Review of the Economic and Environmental Impacts of Producing Waxy Crude Oils

Abstract

Within the oil and gas industry, there is unanimity that wax deposits-driven pipeline blockages are a critical environmental concern and an economic liability of up to billions of dollars. However, a quantitative assessment of such an impact and, especially, of the different individual impacts that add up is absent from the current scientific literature. Such a gap is a deterrent for better-focused research. Given the production transition to heavy and paraffinic oils, harsh climatic zones, and extremely deep offshore oilfields, an extensive investigation is increasingly needed. The current endeavour was inspired by such a challenge and a review of the most recent technical and scientific publications was devised. A PRISMA-inspired and adapted methodology for systematic reviews was adopted. Over two hundred research articles, conference papers, books, theses, reviews, public databases and industry and government agencies reports were considered. As a result, a significant research gap is filled, both with the compilation, critical revision, and systematisation of the dispersed published scientific and technical data on the matter and with the definition of a quantitative economic impact appraisal for the wax deposition issue.

1. Introduction

Wax formation and deposition have caused partial or complete obstructions in pipelines and wells, posing a challenge in producing or transporting paraffinic crude oils. Such phenomena affect the whole value chain of the extractive and transformational sectors for hydrocarbons, from upstream to petrochemical plants [1,2,3,4,5,6].

In fact, the entire multidisciplinary field of Flow Assurance is deemed to prevent, mitigate and remediate obstructions to the optimal flow conditions as well as to avoid accidents, making the research on wax deposition central to environmental, petroleum and process engineering.

Due to the tremendous economic stress that the oil and gas sector is presently under, part of the production has been performed in challenging locations, increasing production risks or transportation risks. The probability of having wax related problems in cold locations is higher than in warmer environments since the wax precipitation can be trigged by low temperature when the crude oils have paraffins in their composition. Fostered by wax deposition mechanisms [7], wax formation occurs as flow temperature is found to be below its intrinsic Wax Appearance Temperature (WAT), also known as the Cloud Point. Below that threshold, wax particles can precipitate and aggregate in the tubing or pipeline frontiers. As a result, flow rates can be profusely reduced, frequently below the endeavours’ economic threshold, or even completely blocked. Wax particles leverage non-Newtonian behaviour and change flow viscosities, either significantly for an oil flow or very significantly for an oil-and-water emulsion [8].

Nevertheless, crude oil will continue to be essential for global economic development, stability, and recovery until a feasible mix of green energy sources can guarantee the energy security.

Different remedial measures and strategies are available for dealing with this flow assurance problem, at a cost. Unfortunately, most current tools are either of limited efficiency, uneconomical, or both. Therefore, past and current research on wax deposition has faced a paradox. On the one hand most, if not all, research initiatives on the matter are justified by the enormous economic and environmental impact of wax deposition and, on the other hand, published scientific literature lacks a systematic review of current knowledge on costs and impacts of wax deposition.

In fact, while most of the available research and reliable data are case-specific, the most significant contributions can be traced back to the early 1980 decade, when significant flow assurance problems in pipelines caused by wax deposition were clearly described. This is the situation that occurred in the Trans Alaska Pipeline System and the North Sea submarine lines [1,9], and operational issues were detailed by ARCO Oil and Gas Co [10]. Although no information has been provided to quantify the economic impact of wax deposition on that Arctic production site, important details about the magnitude of the issue have been shown. It has been demonstrated that flow assurance issues caused by wax deposition would not only occur during the start-up of production, as is typically anticipated, but would also occur over the lifetime. One example was described in the literature when after three years of production wax deposition occurred in the Trans Alaska Pipeline [10]. This waxy crude oil had 14 to 17 percent of wax content [10].

Since the 80s, wax deposition-induced flow assurance problems have been stressed out due to the location of production fields in more extreme environments.

The global value of billions of dollars associated with wax-related problems have been referenced frequently since the middle of the 1990s [11,12,13,14,15,16]. Most of the study utilised offshore Staffa Field’s abandonment, in 1994, as an illustration of the economic effects. The partnership between Lasmo and Ranger was forced to be abandoned and decommissioned due to severe wax problems in the pipeline to Ninian. This incident is one of the most well-known and frequently cited of pipeline obstruction. Despite great attempts to use corrective measures, persistent wax deposition and frequent blockage left no choice but to abandon, at a cost of almost $100 million USD [6,12,17,18,19,20,21,22,23,24,25,26]. In addition, broader analyses have been cited in research articles, mostly based on Gulf of Mexico incidents or on industry reports, such as the Elf Aquitaine’s report [20]. Pipeline segment’s replacement costs related and information about pigging operations are among the topics detailed in the publications [6,12,17,20,22,24,27,28,29]. Furthermore, for particular locations, even for times up to fifty years ago, reliable and comprehensive information regarding incidences related to wax deposition may be obtained [17,30]. This is the situation with the data that has been made public for the offshore oil fields in the Gulf of Mexico and was gathered by regional authorities.

