Reservoir Formation Damage Episode 2 Part 7 ppt

Steps of oil analysis and characterization for paraffin, aromatic, resin, and asphaltene after Leontaritis, ©1997 SPE; reprinted by permission of the Society of Petroleum Engineers... 19

Characteristics of Asphaltenic Oils

As indicated by Figure 14-1 by Philp et al (1995), the boiling andmelting points of hydrocarbons increase by the carbon number Heavycrude oils contain large quantities of higher boiling components, whichcreate problems during oil production (Speight, 1996) Speight and Long(1996) point out that chemical and physical alteration of oils may affectthe dispersibility and compatibility of their higher molecular weightfractions and create various problems, such as phase separation, pre-cipitation and sludge formation during the various phases of petroleumproduction, transportation, and processing

Speight (1996) classified the constituents of the crude oil into fourhydrocarbon groups: (1) volatile saturates (paraffins) and aromatics,(2) nonvolatile saturates (waxes) and aromatics, (3) resins, and(4) asphaltenes Speight (1996) explains that the nomenclature of thepetroleum fractions, such as given in Figure 14-2, is based on thetechniques of separation of the crude oil into its fractions Figure 14-3

by Leontaritis (1997) describes the various steps and techniques involved

Figure 14-1 Effect of n-alkane carbon number on boiling and melting point

temperatures (after Philp, R P., Bishop, A N., Del Rio, J.-C., and Allen, J.,Cubitt, J M., and England, W A (eds.), Geological Society Special Publica-tion No 86, pp 71-85, ©1995; reprinted by permission of R P Philp andthe Geological Society Publishing House)

Toluene

I

Asphaltenes

Carbenes S (Solubles) ;

|

i

3 Methanol

Resins (Polars) \

I

Silica or Alumina

2 Benzene or 1 Heptar Toluene

Aromatics Saturates |

Figure 14-2 Classification of petroleum constituents based on laboratory

fractionation (reprinted from Journal of Petroleum Science and Engineering,

Vol 22, Speight, J G., "The Chemical and Physical Structure of Petroleum:Effects on Recovery Operations," pp 3-15, ©1999, with permission fromElsevier Science)

in the analysis of the crude oil, including cryoscopic distillation (CD),solvent extraction (SE), gas chromatography (GC), high performanceliquid chromatography (HPLC), and gel permeation chromatography (GPC).Table 14-1 by Srivastava and Huang (1997) presents data on the chemicaland physical properties of typical oil samples taken from Weyburn wells.Leontaritis (1996, p 14)* described the heavy fractions of petroleum

as following:

Asphaltenes:

Highly condensed polyaromatic structures or molecules, containing

heteroatoms (i.e., S, O, N) and metals (e.g., Va, Ni), that exist in

petroleum in an aggregated state in the form of suspension and aresurrounded and stabilized by resins (i.e., peptizing agents) They areknown to carry an electrical charge, and thought to be polydisperse.Asphaltenes are a solubility class, hence, they are not pure, identicalmolecules Pentane and Heptane are the two most frequently usedsolvents for separating asphaltenes from crude oil The prefixn-Pentane or n-Heptane asphaltenes refers to the solvent used for

* Reprinted from Leontaritis ©1996, p 14, by courtesy of Marcel Dekker, Inc

Paraffins-Wax Pure

Components

Figure 14-3 Steps of oil analysis and characterization for paraffin, aromatic,

resin, and asphaltene (after Leontaritis, ©1997 SPE; reprinted by permission

of the Society of Petroleum Engineers)

Table 14-1 Chemical and Physical Properties of Weyburn Dead Oils*

878.9 875.9 846.1

Density

@59°C d

846.1 849.2 852.4 858.0 860.9 BS&W, vol %

Molecular Weight, g/g-mol

12.8

4.2 - - Viscosity

@59°C d 4.2

- - -

0.1

230

wt.%

48.5 33.5 13.2

4.8

OilW2"

