Evaluation of corrosion and scaling potentials of oilfield waters in an offshore producing facility, Niger Delta Open Access

Formation waters occur naturally in association with crude oil and are referred to as oilfield produced water (OFPW) once it is separated from the oil for disposal. Since this water occurs naturally with the hydrocarbon fluids, they tend to generate certain problems in oilfield production operations. The large volume of OFPW, which the oil industry is currently contending with, makes the industry appear more like a water industry since it constitutes the largest waste stream on oilfield production facilities (OFPF) (Du et al. 2005). Due to the large volume of OFPW, it is often considered as injection water to enhance hydrocarbon recovery and maintain reservoir pressure, or may also be treated and disposed of into the environment. The former may trigger biogenic souring, corrosion and scaling, while the latter, if not properly treated, may cause bioaccumulation and transfer of potentially toxic elements (PTEs) in aquatic organisms, which would eventually affect the food chain (Konwar et al. 2021). OFPW contains low- and high-molecular-weight petroleum hydrocarbons (HCs), dispersed and dissolved oil, dissolved compounds, suspended solids, naturally occurring radioactive minerals (NORM), dissolved gases, and PTEs (Ozgun et al. 2013; Kpeglo et al. 2016).

Water corrosivity depends on many factors including pH, oxygen content, the presence of metabolizing bacteria, and its suspended solids, which tends to promote microbially assisted corrosion. Scales are inorganic deposits formed due to the precipitation of solids resulting from brines that are present in the oilfield reservoir and production system. The tendency of water to deposit insoluble and protective scales is a function of its corrosivity. Corrosion is the destruction or deterioration of materials due to its reaction with its environment and in an offshore setting, rusting, sweet corrosion, sour and microbiological corrosion are major concerns. Corrosion of metal surfaces is greatly influenced by their chemical composition, electronic properties, microstructure, and passive film properties (Das et al. 2009). Water saturated with CaCO₃ tends to precipitate scale, which may protect against corrosion, which could also produce encrustation in water and injection wells. The degree of CaCO₃ saturation has been used as an indicator of water corrosivity and scaling/encrustation tendency. Metals and their alloys are important materials used in pipeline production for oil fields. Experience and investigation of pipe failures suggest that corrosion of metals, both cast iron and steel, is the most predominant cause of pipe failures (Rajani et al. 1995; Mohebbi & Li 2011). In the last two decades, different studies have pointed out to the oil industry the advantages that aluminium alloys may present for tubular manufacturing compared to steel (Gelfgat et al. 2005; Osorio-Celestino et al. 2020). Since corrosion is linked to almost all pipe failures, it has become a global problem for all stakeholders, in particular engineers and asset managers of buried metal pipes (Goulter 1985; de Sena et al. 2012). The impact of corrosion cuts across so many sectors such as oil and gas, car assembling plants, chemical, electric power, medical and engineering industries (Parangusan et al. 2021). Nwanonenyi et al. (2020) reported the detrimental effects of metal corrosion in industries. Adequate measures that guarantee long-lasting materials must be put in place for efficient service delivery. Different approaches such as the application of native passive films on the surface and other corrosion inhibitory measures are being implemented to minimize corrosion in engineering sites. As is well appreciated, the consequence of pipe failures can be socially, economically, and environmentally catastrophic, resulting in massive disruption of daily life, considerable economic loss, widespread flooding, subsequent environmental pollution and even casualties and so forth (Hou et al. 2016). Corrosion generally occurs due to an electrochemical process that occurs in stages. The rates of corrosion also vary with time, depending on a complex interaction between the material, its environment and circumstances of exposure. In as much as the primary factors responsible for corrosion such as water and oxygen are present, the process is inevitable. The estimation is needed for engineers to be able to predict the lifespan of construction materials in industries. The cost of corrosion may run into billions of dollars every year. Due to the impact of corrosion on business economics, health safety and the environment, this study utilizes conventional water analysis, corrosion and scaling indices to ascertain the water stability conditions. Since aluminium, steel and iron are construction materials that show varying degrees of corrosion due to their physicochemical and electrochemical properties, these metals were used to measure corrosion rates in different formation water and seawater media under laboratory conditions.

