Introduction

Asphaltenes are defined as the fraction of petroleum that is insoluble in n-paraffins (e.g., C5, C7, etc.) but soluble in aromatics (toluene, xylene, etc.). They are consid- ered “bad actors” when crude oils and their fractions are produced, transported, or refined. They have been “blamed” for several operational issues throughout the petroleum value chain such as [1,2]

• Reducing the permeability of oil-bearing formations and the concomi- tant reduction in the volumetric production of wells and percentages of oil recovery

• Increased cost for lifting and transportation due to the high viscosity of asphaltenic crude oils

• Plugging of tubing, pumps, valves, and pipelines due to asphaltene pre cipitation • Causing fouling in heat exchangers and other devices in production and

refinery operations

• Reducing and limiting the yield of residue conversion during catalytic upgrading processes

• Contributing to catalyst poisoning through coke and metal deposition • Being the source of coke during thermal upgrading processes

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On-Column Filtration Asphaltene Characterization

Specifically in upstream operations, the loss in production due to asphaltene plugging could yield financial losses on the order of tens of millions of US dollars, depending on the field that it is being produced. For example, in the Prinos petro- leum production field in Greece, some wells completely ceased flowing in a matter of a few days after an initial production rate of up to 3000 BPD [3]. Other examples of solid precipitation were reported by Carbognani et al. [4] during oil production in Venezuelan fields. The authors identified remedial actions based on deposit charac- terization and developed criteria for selection of the most efficient dispersant addi- tives and solvent mixtures for cleaning production strings [4].

Asphaltene deposition problems during production operations originate from a decrease in the solubility of asphaltenes. As asphaltenes become less soluble, they form floccules that, under the right conditions, can deposit blocking path- ways. In production operations, the decrease in the solubility of asphaltenes is normally related to the decreasing solvent power (ability to dissolve asphaltenes) of the maltenes. Changes in pressure and temperature during the natural declining of production can cause asphaltene precipitation. However, it can also be caused by recovery process and treatments. For example, during carbon dioxide floods and when this gas is mixed with the crudes, asphaltenes can become destabilized and may deposit on the mineral formation or elsewhere in the production system [5]. There are several field examples in which no asphaltene deposition was expe-

rienced during primary production. However, once CO2 was injected downhole,

severe precipitation issues were observed [6–8]. For example, for the CO2 pilot in

an onshore Abu Dhabi Field, three asphaltene removal jobs were carried out dur- ing the life of the pilot (1.5 years). Asphaltene build-up was observed after only 3–4 weeks after cleanup [5]. It is also well known that acidification treatments and other stimulation procedures can modify the solvent power of the maltenes, and therefore induce precipitation. For example, it has been reported that iron- contaminated acid promoted the precipitation of asphaltenes when acidizing cer- tain oil-bearing zones [9].

There are several techniques that can be used to deal with this issue: (i) control of pressure or other operational variables such as comingling, (ii) periodic cleaning (either chemical or mechanical), and (iii) additive injection to prevent asphaltene precipitation.

In 2001, it was reported that each asphaltene cleanup operation using organic solvents with a coiled tubing unit costs, on average, US$200,000 onshore [4]. More recently, in 2012, it has been estimated that each remediation event costs around $500,000 (onshore) or $3,000,000 (offshore) [10]. In fact, in some cases, owing to the severity of asphaltene deposition and based on the economics of the cleanup opera- tion, it was recommended to implement continuous downhole chemical injection of asphaltene inhibitors in producer wells [6].

Additionally, it has been reported that the presence of asphaltenes and its ten- dency to form aggregates plays a fundamental role in the high viscosity (~100,000 cP at room temperature) observed for heavy and extra heavy crude oils (H/XH crudes) [11]. Specifically, it has been shown that by removing the asphaltenes from these materials, a reduction of 2 to 3 orders of magnitude on the viscosity of the maltenes is obtained over the temperature range from 40°C to 80°C [9]. Also, asphaltenes

interact extensively within themselves and with other similar molecules form- ing extended networks, and thus the viscosity of asphaltene-containing petroleum

samples shows an exponential increase with their concentration [12].Several routes

have been investigated in the literature to overcome this issue, such as the use of emulsions [13–15], diluents [11,16], heated pipelines [17], carbon dioxide [18,19], precipitation and separation of asphaltenes [20,21], and use of additives [22–24]. In these routes, caution should be taken in maintaining the asphaltenes in solution so no solid precipitation problems are developed during the transportation and storage of H/XH crudes [11].

It is clear from the previous paragraphs that it is highly desirable for the petro- leum industry to develop methods that can be used to accurately evaluate the effect that different recovery processes and chemical treatments (i.e., acidification, diluents, or additives) might have on asphaltene behavior.

One important step toward this end, namely understanding, mitigating, and solv- ing asphaltene precipitation and transportation issues, is the analysis and charac- terization of the asphaltene fractions present in crude oils and solid deposits. With this goal in mind, in the last few years, we have developed three techniques for asphaltene analysis based on on-column filtration methods [25–29]. In this work, we demonstrate the usefulness of these methods in characterizing produced crude oils and deposits from upstream operations. These analytical techniques are as follows: determination of asphaltene content by the on-column filtration method [25,26], asphaltene solubility profile [27,28], and separation of asphaltenes in solubility fractions [29].