It is challenging to estimate the cost of flow improvers globally. However, some specific cases can be used for reaching a general outlook. For instance, an offshore Indian oilfield is reported to employ pour point depressant, injected along with regular wax mechanical removal [28,31]. Despite its efficiency being arguable, considering the wax accumulation, the cost of the operations is documented. Other reported cases include the offshore Gannet field in the UK, where a solvent was injected and pigging operations were conducted [28,31,32], including even pipelines with exposure to seasonal higher temperatures, such as the Kirkuk (Iraq)—Ceyhan (Turkey) onshore link, where aromatic solvents were used together with pigging operations [31,33].

As oil production became increasingly heterogeneous and complex, economic data on wax deposition-induced problems became even scarcer. Industry reports are usually not released to the public with such detail and many research papers, while addressing the economic issue, usually do it in a qualitative manner or cite older, specific, cases to which costs are known. Therefore, the research gap is evident and widening.

To address such a problem and to evaluate the potential environmental and economic impact of flow assurance problems, namely wax-related issues, it is essential to understand the causes as well as the possible prevention or mitigation measures. Research has been developed over the years on the relevant physicochemical phenomena, however there is not much about the quantification of the impacts.

The current endeavour is a systematic review, deemed to gather, analyse, validate and synthetize the information on wax deposition’s environmental and economic impacts. The task is challenging, given the information scarcity, complexity, and frequent incoherence, as the absence of such a study emphasizes. Yet, it is also critical to ensure that future research is performed over a well-depicted economic and environmental problem, allowing efforts to be directed where the greatest impact can be achieved.

The approach to achieve such a goal includes using a systematic review methodology, adapted to this very specific problem reality, both in terms of temporal scope and data sources.

2. Methodology

The Identification, Screening, Sorting, Eligibility Assessment, Information Extraction, Qualitative Synthesis, and Discussion stages of the literature research on the Economic and Environmental Impacts of Wax Deposition were carried out between July 2020 and May 2022, as more clearly and systematized in Figure 1. While addressing the case-specificity of this scientific subject, adherence to known standards for systematic reviews, such as PRISMA [34], was pursued. Recent review articles, such as [35] were used as guidelines, and later double-checked against the methodology depicted in [36].

There has been a thorough search of the scientific literature. Peer-reviewed and published works, including accepted master’s and doctoral theses, journal research articles, conference proceedings, review articles, peer-reviewed book chapters, and review articles, were taken into consideration. Yet, such an envelope was not enough to portray the reality for environmental impacts and, especially, for economic impacts. Hence, industry reports, (U.S. and Canada) publicly released governmental agencies documents and reliable databases were also searched.

Considering the extreme scarcity of information, as well as the fact that a previous systematic review was not found, covering the entire body of knowledge on the examined matter was found crucial. Hence, the temporal scope for literature search was defined as the period between 1980 and 2021. However, to provide the historical framework of wax deposition and its economic and environmental impacts a few older references were added, covering the period of 1880 to 1980.

Journal and conference papers were redundantly searched in Springer Online, Scopus Online, Mendeley Desktop, Wiley Online, and Taylor and Francis Online search engines. Moreover, universities repositories were also investigated. Industry reports were searched through companies’ websites and reports were retrieved from government agencies pages. Nevertheless, document identification and finding through articles reading was a critical part of the endeavour.

Table 1. Search dimensions.

Stage	Included	Excluded
5 Identification	n = approximately 400 to 450, of which 336 were eligible
6 Screening	n = 311	n = 25
7 Sorting	n = 278	n = 33
8 Eligibility	n = 214	n = 64

shows the search dimensions.

Duplicate entries, title and content mismatches, corrupted files, and document accessibility issues were among the screening criteria for removal. Items that didn’t meet the criteria for the Sorting step included documents whose content didn’t quantify economic or environmental implications. The eligibility review was concentrated on the content of the document, rejecting items without a particular relevance, without crucial information, with any recognized methodological flaws, or with potential for strong bias because of sponsorship.

An effort has been made to conduct inclusive research, avoiding the exclusion of poorly written papers and linguistic bias. Some non-English articles, whose machine translation attempts did not succeed, could not be included.

3. The Problem of Wax Deposition in Energy Production Industries

Since the earliest times of production, waxy crude oils have been known. As early as 1788, paraffin oil was described as a waxy, white, and semi-transparent substance [37]. It has reportedly been extracted from Rangoon mineral oil in 1788, from wood tar in 1830, and from coal tar in 1835, among other sources, until its commercial production was accelerated in 1850 [37]. Later, as the oil industry developed, several references to the wax deposition problem arose, such as Reistle Jr. [38], Jessen and Howell [39], and Hunt Jr. [40], to mention some of the most impactful.

