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Required viscosity values to assure proper transportation of crude oil by pipeline José A. D. Muñoz, Jorge Ancheyta, and Luis C. Castañeda Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01908 • Publication Date (Web): 20 Sep 2016 Downloaded from http://pubs.acs.org on September 22, 2016
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Required viscosity values to assure proper transportation of crude oil by pipeline José A. D. Muñoz, Jorge Ancheyta, Luis C. Castañeda Instituto Mexicano del Petróleo, Eje Central Lázaro Cárdenas Norte 152, Col. San Bartolo Atepehuacan, Mexico City, 07730, Mexico, Email:
[email protected] Abstract Transportation of crude oils has recently received much attention than in the past, the main reason for this is because the changes that petroleum production has experienced. The increasing availability of heavy and extra-heavy crude oil and the depletion of the production of light crude oil have motivated the mid-stream oil sector to search for methods to reduce the viscosity to values that assure the transportation of crude oils by pipeline. The target for viscosity varies according to the type of crude oil and the region where it is transported. In order to define and propose a range of viscosity, the different reports in the literature were reviewed and it was concluded that the maximum value of viscosity to allow crude oil transportation ranges between 250 and 400 cSt at 37.8ºC. Keywords: viscosity, crude oil, transportation 1. Introduction According to worldwide definition, the API gravity of heavy crude oil ranges from 10° to 20°, while extra-heavy crude oil and bitumen exhibit value less than 10°. In addition, heavy crude oil and extra-heavy crude oil reach values of dynamic viscosity between 100 and 10,000 cSt while the dynamic viscosity of bitumen is always greater than 10,000 cSt [1]. Various studies indicate that the viscosity of extra-heavy crude oil or bitumen depends on
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the concentration and type of asphaltenes that are present in the crude oil and not by their chemical composition [2, 3]. Bitumen exhibits higher viscosity than extra-heavy crude oil at pressure and temperature of the reservoir. While the former does not move at reservoir temperature, the latter has some degree of mobility [4], which is the ratio of the effective permeability to the oil flow to its viscosity. These oils possess high viscosity and low API gravity. Due to this, crude oils can cause a series of complications during the production, separation, transportation by pipeline, and refining [4, 5]. The high viscosity of heavy petroleum is an important issue that adversely affects the up-stream recovering, mid-stream surface transportation, and down-stream conversion processes. As example, Table 1 shows API gravity and viscosity of various heavy crude oils. It is observed that an increase in API gravity is not directly proportional to a decrease in viscosity and other structural factors affect this fluid property [6, 7]. Nowadays, the worldwide resources of heavy petroleum and bitumen duplicate those of the conventional light crude oil. The transportation of heavy crude oil and bitumen is a challenge due to their incapacity to flow easily. Without reducing the viscosity, the transportation of heavy crude oil and bitumen viscosity is difficult, which is due to the high demand of energy required to handle the high-delta P in pipelines [7]. For instance, Figure 1 shows oil viscosity as function of reservoir temperature for the two largest deposits in the world: Athabasca bitumen in Canada (1,000,000 cP; ~ 980,000 cSt at 11°C) and Orinoco extra-heavy crude oil in Venezuela (1,500 to 3,000 cP; ~ 1,600 to 3,200 cSt at 53°C). Table 2 report the values of viscosity and density for different heavy oils and their mobility at reservoir conditions. Oils with API gravity higher than 18 and viscosity lower than 100
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mPa-s (~ 111 cSt) exhibit mobility, while oils with API gravity lower than 18 and viscosity higher than 100 mPa-s do not move [8]. Oil transportation involves a highly technical and complex operation. The most important problem in pipeline transportation is the high viscosity of the fluids that need efficient and economical methods to be transferred [9, 10]. Other reasons are the high concentrations of asphaltenes and the associated problems of incompatibility during the blending of crude oils and instability during storage. The following different types of heavy crude oils and approaches for their production have been reported based on viscosity [4]: •
Class A (Medium heavy crude oils). Those with viscosity in the range of 10-100 cP (~ 11-111 cSt)