Density kg/m 3

854.9 842.4 - 813.1 - Density

@61°C d

813.1 816.4 819.6 822.9 829.3

Viscosity mPa*s

4.60

-2.35

Viscosity

-@61°C d

2.35 2.49 2.62 2.76 3.04

0.2 203

wt.%

55.3 31.1

9.6 4.0

Density kg/m 3

869.2 864.4 - - 839.4

Density

@63°C d

839.4 842.4 845.2 848.4 854.7

Viscosity mPa«s 11.76

9.40

-

-3.15

Viscosity

@63°C d 3.15 3.26 3.37 3.49 3.71

0.5%

215

wt.%

48.4 33.5 13.2

4.9 'Collected from Weyburn well 14-17-6-13 W2M

" Collected from Weybum well 3-11-7-13 W2M

0 Collected from Weybum well Hz 1 2-1 8-6-13 W2M

d Reservoir temperature for the oil samples

* Srivastava and Huang, ©1997 SPE; reprinted by permission of the Society of Petroleum Engineers

their separation The composition of n-Pentane asphaltenes is ent from that of n-Heptane asphaltenes

differ-Resins:

Aromatic and polar molecules, also often containing heteroatoms andmetals, that surround the asphaltene structures and are dissolved inthe oil and help keep the asphaltenes in suspension They are surfaceactive and, at some thermodynamic states, form their own reversiblemicelles They are polydisperse and have a range of polarity andaromaticity Resins are considered to be pre-cursors to asphaltenes

Paraffin Waxes:

Primarily aliphatic hydrocarbons (both straight and branched chain)that change state from liquid to solid during conventional oil produc-tion and processing operations In addition to aliphatics, fielddeposits usually contain aromatic, naphthenic, resin, and asphaltenic

molecules as well The combined mass is called wax Paraffin waxes

usually melt at about 110°-160°F Field waxes contain moleculesthat can have melting points in excess of 200°F

Asphalt:

The residual (non-distillable) fraction of crude oil that containssuspended asphaltenes, resins, and the heaviest aromatic and para-ffinic components of oils Propane has been traditionally a veryefficient and convenient solvent for separating asphalt from petroleum.Although, the latest commercial processes use other more efficientsolvents for asphalt separation

Leontaritis (1997) describes that: "Since waxes, asphaltenes and mostresins are solid in their pure form and the other oil molecules are inliquid form, the overall crude oil mixture is a liquid solution of waxes,asphaltenes, and resins in the remaining liquid oil In general, thewaxes and resins are dissolved in the overall crude oil Whereas theasphaltenes are mostly undissolved in colloidal state."

Anderson et al (1997) state: "Petroleum asphaltenes are defined as thesolids precipitating from a crude oil upon addition of an excess of a lighthydrocarbon solvent, in general n-heptane or n-pentane." Therefore, forpractical purposes, the crude oil is considered in two parts The first partconsists of the high boiling-point and polar asphaltic components Thisfraction of the crude oil creates various deposition problems during theexploitation of petroleum reservoirs The second part is the rest of thecrude oil, referred to as the deasphatened oil or the maltenes This fraction

of the crude oil acts as a solvent and maintains a suspension of theasphaltenes in oil as illustrated in Figure 14-4 by Leontaritis (1996).However, ordinarily, the asphaltenes do not actually disperse in themaltene unless some resins are also present in the crude oil The resinshelp asphaltenes to disperse in oil as a suspension by means of thehydrogen-bonding process and the irreversible acid-base reactions of theasphaltene and resin molecules (Speight, 1996; Speight and Long, 1996;Chang and Fogler, 1994, 1996) Therefore, Leontaritis et al (1992) pointout that: "An oil that contains asphaltenes will not necessarily causeasphaltene problems during recovery and processing." Leontaritis et al.(1992) draw attention to the fact that the Boscan crude of Venezuela hasnot created any asphaltene problems, although it has a large fraction (over

r ,*

ASPHALTENE PHAS

Figure 14-4 A proposed model for asphaltenic oils (after Leontaritis, ©1996;

reprinted by courtesy of Marcel Dekker, Inc.)

17% by weight) of asphaltenes (Lichaa, 1977) Whereas, the Messaoud oil has created severe asphaltene problems, although it has only

Hassi-a smHassi-all frHassi-action (0.1% by weight) of Hassi-asphHassi-altenes (HHassi-askett Hassi-and THassi-arterHassi-a,1965) In fact, de Boer et al (1995) have concluded that light to mediumcrudes containing small amounts of asphaltenes may create more asphalteneprecipitation problems during primary production Nghiem and Coombe(1997) explain: "Heavier crudes that contain a larger amount of asphaltenehave very little asphaltene precipitation problems as they can dissolvemore asphaltene." Leontaritis et al (1992) state that: "Asphaltene floccula-tion can be prevented by addition of resins and aromatics." The investi-gations of Chang and Fogler (1994, 1997) using model chemicals forresins have verified this statement