Even if phenomenological knowledge was not sufficiently profound, namely on the deposition mechanisms, for making optimal flow assurance decisions [41], the 1980 decade brought the generalisation of chemical treatments on economic grounds. For instance, a specially formulated chemical compound (mixing an oil soluble paraffin dispersant with a esterified olefin/maleic anhydride copolymer in equal parts) helped one oilfield in Mississippi to save tens of thousands of dollars compared to the hot oiling procedure [42]. Reports about the incidence of wax deposition in pipelines and wells have followed throughout the decade. For instance, Agrawal et al. depicted such problems in Indian offshore highly waxy (up to 14%) crude oil [43]. Hsu et al. found wax deposition in turbulent flow conditions [44]. Kadir and Ismail reported incidents with deposits of highly waxy oil (up to 25%) in Malaysian oilfields which led to extensive maintenance and downtime [45]. Marques et al. [46] and Machado et al. [47] described persistent wax deposition incidents in Brazilian Campos basin oilfields. Adewusi referred the flow assurance problems occurred in Nigeria with waxy gas condensates and alerted for the potentially disastrous consequences of pump failures due to wax deposition [48]. Elsharkawy et al. reported wax deposition problems in Middle East oilfields [49], which led to the increase in pumping cost, as well as, in pipeline plugging risk. Hammami and Raines highlighted that wax related problems are more prevalent in offshore oilfields [50]. Despite this extensive published research about wax deposition, they generally ignore the economic quantitative assessment.

The crude oils composition varies according to their raw natural resources and the conditions they were subject over the time. According to their composition it is possible to perform a rough characterisation per type. Knowing that the crude oil production around the world reached 82.3 million of barrels per day, and the paraffinic crude oil was approximately 16.4 to 27.1 million barrels per day [31,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71], it is possible to estimate the percentage of produced paraffinic crude oils as 20% to 33% of paraffinic oil.

A complete supply chain, from the reservoir to the industrial plant, can include oil fields or pipelines immersed in cold locations. Figure 2 represents the crude oil production per location [31,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71]. A classification as warm or cold is simplistic, yet necessary to cluster and analyse data. Hence, arctic and frozen sites have been grouped as cold sources and tropical and arid sites as warm sources. To such an end, countries (or states in the case of the U.S.) have been classified as cold and warm and its production summed in the corresponding group.

As it is possible to observe, most of the crude oil is produced in warm environment. However, 22% is found in cold environments.

While offshore production saw a significant increase in new ventures in the late 20th century, this change was considerably accelerated in the new century [17]. The change was prompted by the finding of significant reserves in more remote and deep basins, such as those in the Gulf of Mexico [72], China Sea [58,73], North Sea [74], and the West Africa, including Niger Delta [75,76,77], and Brazilian coast [78].

In fact, as shallow-water offshore production depleted in many sites, deepwater and ultra-deepwater production has been used to compensate the global demand [79]. It increased approximately one quarter in a decade, between 2005 and 2015 [31].

Numerous flow assurance problems were introduced with the shift to offshore deepwater and ultra-deepwater production. Wax deposition is one of these problems. Therefore, recent wax induced obstructions can be found not only in the previously mentioned locations but also in South Sudan, Uganda, Kuwait and Qatar [80].

Interestingly, wax deposits has been found even in gas production, for instance in Iran [81]. Despite this problem, total obstruction of lines remains rare [82]. The extraordinary length of pipelines required to deliver oil and gas to shore or to storage and transport, as well as the low temperatures at which fluids must be moved, are the primary causes of this heightened tendency for wax-related issues [17,83,84].

With increasing production depth, there is a nonlinear rise in the costs of handling wax-related issues [78,85]. Yet, that is only one part of the problem, as the problem complexity, the difficulty in attaining accurate data [86], and the time such experiments take have deterred the scientific community from reaching a common understanding about the wax formation and deposition mechanisms relative importance [7,85,87,88].

Moreover, effects due to flow regime and emulsions formation are increasingly regarded as a significant complexity source impeding a clearer phenomenon understanding [7,8,89,90].

To face these flow assurance problems, several proven and experimental techniques are available [91]. However, not all have the same prevalence. Mechanical removal has been favoured in the industry in earlier days, yet it may become too challenging from the technical point of view to be employed through long pipelines in deep and ultra-deep waters. At the same time, it also requires penalising production downtime [31]. Regarding the former, it is worth referring that flowlines’ mechanical removal (pigging) requires launching and recovering the instrument (pig). If the flow lines are looped for round-trip pigging, this can result in high costs for installing the second line to close the loop. [92]. Chemical flow improvers have been profusely used, even in association with other techniques. Nevertheless, such chemicals are case-specific and must be designed, both in their composition and dosage, for each oilfield and adjusted over time. The dependency on the crude oil composition has been widely regarded as a significant factor for chemical flow improvers efficiency [93,94]. Moreover, many flow improvers are either of limited efficiency, such as poly(ethylene-co-vinyl acetate), poly(maleic anhydridealt-1-octadecene) with inhibition rates of 23% and 8%, approximately, [95], and acrylate ester copolymers with alkyl side chains with inhibition rates between 25% and 55% [31,96], or require significant concentrations, such as polyacrylate polymer up to 2000 ppm [97] or being injected at high temperatures [92], with high costs for tubing, to assure the wax deposition complete prevention. Bio-based pour point depressants are an interesting solution due to their biodegradability and non-toxicity [80]. However, those have not yet been widely accepted as a better solution than the classical chemical flow improvers. Under these circumstances, the occurrence of incidents of abrupt production decrease and even blockage due to wax deposition in wells where chemical flow improvers were being employed is not surprising. For example, a well in subsea Coulomb field in the Gulf of Mexico, in which monoethylene glycol was continuously injected under a strategy of wax-in-place for managing a limited wax deposition rate, had to temporarily closed due to excessive wax accumulation in the production first month [98]. The reported issue was the misevaluation of significant differences in wax content among different wells.

