed water. No damage was noted in high permeable cores (> 100 mD) even after the injection of 600 PV of the same water.
The results of the study identifies various types of scale related to mixing GOSP disposal water and aquifer water, and determine conditions where these waters can be injected. Also, the study highlights an unexpected benefit from injecting H2S containing waters into carbonate reservoirs.