Preliminary geochemical characterization of saline formation water from Miocene reservoirs, offshore Niger Delta

The Niger Delta is one of the most prolific hydrocarbon provinces in the world, but information on basinal hydrogeology is scanty. Oilfield brines from nine producer wells in the Miocene reservoirs (2,472.25–3,532.48 m.b.s.l.), offshore Niger Delta, have been investigated along with two seawater samples to understand their hydrogeochemical characteristics in relation to the host rock mineralogy. Chemical analysis revealed that the waters are slightly alkaline and can be generally classified as saline water of the Na–Cl type based on their total dissolved solids (TDS). On the basis of bicarbonate, chloride, and sulphate ions, they are shown to be of connate origin. Relative abundance of major ions is in the following order: Na > Ca > Mg > K and Cl > HCO₃ > SO₄. Saturation indices (SI) of selected mineral phases calculated using PHREEQC indicate that the dissolution of iron oxide and carbonate minerals may contribute major ions in the formation water. The preponderance of alkali elements suggests the presence of feldspars, which could have resulted from sediments through which the water flows. An inverse relationship is observed between the resistivity of formation water and its TDS, which could be used to calculate resistivity values of formation waters in the area if the TDS contributions are known. Chemical data suggest that the formation waters were derived from seawater, dominantly altered by reverse ion-exchange processes and subsequently by water-rock interactions. Multivariate statistical analyses (correlation and factor analysis) indicate multiple sources of enrichment of ions in the formation waters.

Microbial Methane Production Associated with Carbon Steel Corrosion in a Nigerian Oil Field

Microbially influenced corrosion (MIC) in oil field pipeline systems can be attributed to many different types of hydrogenotrophic microorganisms including sulfate reducers, methanogens and acetogens. Samples from a low temperature oil reservoir in Nigeria were analyzed using DNA pyrotag sequencing. The microbial community compositions of these samples revealed an abundance of anaerobic methanogenic archaea. Activity of methanogens was demonstrated by incubating samples anaerobically in a basal salts medium, in the presence of carbon steel and carbon dioxide. Methane formation was measured in all enrichments and correlated with metal weight loss. Methanogens were prominently represented in pipeline solids samples, scraped from the inside of a pipeline, comprising over 85% of all pyrosequencing reads. Methane production was only witnessed when carbon steel beads were added to these pipeline solids samples, indicating that no methane was formed as a result of degradation of the oil organics present in these samples. These results were compared to those obtained for samples taken from a low temperature oil field in Canada, which had been incubated with oil, either in the presence or in the absence of carbon steel. Again, methanogens present in these samples catalyzed methane production only when carbon steel was present. Moreover, acetate production was also found in these enrichments only in the presence of carbon steel. From these studies it appears that carbon steel, not oil organics, was the predominant electron donor for acetate production and methane formation in these low temperature oil fields, indicating that the methanogens and acetogens found may contribute significantly to MIC.

Comparison of microbial communities involved in souring and corrosion in offshore and onshore oil production facilities in Nigeria

Samples were obtained from the Obigbo field, located onshore in the Niger delta, Nigeria, from which oil is produced by injection of low-sulfate groundwater, as well as from the offshore Bonga field from which oil is produced by injection of high-sulfate (2,200 ppm) seawater, amended with 45 ppm of calcium nitrate to limit reservoir souring. Despite low concentrations of sulfate (0–7 ppm) and nitrate (0 ppm), sulfate-reducing bacteria (SRB) and heterotrophic nitrate-reducing bacteria (NRB) were present in samples from the Obigbo field. Biologically active deposits (BADs), scraped from corrosion-failed sections of a water- and of an oil-transporting pipeline (both Obigbo), had high counts of SRB and high sulfate and ferrous iron concentrations. Analysis of microbial community composition by pyrosequencing indicated anaerobic, methanogenic hydrocarbon degradation to be a dominant process in all samples from the Obigbo field, including the BADs. Samples from the Bonga field also had significant activity of SRB, as well as of heterotrophic and of sulfide-oxidizing NRB. Microbial community analysis indicated high proportions of potentially thermophilic NRB and near-absence of microbes active in methanogenic hydrocarbon degradation. Anaerobic incubation of Bonga samples with steel coupons gave moderate general corrosion rates of 0.045–0.049 mm/year, whereas near-zero general corrosion rates (0.001–0.002 mm/year) were observed with Obigbo water samples. Hence, methanogens may contribute to corrosion at Obigbo, but the low general corrosion rates cannot explain the reasons for pipeline failures in the Niger delta. A focus of future work should be on understanding the role of BADs in enhancing under-deposit pitting corrosion.