Likewise, wax deposition-related incidents in wells and pipelines where mechanical removal is employed continue to be frequent. There are several reports of pigs getting stuck in wax deposits [99,100], including one incident in one Gulf of Mexico oilfield, from which the costs of production downtime and intervention for removing the stuck instrument have been made publicly available [31]. Furthermore, the high-pressure effects of mechanical removal upon the infrastructure have been pointed out as a major reason for not performing pigging operations on old pipelines [101].

On the other hand, too frequent pigging has been appointed to cause unnecessary costs, mainly due to deferred production, not the technique itself [16], which can reach tens of millions of dollars [99]. For these reasons, mechanical removal and chemical flow improvers injection are more frequently complemented with insulation and heating techniques [99].

Other recent costly developments and applications include pipe-in-pipe or insulated pipelines, active heating systems [102], and wax deposition control through operational conditions rather than its formation. The latter include keeping the production above one well-defined threshold, as in the Trans-Alaska pipeline system [103,104,105]. Several other cases can be found, where depending on the infrastructure, such as insulation systems, production thresholds have been defined for avoiding flow assurance problems [82].

Wax-related issues are a prevalent concern in some oilfields. While technological developments must be recognised, the fact is that the continuous increase in production until very recently, along with the need to resort to waxier and heavier reservoirs in increasingly less reachable and colder fields, is not allowing the solution of the problem only through adequate prevention.

Figure 3 represents the crude oil production divided into accessible and inaccessible conditions [31,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71].

The most common costs include the ones with preventive measures, with downtime for maintenance and with deferred production due to sectional decrease [106], even if incidents with the too frequent temporary shutdown for maintenance currently happen in several oilfields and, in many cases, are responsible for discontinuing operations. In these circumstances, incidents with not despising environmental impacts are very infrequent [107].

Even though applying dangerous chemical inhibitors and mistakenly releasing extracted wax might have some negative effects on the environment, massive oil spills are the only event that can have catastrophic repercussions. Even with completely obstructed infrastructures, it has not been the case. However, if drastic measures are performed the sentimental risk of that procedure will increase.

As previously mentioned, there are also wax deposition problems in gas production, even if those that can be regarded as very specific cases related to local production conditions. Furthermore, wax formation and deposition can merge with non-paraffinic components effects, such as asphaltenes, scales, or gas hydrates, to name the most frequent. Leaving behind these details, which are extremely important for understanding the phenomena but absent from the economic quantifications broadly attributed to wax-related problems, we focused this review on oil fields, as well as on the costs and environmental impacts associated with wax control as reported in industry or research publications, without analysing each case’s phenomenology.

4. The Cost of Spills

Data has been gathered primarily from published and peer-reviewed research articles for its reliability and accuracy. Yet, despite the widely acknowledged economic importance of wax deposition, it has been extremely difficult to attain quantitative information from such sources. Therefore, significant industry or government agency reports have also been considered, provided source reliability has been cross-checked.

When obstructions caused by wax deposition need site operations and downtime, a cost of millions of dollars is anticipated [84]. Although the expenses of the most known occurrences were far greater, it is estimated that the average direct cost of each incident is approximately 40 million USD [17].

Several researches indicated a yearly cost of billions of dollars from an industry-wide and global viewpoint [11,13,14,15,16,18]. However, this minimal value probably just considers upstream activities and direct costs.

Available data in the United States of America’s dates back to a time when internal production sources were much less dependent on offshore and deep offshore, as well as from extreme climates. Hence, an annual cost of 4.5 to 5 million USD was estimated for domestic production in 1969 (approximately 0.117% of the US production for that year) [30,31]. The Prudhoe Bay oil field on Alaska’s North Slope only began producing in 1977 [104,105].

Although there haven’t been any major oilfield mishaps directly attributable to wax deposition issues, it is generally known that full well and pipeline blockages necessitate extensive removal and replacement operations, particularly in deepwater and ultra-deepwater offshore. Even while accidents are infrequent, they might have severe effects on the ecosystem and environment.