To manage a potential scaling problem, it is essential to know where and how much scale forms during oil and water production (Osorio-Celestino et al. 2020). Many studies have shown that PW has high TDS content, which is the major cause of scale formation and pipe clogging (Arthur & Bruce 2005; Merdhah & Yassin 2007; Al-Ghouti et al. 2019; Al-Samhan et al. 2020; Bolaji et al. 2021). The Niger Delta is a beehive of oil exploration and production (E & P) activities in Nigeria and the volume of OFPW is high. There is scant literature on the effect of OFPW on construction materials used in the area. The present study aims to elucidate the corrosion and scaling potential of these waters on iron, steel and aluminium. This work examines the chemical characteristics of the Freeman OFPW to evaluate the corrosion behaviour of steel, aluminium, and iron in the aqua media, and determine the scaling potentials and corrosion kinetics of the OFPW. This research will provide further information for engineers in ascertaining the integrity and predicting the lifespan of materials used in the construction of OFPF.

The study area is located in the production zone of the Freeman field in the southwestern part of the Niger Delta, about 120 km offshore on the continental slope, at water depths of about 950–1,200 m (Figure 1). The reservoirs are structurally and stratigraphically trapped mud-rich unconfined turbidite sands within the Akata-Agbada petroleum system, located in a mid-lower slope depositional setting and are of Lower–Upper Miocene age within the prograding siliciclastic system. The Tertiary section of the Niger Delta is divided into three formations: the Akata, Agbada and Benin Formations, the type sections which are described by Short & Stauble (1967) and summarized in many other papers (Weber & Daukoru 1975; Avbovbo 1978; Evamy et al. 1978; Ejedawe 1981; Whiteman 1982; Knox & Omatsola 1989; Doust & Omatsola 1990; Beka & Oti 1995; Bolaji 2020; Bolaji et al. 2021; and references cited therein).

In this study, five indices were used to identify the corrosiveness and scaling potentials of the formation water and seawater samples. These include Langelier saturation index (LSI), Ryznar stability index (RSI), Larson–Skold index (L–S index), Puckorius scaling index (PSI), and aggressiveness index (AI).

Values of A, B, C and D are determined according to Piri et al. 2008; Table 1. Constant A is determined using the TDS table by taking the value that corresponds to the measured total filterable residue or the estimated TDS. Constant B takes into account the effect of temperature. Value C is obtained by the corresponding value to the calcium hardness (in mg/L CaCO₃) of the sample. Value D is obtained from the hardness table by reading the measured value for total alkalinity (in mg/L CaCO₃) of the sample (Piri et al. 2008; Fazlzadehdavilb et al. 2009).

This index has been correlated to observed corrosion rates and the type of attack. The L-S index might be interpreted by the following guidelines: index < <0.8, chlorides and sulphates probably will not interfere with natural film formation 0.8 < <index < <1.2, chlorides and sulphates may interfere with natural film forming. Higher than desired corrosion rates might be anticipated. Index > >1.2, the tendency towards high corrosion rates of a local type should be expected as the index increases.

Analytical results of the measured physicochemical parameters are presented in Table 2. Produced water pH values range from 7.32 to 8.38, slightly alkaline to alkaline in nature, while those obtained for the raw seawater (SW_R) and treated seawater (SW_T) are 7.76 and 6.63 respectively. pH value less than 4 encourages corrosion as protective oxide film dissolves, while solutions with pH values between 4 and 10 have little effect on corrosion. Higher values above 10 make the metals passive, thereby reducing corrosion. Total alkalinity (T. Alk) ranges between 760 and 1,760 mg/L for formation water samples, while 178 and 360 mg/L were obtained for SW_R and SW_T samples respectively. The mean Total Hardness (TH) concentration of 3,444.44 mg/L for the formation water samples indicate the presence of more magnesium rather than calcium, while TH in seawater samples slightly exceeds the peak value of 6,000 mg/L obtained for the formation waters. Chloride, bicarbonate, and sulphate are the dominant anions, with an average of 12, 316.15 mg/L, 1,447.73 mg/L, and 877.78 mg/L respectively. Sodium and calcium on the other hand predominate the cations, averaging 7,426.79 mg/L and 556.67 mg/L respectively. Total dissolved solids (TDS) for the produced water samples ranges between 14,537 and 31,761 mg/L (Av. 22,317.49 mg/L), while 36,184 mg/L and 34,658 mg/L were obtained for the SW_R and SW_T samples. The analyses revealed that the principal hydrochemical is Na-K-SO₄-Cl where the chemical properties of the waters are dominated by alkali elements. The water chemistry revealed that the formation waters are generally hard, saline water, dominantly of the Na-Cl type based on its TDS concentrations (Sawyer et al. 1994; Bolaji et al. 2021).