Leontaritis (1996, p 13) describes that " asphaltene particles ormicelles aggregate or flocculate into larger aggregates or floes .Asphaltene flocculation can be both reversible and irreversible Paraf-fin waxes, on the other the hand, exhibit the phenomenon of crystal-lization Wax crystallization is generally a reversible process However,paraffin waxes more than often precipitate together with resins andasphaltenes (which are said to be responsible for the observed irrevers-ible thermodynamic phenomena) Hence, some wax precipitation isoccasionally reported as irreversible." Leontaritis (1996) points out thattemperature and composition have a large affect and pressure has a smallaffect on the solubility of wax in oil Leontaritis (1996) explains that thebehavior of wax in oils can be determined by means of the cloud andpour points.

Ring et al (1994) defined the cloud point as "the equilibrium ture and pressure at which solid paraffin crystals begin to form in theliquid phase." Leontaritis (1996) states: The "pour point is defined asthe lowest temperature at which the fuel will pour and is a function ofthe composition of the fuel."

tempera-Mechanisms of the Heavy Organic Deposition

In this section, the mechanisms of the heavy organic deposition ing to Mansoori (1997) are described Mansoori (1997) states that organicdeposition during petroleum production and transportation may occur byone or several of the following four mechanisms:

accord-1 Polydispersivity effect As depicted in Figure 14-5 by Mansoori

(1997), a stable state of a polydispersed oil mixture can be attainedfor a certain proper ratio of the polar to nonpolar and the light toheavy constituents in the crude oil at given temperature and pressureconditions Thus, when the composition, temperature, or pressureare varied, the system may become unstable and undergo severalprocesses Figure 14-6 by Mansoori (1997) depicts the formation

of micelle-type aggregates of asphaltene when polar miscible pounds are added into the system Figure 14-7 by Mansoori (1997)describes the separation of the asphaltenes as a solid aggregatephase when more paraffinic hydrocarbons are added into the system

com-2 Steric colloidal effects At high concentrations, asphaltenes tend to

associate in the form of large particles, as depicted in Figure 14-8

by Mansoori (1997) In the presence of some peptizing agents, such

as resins, these particles can adsorb the peptizing agents and becomesuspended in the oil

Figure 14-5 Heavy organics in petroleum crude (straight/curved line =

paraffin molecules, solid ellipse = aromatic molecules, open ellipse = resinmolecules, and solid blocky forms = asphaltene molecules) (reprinted from

Journal of Petroleum Science and Engineering, Vol 17, Mansoori, G A.,

"Modeling of Asphaltene and Other Heavy Organic Depositions, pp

101-111, ©1997, with permission from Elsevier Science; after Mansoori ©1994SPE; reprinted by permission of the Society of Petroleum Engineers)

Figure 14-6 Colloidal phenomenon activated by addition of a polar miscible

solvent (solid ellipse = an aromatic hydrocarbon) (reprinted from Journal of Petroleum Science and Engineering, Vol 17, Mansoori, G A., "Modeling

of Asphaltene and Other Heavy Organic Depositions, pp 101-111, ©1997,with permission from Elsevier Science)

Aggregation effect When the concentration of the peptizing agent

is low and its adsorbed quantity is not sufficient to occupy theparticle surface completely, several particles can combine to formbigger particles as depicted in Figure 14-9 by Mansoori (1997) Thisphenomenon is called flocculation When the particles become

Figure 14-7 Flocculation and precipitation of heavy components by addition

of a non-polar miscible solvent (dashed line = a paraffin hydrocarbon)

(reprinted from Journal of Petroleum Science and Engineering, Vol 17,

Mansoori, G A., "Modeling of Asphaltene and Other Heavy Organic tions, pp 101-111, ©1997, with permission from Elsevier Science; afterMansouri ©1994 SPE; reprinted by permission of the Society of PetroleumEngineers)

Deposi-Figure 14-8 Steric colloidal phenomenon activated by addition of paraffin

hydrocarbons (reprinted from Journal of Petroleum Science and ing, Vol 17, Mansoori, G A., "Modeling of Asphaltene and Other Heavy