The Deepwater Horizon oil disaster, which occurred in April 2010, brought to light the world’s present awareness of environmental challenges and legislation. This incident overwrote the industry’s common understanding of significant accidents’ economic impacts, built upon prior accidents, such as the Exxon Valdez oil spill (which happened in March 1989). When licensed to BP to drill at the Macondo Prospect, the Deepwater Horizon drilling rig had a blowout that resulted in the greatest oil leak ever recorded [108]. Considering its location, the spill’s uncontrolled nature altered the paradigm, created a previously unheard-of level of difficulty in determining the effects on the environment without a proper baseline, and raised the possibility that future accidents would result in increasing costs [109,110,111]. To provide the most accurate estimate currently available for the costs associated with litigation and the environment caused by a big oil catastrophe, the Deepwater Horizon can be used as an example. The sum of the costs that BP spent was almost 60 billion USD and the percentual distribution by type is presented in Figure 4.

Due to incomplete assessments of the effects on seagrass species and long-term effects on big fish species, deep-sea corals, sea turtles, and cetaceans, it is possible that these effects were underestimated [112].

5. Operational Costs to Avoid Deposition

Mechanical removal of wax deposits is a typical preventative strategy to avoid total blockage [91], albeit it is not always effective. Most of the costs of applying this technology are related to the downtime required for the operations. Even if the technology is not expensive, sometimes it has certain needs, such as looping flowlines [92] or sufficient launching and reception conditions. High-frequency pigging can cost tens of millions of dollars per intervention [99], which is more expensive than competing solutions, such as insulation and chemical flow improvers [92].

According to an industry estimate from 2002, the cost of production delays caused by pigging for a 29 km line at a crude oil price of $20 per barrel is approximately $17,000 USD per km [17].

It is crucial to emphasise that the cost of using chemical flow improvers varies depending on the circumstances. However, calculations have been released that show adding Pour Point Depressants would cost 15 million USD annually [28,31].

Repairing a blocked subsea line, which may encompass pipeline removal and replacement, as well as employing divers, can cost between five and six hundred thousand USD [20,28,29]. However, the cost of repair is estimated to be at least $5 million USD for every incidence [12,24,28].

Production rate and oil price have an impact on the cost of production loss due to downtime. However, some estimates range from approximately 625,000 USD each day [12,22,24,28], and between 30 and 40 million USD every occurrence [6,20,24].

In fact, it has been stated that the cost of downtime is more than the intervention costs to fix or replace a clogged pipe or well, whether as a result of wax build-up or another phenomenon [113,114].

A critical factor to assess economic and societal impacts of a given occurrence in an industry is the number and wages of the workforce. Nearly 4.7 million people were working permanently in the oil and gas fields and production sector as of 2019 [115], with a considerable pay gap between their locations and roles. An administrator in the Middle East made 14 713 USD per year, whereas a drilling supervisor in Australasia made 185 384 USD per year [116], according to global average values based on six years of experience.

Despite the paucity of data sources, the investigation of attained results concerning each cost source was able to identify wide ranges, which are related to the diversity and complexity of operations. This is consistent with the reported difficulty on assessing the economic impacts of potential incidents due to wax deposition.

6. Information about Oil Production

The most important factor to consider when determining the economic effects of production collapse is production value.

In addition, several incidental costs are seen as becoming increasingly significant, particularly for their impacts on the environment and society. Deferred and permanently lost production are also anticipated to be more costly than treatments.

However, it is necessary to know the oil value and production rates across time to calculate production value and, to enable its analysis, data shall be consolidated onto a few categories.

These will necessarily be broad categories that do not consider many important factors, such as the interaction with scales and other substances that can cause blockages, heavy oils, high water fractions, multiphase flows, Enhanced Oil Recovery, among many other factors. However, such a categorisation is crucial to being able to calculate economic costs broadly rather than relying on a case-by-case approach.

Hence, the herein proposed categorisation consists of (i) arctic and permanently or temporarily frozen sites in cold sources, (ii) tropical and hot arid sites in warm sources, (iii) near onshore and shallow offshore at accessible conditions and (iv) deep offshore and long pipelines in inaccessible conditions. Criteria for such categorisation are certainly arguable, as there are not complete worldwide lists from where one can attain and filter information with a high degree of accuracy.

Given the existing understanding of the world’s oil reserves [53,70,117], one must comprehend that future oil fields are expected to have a substantially greater proportion of less accessible, cold climate and paraffinic oils.

The quantity of oilfields, average production, and average number of active wells are further pertinent factors to consider when assessing the economic effect of waxy crude oils [118,119,120,121,122,123,124]. Considering the average values between 2010 to 2019, it was found that more than 650000 oilfields exist worldwide. The average daily production of the 30 largest oilfields was found to be 750000 barrels (27 percent of global production). The output from the other fields is substantially lower. The average number of active wells per oil field varies considerably. Usually between 10 and 20, although it might reach hundreds.

Because of large swings brought on by prospected oil prices, field production, and technical advancements, the actual field life may differ from the predicted one. The typical projected field life spans twenty to fifty years [125,126,127]. However, many current wells continue to run much beyond these horizons [128], while others could be terminated early owing to adverse market circumstances [129].