The LSI determined for the produced water samples ranged from −0.67 to 1.91. Except for the treated seawater (SW_T) sample, LSI was generally greater than zero, indicating super saturated water with scaling tendencies (Figure 2). Based on this index, SW_T is not saturated and has no corroding tendency, hence, it is considered suitable as injection water. However, the remaining PW samples are generally supersaturated with respect to CaCO₃ and may form scale. Although water temperature has a positive effect on LSI, i.e. as water temperature increases, LSI becomes more positive. In this study, LSI values revealed that the majority of the PW are supersaturated with respect to CaCO₃ and scale forming may occur on the pipelines, thereby restricting the free flow of HC fluids. Undersaturated waters (e.g. SW_T) changes the chemical equilibrium of CaCO₃, favouring the dissolution of protective coatings in pipelines and equipment (DeMartini 1938).

Ryznar saturation index ranges between 4.1 and 5.5 for PW samples and 6.2–8.0 for SW samples (Figure 3). RSI less than 5.5 indicate heavy scale formation while values greater than 8.5 indicates that the water is aggressive, that is such water actively seeks to gain minerals and metals which results in the corrosion of iron and copper surfaces and causes dissolving concrete and stone surfaces (Konwar et al. 2021). The Freeman PW samples show rigorous scaling tendency based on the RSI. SW_R is balanced with no scaling and corrosive tendency, while SW_T has a corrosive tendency. The SW_T which show a seemingly neutral has the highest RSI value.

L–S index is generally greater than 1.2 indicating that high rates of localized corrosion can be expected in all the studied water samples (Figure 4). PSI values in the OFPW samples range from 2.79 to 6.34. PSI values generally suggest that scaling is likely to occur in the PW and SW_R samples, while SW_T shows little scaling and corrosive tendency (Figure 5). AI range from 11.47 to 14.04 (Figure 6). According to the AI, produced water and SW_R samples are non-aggressive with the ability for scale formation, while SW_T is moderately corrosive and aggressive.

Steel has the highest CR in B-25b with value 3.84 × 10⁻³ mg cm⁻² h⁻¹, while the least value was 1.49 × 10⁻³ mg cm⁻² h⁻¹ in B-51a (Table 3). Iron has the highest CR in B-30 with a value of 3.29 × 10⁻³ mg cm⁻² h⁻¹ while the least value was 1.24 × 10⁻³ mg cm⁻² h⁻¹ in SW_T. The highest and least CR were 2.48 × 10⁻³ mg cm⁻² h⁻¹ in SW_R and 0.37 × 10⁻³ mg cm⁻² h⁻¹ in B-24a for aluminium. The highest CR was observed in steel with a value of 3.84 × 10⁻³ mg cm⁻² h⁻¹, while the least CR occurred in aluminium with a value of 0.37 × 10⁻³ mg cm⁻² h⁻¹. The corrosion suffered by iron plates is of the general type (uniform attack). The corrosion rate of iron varied from month to month. Iron has the highest CR in all the water samples except B-25b, B-14, B-01 and SW_T where steel CR was higher. The corrosion suffered by steel plates was mainly of a general type attack. Aluminium has the least CR in all the water samples compared to other metals investigated. It is a very reactive metal but mild to corrosion. Of all metals investigated by Natesan et al. (2006), aluminium was also the least corroded. No significant attack was observed on aluminium panels. Corrosion observed in B-01 was due to atmospheric gases and water which aids the corrosion process. An increase in salt content (salinity) also increases the rate of corrosion.

Chemical analysis revealed that the studied formation water and seawater samples are slightly alkaline and generally classified as hard, saline water based on its TDS. The waters are of the Na-Cl type with Na-K-SO₄-Cl as the principal hydrochemical facies. Based on the computation of the different corrosivity indices, there is a likelihood of some of the water to form mild to severe scales, while the seawater samples are classified as ‘non-aggressive’ and ‘aggressive’. In most cases, the rate of corrosion of the metals followed the first-order rate constant in some of the samples, and second-order in others within the first seven days. It was observed that the rate of corrosion follows this order: steel > iron > aluminium. The potential heavy and intolerable corrosion associated with the use of these seawater samples as injection waters is a potential risk that must be handled by adequate treatment. A comprehensive analysis of steel, iron and aluminium corrosion in oilfield waters was presented in this paper. From the results available it has been established that aluminium was the least corroded while the iron was the most corroded. It can be concluded that the results of cumulative corrosion rate and kinetics study presented in the paper can contribute to the body of knowledge of corrosion behaviour of the metals and aid in the prediction of long-term properties of the metals in construction. This knowledge can enable a more accurate prediction of failures of metal pipes.

The authors wish to appreciate Shell Nigeria for providing fluid samples for this study. Comments from anonymous reviewers, which greatly improved the quality of this work are gratefully acknowledged.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.