Engineer-Organic Depositions, pp 101-111, ©1997, with permission from ElsevierScience; after Mansouri ©1994 SPE; reprinted by permission of the Society

of Petroleum Engineers)

sufficiently large and heavy, they tend to deposit out of the solution

as depicted in Figure 14-10 by Mansoori (1997)

4 Electrokinetic effect As explained by Mansoori (1997), during the

flow of oil through porous media and pipes, a "streaming current"and a potential difference are generated because of the migration

Figure 14-9 Migration of peptizing molecules (solid arrows) by change of

chemical potential balance (reprinted from Journal of Petroleum Science and Engineering, Vol 17, Mansoori, G A., "Modeling of Asphaltene and Other

Heavy Organic Depositions, pp 101-111, ©1997, with permission fromElsevier Science; after Mansouri ©1994 SPE; reprinted by permission of theSociety of Petroleum Engineers)

Figure 14-10 Flocculation and deposition (big arrow) of large heavy organic

particles (reprinted from Journal of Petroleum Science and Engineering, Vol.

17, Mansoori, G A., "Modeling of Asphaltene and Other Heavy OrganicDepositions, pp 101-111, ©1997, with permission from Elsevier Science; afterMansouri ©1994 SPE; reprinted by permission of the Society of PetroleumEngineers)

of the charged particles of the asphaltene colloids The asphalteneparticles are positively charged but the oil phase is negativelycharged, as depicted in Figure 14-11 by Mansoori (1997) Therefore,the negative upstream and positive downstream potentials are gen-erated along the pipe to resist the flow of the colloidal particles, as

Flowingcrude oil

Chargedheavy organi

particles

Conduit

Figure 14-11 Streaming potential generated by oil flow in a pipe (reprinted

from Journal of Petroleum Science and Engineering, Vol 17, Mansoori,

G A., "Modeling of Asphaltene and Other Heavy Organic Depositions, pp.101-111, ©1997, with permission from Elsevier Science; after Mansouri

©1994 SPE; reprinted by permission of the Society of Petroleum Engineers)

depicted in Figure 14-12 by Mansoori (1997) This, in turn, induces

a back diffusion of the colloidal asphaltene particles Mansoori(1997) points out that the deposition of the polar, asphaltene by theelectrokinetic effect and the non-polar paraffins by the dynamic pourpoint crystallization effect, could occur simultaneously when the oilcontains both asphaltenes and paraffins

Asphaltene and Wax Phase Behavior

and Deposition Envelopes

In this section, a brief summary of the review of the asphaltene andwax phase behavior by Leontaritis (1996) is presented

Figure 14-12 Electrokinetic deposition in a pipeline (reprinted from Journal

of Petroleum Science and Engineering, Vol 17, Mansoori, G A., "Modeling

of Asphaltene and Other Heavy Organic Depositions, pp 101-111, ©1997,with permission from Elsevier Science)

Accurate measurement of the asphaltene and wax phase behavior isexpensive and requires sophisticated techniques for proper handling ofthe reservoir fluid samples and laboratory testing of the recombinedreservoir fluids Therefore, Leontaritis (1996) suggests that phase diagramscan be more economically and rapidly developed by simulation with alimited number of actual data required for tuning and calibration Leontaritis(1996) demonstrated this exercise by applying the Thermodynamic-Colloidal model by Leontaritis (1993) Nghiem and Coombe (1997) state:

"Above the saturation pressure, the precipitation is solely due to pressure,while below the saturation both pressure and composition affect theprecipitation behavior." However, more research is needed in this area.Leontaritis (1996) points out that wax crystallization and asphalteneflocculation phenomena occur at low and high temperatures, respectively.Then, he hypothesizes that these two phenomena should, therefore,represent the two extreme cases of the phase behavior and there should

be continuously varying intermediate phase behavior in between these twoextremes depending on the composition of the heavy fractions of thecrude oils, as schematically shown in Figure 14-13 by Leontaritis (1996).Although it sounds reasonable and there is some evidence of support for thishypothesis, more research is obviously needed to verify this hypothesis.The schematic Figures 14-14 and 14-15 by Leontaritis (1996)depict the features of typical asphaltene and wax deposition envelopes,respectively As explained by Leontaritis (1996), the phase diagrams ofthe asphaltenic fluids typically do not have a critical point, because theasphaltenic fluids can only have bubble-point lines and no dew-pointlines, as they cannot vaporize Leontaritis (1996, 1998) refers to thelocus of the thermodynamic conditions for asphaltene flocculation as the