Further research was conducted, as local data became crucial for utilising well-documented occurrences and cost data. Hence, to match the very scarce available useful data on wax deposition, described in Section 6, production data was gathered for key years, some of those already far in the past. According to the information provided in different publications [7,17,130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149,150,151,152,153,154], in 1969 the USA oil production was 3372 million bbl, and it represented 4282 million USD. For the same year, the estimated world oil production was 20323 million bbl, with a value of 25810 million USD. Between 1992 and 2002 the Gulf of Mexico (USA) oil production was 4640 million bbl (Figure 5) and world oil production was 259317 million bbl. In 1994, the world oil production was 346762 million USD. In 1994, the number of subsea pipelines in the Gulf of Mexico (USA) was among 1500 to 2500. Concerning the wells, its usual inner diameters range between 50 mm and 120 mm and, in special cases, it can go up to 500 mm. The flowlines usual inner diameters range between 200 mm and 600 mm. According to the information between 1979 and 2015, The pipelines average length was 84 km.

7. Information about Wax Deposition in the Oil & Gas Industry

Despite some recent advancements, the relative relevance of several wax mechanisms is still not entirely understood [25]. While the first stage of deposition, wax crystallisation, depends greatly on the highly variable oil composition, deposition itself can happen or not depending on a few factors, particularly climatic and system-related factors, such as geometry, flow regime and pattern, and water content [7,155,156].

It is difficult to provide a precise range for the usual wax deposition rate. However, a rough estimate could be made for the time-stabilised thickness of wax deposition, using the experimental data obtained by several researchers [12,22,157,158,159].

Only in a few localised regions, especially the Gulf of Mexico (USA) fields, has the presence of wax deposition issues been thoroughly studied. Available data shows that in 1994, wax deposition in these areas caused the obstruction of 14 subsea pipelines [24]. Between 1992 and 2002, 51 severe occurrences of wax obstruction occurred [17,23,28,29]. In this period, 1.79 percent of the oil produced globally was produced at this offshore location.

The vast variety of severity magnitudes of such an issue and the challenge of differentiating wax-related flow assurance blockages produced by other causes make it impossible to quantify the global fraction of oil-producing locations impacted by wax deposition. However, Thota and Onyeanuna estimated that wax deposition issues had an impact on 85% of global oil production [160].

To assess the range of expected values, it is crucial to quantify the economic effects of wax deposition over the whole industry. Wax deposition has an annual economic effect of more than USD 2 billion worldwide [11,13,14,15,16,18]. Moreover, wax deposition has a 40 million USD economic effect each blockage incidence [17].

Knowing case-specific data is important to study the global potential economic impact. As such, it has been found that a North Sea incident in 1994 that resulted in abandonment had an economic effect of $100,000,000 (0.00003 percent of the total value of world output for the year) [6,12,17,18,19,20,21,22,23,24,25,26,28].

The investigation deemed to garner a cost for some of the aforementioned factors found that, for an oil price of $20 per barrel, the preventative and corrective steps for mechanical removal of wax deposits cost seventeen thousand USD per km [17]. The estimated annual cost of injecting Pour Point Depressants injection for preventative and corrective treatments is fifteen million USD [28,31]. The price per kilometre to replace a blocked subsea pipe ranges from five to six point two hundred thousand USD [20,28,29]. Furthermore, according to the literature, production loss due to downtime costs are approximately six hundred thousand USD each day [12,22,24,28]. Additionally, it is stated that the production loss due to downtime is larger than 30 to 40 million USD each event [6,20,24].

8. Economic and Environmental Implications of Flow Assurance Problems

While the search, validation, compilation, and synthetization of published data is described in the former sections, a further step in adding value to this review is deriving scenarios for the global magnitude of the problem. To such an end, the information synthetized in the former sections, including on operational costs to avoid deposition is paramount.

Attending to the available literature, one can assert that, when possible, the economic implications of flow assurance problems can be extrapolated for a wider scope based on individual case studies. Among the few examples discovered, one may consider the analyses done for determining the economic effect of scale deposits. Utilised methodologies include performing numerical simulations to production datasets depicting several conditions. The benefit of using a scale deposition control mechanism at the Veslefrikk field, in the North Sea, was estimated to be more than 120 million USD per year, over a nine-year period [161]. However, this is a case-specific study.

Even so, extrapolations are routinely used when better predictions are unavailable. Input-Output analysis, which Leontief popularised, is one of the most well-known methodologies for economic impacts appraisal [162,163]. Beyond the more common industrial applications, it has been shown adequate for analyses with interdependencies, including environment, sustainability, energy-related issues, and industrial applications [164,165,166,167,168,169]. It has been used to evaluate the effects on the extractive industry [170]. Other approaches may be applied, as the contingent valuation method for non-financial values [171,172,173,174].