•
Class B (Extra-heavy crude oils). Those having viscosity in the range of 100-10,000 cP (~ 111 to 10,150 cSt)
•
Class C (Bitumen). Hydrocarbons with viscosity higher than 10,000 cP (~ 10,150 cSt) and API gravity less than 7°, which are immovable at reservoir conditions. They require thermal recovery methods (steam injection or mining techniques)
•
Class D (Bituminous shales). They are considered as source rock, and are extracted using mining or in situ techniques
Classes A and B, which API gravity ranges from 25 to 7, comprise oils that can be recovered by cold production. Figure 2 shows the API gravity of some samples for classes A, B and C. Previous experimental studies have demonstrated that the viscosity of heavy oil depends on three factors: volume fraction, chemical structures, and physicochemical properties of the
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recovered asphaltenes, these latter being the heaviest and most polar components present in the composition of heavy oil [11-13]. The effect of the content and quality of asphaltenes on the viscosity of heavy oil has been studied with the following conclusions: the viscosity of a liquid is dependent on the molecular attraction and is influenced by the polarity and molecular weight, mainly added by asphaltenes and resins. Any concentration of asphaltenes higher than 4 wt% forms a colloidal system that determines the viscosity. Moreover, asphaltenes tend to associate in diluted solutions and they are associated with resins by intermolecular hydrogen bonds. The origin of the high viscosities in heavy oil comes from such an interaction [14, 15]. Heavy crude, extra-heavy crude and bitumen present problems for transportation via pipeline due mainly to their high viscosities. Typically, they are transported after the addition of lighter hydrocarbons (dilution). The amount of added diluent is a key factor because of its higher cost compared with that of the crude oil alone. So that proper definition of the amount of crude oil and diluent to be blended is crucial to allow the oil to be transported at the lowest cost. Flow properties, particularly viscosity, are of high relevance to define the most appropriate quantities of the blend components. In this sense, maximum values of viscosity required for transportation have been reported by several authors, which are based on field experience, laboratory experiments and local regulations. It is then the objective of this work to discuss the different literature reports on this topic in order to define and propose a range of viscosity to allow for the crude oils to be fluidly transported and thus minimizing the use of diluents with the consequent economic benefits. 2. Reports of crude oil viscosity values for transportation The following reports are found in the literature regarding proposed values for viscosity of crude oil to allow its transport by pipeline: 4 ACS Paragon Plus Environment
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In the US patent 4343323, assignee to Research Council of Alberta, Canada in 1982, it is considered essential for pipeline transportation of crude oil to have a viscosity lower than 200 cSt when measured at 15°C [16]
Canada and Europe recommend a viscosity value of 25 cSt at 50°C for crude oil and Venezuela states a viscosity of 180 cSt at the same temperature [17]
Europe and North America report a viscosity specification of 25 cSt at 50ºC for crude oil transportation [18]
For diluted extra heavy oils with naphtha or lighter crude for transportation by pipeline is API gravity 19 and viscosity < 350 cP (~ < 355 cSt) for Canada and 400 cP (~ 420 cSt) for Venezuela [19].
In Canada, the dynamic viscosity for pipeline transportation is 350 cP (~ 355 cSt) at 11ºC [20].
Canadian specifications for transportation by pipeline of heavy crude oil and bitumen are: 19°API min, 350 cSt max at 7°C and 0.5 vol% max of basic sediment and water (BS&W) [21]
The specifications of transportation of heavy oil depend on the lease agreement between the producer and the transportation company. Generally, the viscosity, gravity and sulfur for transportation purposes are in the range of 350 cSt, 19ºAPI and < 2wt% respectively [22].
Pipeline specifications for crude oil transportation were defined in 2009 tube: 350 cSt at 11°C and 19°API, for viscosity and API gravity, respectively [23]
Heavy crude oils that require to be transported need to be upgraded to a viscosity specification, i.e. < 250–300 cSt at pumping temperature conditions [24].
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The viscosities for the heavy crudes oil at room temperature vary from 100 mPa s (~111 cSt) to more than 105 mPa s (~ 101,500 cSt). Generally, crude oil with viscosity
