Iranian Journal of Oil and Gas Science and Technology

Abstract The asphaltene and metal naphthenate components of crude oil samples from 10 wells within an oil reservoir were determined using different analytical techniques. The asphaltene content of the crude was determined by gravimetric analyses using American Standard for Testing and Material (ASTMD) 6560 to obtain its weight concentration. In contrast, the metal naphthenate components were determined by obtaining the metal ion concentration of the produced water and the naphthenic acid concentration of the crude using atomic absorption spectrometer (AAS) and potentiometric titration respectively. The results show that the asphaltene content of the crude samples ranges from 2.0000 to 8.000 wt %, while the naphthenic acid concentration indicated by the total acid number (TAN) ranges from 0.3000 to 1.4600 mg/KOH/g. All the crude samples possess asphaltene components and the propensity to form calcium and sodium naphthenate scale deposits with a Ca₂⁺ concentration between 32.5000 and 94.5000 mg/L and an Na⁺ concentration between 27.7 and 105.1 mg/L respectively. However, the formation of naphthenate scale deposits highly depends on the pH of the produced water of the crude, which makes well FT01 less likely to form naphthenate scales since it has a pH of less than 6; in other words, the produced water pH and availability of cations play an essential role in the formation of naphthenate scales. Calcium naphthenate scale formation is more favored at a brine pH higher than 6, while sodium scale formation is favored at a pH of approximately 8.5. An increase in produced water pH during crude oil production is usually caused by depressurization and CO₂ release. Both asphaltene and naphthenate deposits are directly proportional to the specific gravity of the crude and inversely proportional to the API gravity, implying that both components reduce the quality of the crude. Asphaltene and metal naphthenate solid deposits in the crude can cause many flow assurance difficulties, such as blocking expedition lines, pore plugging, wettability, crude oil parameter alteration, and reduction in oil recovery.

Abstract The geochemical investigation of trace metals in crude oils from some producing oil fields in the Niger Delta, Nigeria, was carried out to ascertain the petroleum source rocks and organic matter deposits. The concentrations of trace metals in crude oil samples obtained from eight producing fields from Niger Delta, Nigeria, were analyzed using a 700 model Perkin Elmer atomic absorption spectrophotometer. The results showed the following ranges for the trace metals: Cu (0.01–0.04 mg/kg), Fe (0.05–5.90 mg/kg), Ni (0.09–0.72 mg/kg); and V (0.008–1.05 mg/kg). Pb and Zn were lower than 0.01 mg/kg. Trace metal ratios such as V/Ni, V/Fe, and (V/V + Ni) were used to unravel the genetic correlation among the oils. All the crude samples except the sample from Nembe South-2 have a V/N ratio lower than 1.0, indicating that the organic materials produced the petroleum source rock. A cross plot of V/Ni revealed two genetic families for the crude oils, derived from a terrestrial and marine origin, which was confirmed by the ternary plot of V, Ni, and Fe, discriminating the crude oils from the producing fields into two distinct groups. The V/(Ni + V) of smaller than 0.5 showed that most crude oils were deposited in an oxic environment. A cross-plot of V/(Ni + V) and V/Fe showed a weak correlation, suggesting that it could not substitute for the V/Ni ratio in determining the origin and depositional environment of crude oil samples. Therefore, an in-depth knowledge of the concentration of trace metals, especially vanadium and nickel, within an environment during oil exploration is essential in developing new oil locations.