There are several suitable approaches for determining the economic effects of wax deposition. The Bottom-Up approach is used for listing costs in a cluster [175]. The Market Value approach [176] may be used multiplying the production lost by the price of oil. The Top-Down approach begins by identifying macro-elements and then refined to more individualised expenses [177]. The mix approach can also be applied joying of Bottom-Up and Top-Down methods [178]. Additionally, less analytical methods, such as directly posing specific questions using the Questionnaire-Type technique [177,178,179,180], or utilising data from other research using the Benefits Transfer method, can be used to improve the knowledge filling the gaps and validate the material in such a comprehensive study [175,181,182,183]. The latter serves as an introduction to meta-analyses.

Based on this systematic review, a research article by these authors was able to establish several global scenarios, concluding that wax deposition economic impact might range from 15 to 331 billion USD per year, with an expected value of 48 billion USD [184].

Assessment of socioeconomic implications has historically relied on extensive fieldwork supported by the community [185,186,187], or on demographic data [188], and it is often condensed to social indicators [189,190,191]. Nevertheless, most evaluations are usually site-specific [191,192,193,194]. Because of the size, complexity, interconnectedness, and dynamics of the extractive industries, meta-analyses have been used for appraising socio-economic impacts [195], even though data is uncertain [196].

Environmental impact assessments are, as well, frequently, if not always, restricted to affected areas and heavily influenced by regional circumstances [197,198,199,200,201].

Hence, the environmental impact assessment of O&G industry accidents is essentially case-based. Until the last decade, knowledge was scarce and mostly based on Exxon Valdez spill. Such a case was instrumental in proving that long-term ecosystem contamination, including cascades of indirect and somehow unexpected effects, would add to the well-perceived acute mortality [202].

Yet, the Deepwater Horizon (DWH) accident dramatically changed this scarcity in 2010. Compared with the Exxon Valdez spill, DWH released more than twelve times more oil and induced more dramatic permanent damage to the ecosystems [203].

DWH rapidly became the landmark for oil spills environmental impact assessment, given its magnitude [204] and long-lasting effects [205,206]. It leveraged significant studies on coastal flora, including thorough investigations on salt marches, leading to the conclusion that contamination is much more expressive on the marsh edges and varies significantly with oiling intensity. Moderately oiled marshes have been found to recover within two-and-a-half years [207], while heavily oiled ones may not recover at all, also due to induced shoreline erosion [208] and shoreline clean-up, leading to the loss or morphological modification of species, such as periwinkles [209]. However, the effects upon marshes have frequently been found not to live up to the worst expectations [210], chiefly due to the role of oil emulsification in fostering the formation of plumes with a tendency to sediment in the deep sea [211].

Studies further ranged from phytoplankton to deep-water coral communities. The former has undergone significant modifications, as interactions between bacteria and phytoplankton are affected by the oil spill, which leverages the growth of oil-degrading strings, including marinobacter, which, in turn, will foster the production of exopolymeric substances [212]. Regarding the latter, coral colonies have shown several symptoms of oil-related impacts, such as mucous production, sclerite enlargement, bleached commensal ophiuroids and tissue loss [213], beyond the obvious contamination with flocculated oil.

A metanalysis by Kimes et al. found microbial transformation in the aftermath of DWH spill to be rapid and robust for aromatic and low-weight aliphatic hydrocarbons attenuation but not for oxygenated oil components [214].

The research on fauna has been especially profuse, allowing to conclude that, even if megafauna abundance was not impacted near the contamination origin, its homogeneity increased as did the arthropods density, suggesting the loss of some species [215,216]. The salinity decrease led to a critical reduction in oyster populations, which can amount to more than three billion individuals [217]. Hence, not only were the heavily oiled birds lost to the spill, as profusely documented but many lightly oiled individuals, including in the American oystercatchers population, have been critically affected, as depicted by a Haematological Indices assessment [218]. Other long-lasting effects include genomic modifications in killifish [219] and increased zooplankton community density following the spill [220]. On the other hand, loggerhead foraging sites have not been changed despite the contamination with oil and dispersants [221].

Clean-up workers’ health data has also been gathered from the DWH aftermath, showing immediate signs of stress, rashes, heat strokes and flu symptoms [222], which can extend to long-lasting effects such as DNA damage, considering both the findings of health checks to populations affected by previous spills [222] and the DHW oil exposition immunotoxic effects upon vertebrates [223].

The sociological aspect of the environmental problem has also been studied, highlighting the susceptibility of the community to shape its perception of the spill response by media coverage and BP official communication [224].

Regarding sediment dynamics, conclusions from DWH vicinity show that oil sinking occurred in the form of marine oil snow (MOS), and that the formation of MOS was promoted by plankton dynamics [225]. Moreover, it has been found that deposited MOS on the seafloor accounted for nearly 8% of the spilt oil, mostly in a layer of less than one centimetre, with heavier components concentrated near the well, and that after four years, the MOS mass had decreased by 80% to 90%, confirming the short-term deposit hypothesis [226,227].

Further developments in remediation approaches and, especially, modeling techniques were due to the studies performed for DWH spill. This included simulation methods for oil burning emissions [228], and oil and dispersants fate [229,230,231]. Remediation methods research found some developments as dispersants and sorbents with lesser toxicity [232], forensic methods [233] and screening techniques [234].

However, other recent environmental accidents associated with the O&G industry also provided case-based insights worth mentioning. One example can be found in Niger Delta pipeline systems’ recurrent oil spills, namely in Delta, Rivers and Bayelsa states where, after the remediation operations, soils have been shown to face contamination by hydrocarbons in clear exceedance of the allowable limits [235,236]. Soil contamination, along with pipeline downtime due to the spills, have had significant impacts on the local communities’ livelihood.

Recently, an oil spill was reported at the Brazilian Alagoas and Sergipe coast. Despite its unknown origin, forensic research with Spill, Transport and Fate Models (STFM) was able to determine the likelihood of having been caused by a leakage in an oil tanker ranging from Panamax to Suezmax [237]. As a result, water and marine fauna were contaminated with fluoranthene, phenanthrene, acenaphthalene, fluorene and naphthalene [238].

The coast of Bohai Sea has also been recently investigated for environmental impacts of oil-related activities. Since the major Penglai 19-3 oil spill in 2011, minor spills have been reported almost annually, with the outcome of degrading the sea’s coastal area environmental conditions and significantly decreasing the edible fauna, such as shellfish, crustaceans, sea cucumbers, sea urchins and algae [239].

The analysis of the MV Wakashio oil spill in Mauritius fostered the understanding of the environmental hazards of very low sulphur oil spills, suggesting that these fuel oil spills may be less harmful to the environment [240].

Beyond single case analyses, metanalyses and comparative studies have been used to gather more representative conclusions on the environmental impacts of oil and gas production activities. Some of the more significant of such endeavours include understanding oil-consuming microorganisms, especially deep-sea g-Proteobacteria, as intrinsic bioremediation mechanisms [241,242]. Moreover, spilt oil toxicity has been shown to play a significant role in cholesterol depletion in fish species, which, in turn, is regarded as a plausible cause for neuronal, cardiac, and synaptic function effects [243].

The definition of methodologies and thresholds to assess environmental impacts of oil contamination events is, however, much more complex and, therefore, scarcer in the published scientific literature. Nonetheless, some crucial endeavours can be found in Suslick and Furtado’s [244] Multi-Attribute Utility Theory (MAUT) for decision models concerning petroleum exploration, duly accounting for the environmental level, in Bock et al.’s Relative Risk Methodology [245] for integrating the dispersant injection within oil spill response decisions, in the establishment of baselines for spill damages quantification [246], in the developments in oil spill toxicity models, by Hook [247], suggesting that the reported lack of consistence between predictions and site observations on plankton life can be enhanced by better accounting for the differences among laboratory and whole ecosystem conditions, or in the review of the role of interactions between dispersed oil, oil and sediments in the fate and transport of spilt oils [248]. Recent review publications based on multiple case studies have shed light on treatment technologies [249], including the suggestion of incorporating environmentally friendly agents and enhancing information control technology for produced waters that can be applied to spills remediation strategies, as well as on spilt oil removal devices [250], namely the sorbent-based ones, such as calcium carbonate, calcium stearate, exfoliated graphite, expanded perlite, functionalized silica aerogel, hydrous aluminium silicate, organo-clay, rice husk, rice straw, silk fibre, silkworm cocoon, sugarcane bagasse or vermiculite, with perspectives of recovering up to 90% of the leaked oil.

9. Conclusions

As an answer to the increasingly imminent need to quantify the significant economic and environmental problem of wax deposition and blockage in oil transport infrastructure, this review depicts an extensive literature review and big data analysis to support a deeper understanding of the influential factors and to quantify the problem. Specifically, a historical review of wax deposition is offered, as well as of flow assurance incidents. Data collection, validation and systemisation have been performed both for paraffinic oil production and for economic impacts of flow assurance incidents and maintenance operations. Moreover, a characterisation of oil production regarding the parameters of interest for wax deposition, namely the temperature, accessibility, and the paraffinic nature, was performed. Case-specific investigations, most comprehensively on the Deepwater Horizon spill, allowed for a discussion of the accident’s economic costs, by nature, as well as on its environmental impacts. As a key contribution of this work, primarily, one can count the establishment of layers of impact as a method for the overall impact. Yet, adding the results of the investigation of other oil spills and metanalyses on the matter, it was possible to identify two trends in the research literature. The first shows that the impacts on the affected ecosystems are much more profound than the usually accounted for acute mortality and extend to genetic modifications in the affected species. The second proves the role of the very same ecosystems in providing means to mitigate the spill effects, primarily through microbial and bacterial reactions, ensuring resilience in some cases to unexpected levels. The key limitations of this work can be found in the scarcity of verifiable data on economic impacts and conclusive data on environmental impacts. Hence, further research on this matter is expected, as well as the employment of the herein-depicted context to leverage the assessment of the economic effects in specific cases.