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AN ALTERNATIVE APPROACH TO THE SELECTION OF

PSEUDOCOMPONENTS FOR MODELLING IN-SITU

COMBUSTION

K. HUTCHENCE N. FREITAG

this article begins on the next pageFF

SS92-21 PETROLEUM SOCIETY OF CIM and CANMET PAPER NO 21 THIS IS A PREPRINT - SUBJECT TO CORRECTION AN ALTERNATIVE APPROACH TO THE SELECTION OF PSEUDOCOMPONENTS FOR MODELLING IN-SITU COMBUSTION K. Hutchence N. Freitag Saskatchewan Research Council PUBLICATION RIGHTS RESERVED THIS PAPER ISTOBEPRESEN TEDATTHE FOURTH PETROLEUM CONFERENCE OFTHE SOUTH SASKATCHEWAN SECTION, THE PETROLEUM SOCIETY OF CIM, HELD WITH CANMET IN REGINA OCTOBER 7-9, 1991. DISCUSSION OF THIS PAPER IS INVITED. SUCH DISCUSSION MAY BE PRESENTED AT THE TECHNICAL MEETING AND WILL BE CONSIDERED FOR PUBLICATION IN CIM JOURNALS

IF FILED IN WRITING WITH THE TECHNICAL PROGRAM CHAIRMAN PRIOR TO THE CONCLUSION OF THE MEETING. ABSTRACT An bwrtant step in the numerical simulation of in-situ combustion for enhanced oil recovery is the selection of the pseudocomponents used to represent the oil Frequently the oil is separated according to distillation cuts. In adwtion, a hydrocarbon pseudocompone nt called "coke" is used to present the heavy residue that results from the reactions immediately ahead of a combustion zone. While this approach provides for sonar of the general changes that occur during in-situ combustion,it provides a poor basis for representing the overall set of chemical reactions. Chemical principles and the results of several publications from diverse sources suggest that a modified SARA analytical approach presents a sound basis for representing in-situ combustion chemistry. The poiential for such an approach is discussed and a set of pseudoco"onents is proposed that could plausibly represent all the nwjor physical and che@cal changes that oil experiences in the in-situ combus@n process. F@ly, the steps that are foreseen to success ully 4Wlement such an approach are outlined.INTRODUCTION In-situ combustion may be a good option for enhanced oil recovery of many reservoirs. UnfOn~tely, its use is diminished by the lack of reliable methods for predicting performance. Usefully accurate p@ction of performance almost ce~y req~ numerical simulation. The complicated interactions that control firefloods vary with oil and reservoir types, and can not normally be repm~ted in simpler forms. Many of these interactions depend on the chemical reactions that dominate in-situ combustion. Consequently, the selection of a suitable set of

pseudocomponents and reactions is vital to the establ~ent of an accurate numerical model for firefloods. In the h for a suitable basis to express the chemical mwtions of fimfiooding, a thought-provokin g publication by Ciajolo and Barbella' was found. Their thermal gravimetric analysis data suggested that SARA (saturates, aromatics, resins and asphaltenes) analysis offered this sought-after basis for pseudocomponent selection. Various data in another publication by BaJ provided a separate indication that the oxidation and cracking behaviour of a wide variety of oils

corresponded to their SARA analysis.

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PETROLEUM SOCIE1Y OF CIM and CANMET

THI S I SA PREPR IN T - SU BJECT TO C OR RECTIO N

PAPER NO 21

AN ALTERNATIVE APPROACH TO THE SELECTION OFPSEUDOCOMPONENTS FOR MODELLING IN-SITUCOMBUSTIONK. Hutchence

N. FreitagSaskatchewan Research Council

PUBLICATION RIGHTS RESERVEDTHIS PAPER IS TO BE PRESENTEDAT THE FOURTH PETROLEUM CONFERENCEOFTHESOUTH SASKATCHEWAN SECTION,THE PETROLEUM SOCIE1Y OF CIM, HELD WITH CANMET IN REGINA OCTOBER 79, 1991. DISCUSSION OF THIS PAPER ISINVITED. SUCH DISCUSSION MAY BE PRESENTED AT THE TECHNICAL MEETING AND WILL BE CONSIDERED FORPUBLICATION IN CIM JOURNALS IF FILED IN WRITING WITH THE TEC HN IC AL PROGRAM CH AI RM AN PRIOR TO THECONCLUSION OF THE MEETING.

ABSTRACTAn important step in the nwnerical simulalion of in-situ

combustion for enhanced oil recovery is the selection of thepseudocomponents used to represent the oil. Frequently theoil is separated according 10 distillation cuts. In addition,a hYdrocarbon pseudocomponent called "coke" is used topresent the heavy residue that results from the reactionsimmediately ahead of a combustion zone. While thisapproach provides for some of the general changes thatoccur during in-situ combustion. it provides a poor basisforrepresenting the overall set of chemical reactions.

Chemical principles and the results of severalpublications from diverse sources suggest that a modifiedSARA analytical approach presents a sound basis forrepresenting in-situ COmbUStiOIl chemistry. The potentialforsuch all approach is discussed and a set ofpseudocompollents is proposed that could plausiblyrepresent all the major physical and chemical changes thatoil experiences in the in-situ COmbUStioll process. Finally.the steps that are foreseen 10 successfully implement suchall approach are outlined.

INTRODUCTIONIn-situ combustion may be a good option for enhance

recovery of many reservoirs. Unfortunately, its usdiminished by the lack of reliable methods for prediperformance. Usefully accurate prediction of performalmost certainly reqnires numerical simulation.complicated interactions that control frrefloods vary witand reservoir types. and can not normally be representesimpler forms. Many of these interactions depend onchemical reactions that dominate in-situ combusConsequently, the selection of a suitable setpseudocomponents and reactions is vital to the establishmean accurate numerical model for frrefloods.

In the search for a suitable basis to express the chemreactions of frreflooding, a thought-provoking publicatioCiajolo and Barbella' was found. Their thermal gravimanalysis data suggested that SARA (saturates, aromatics, rand asphaltenes) analysis offered this sought-after basipseudocomponent selection. Various data in anopublication by Bad' provided a separate indication thaoxidation and cracking behaviour of a wide variety ofcorresponded to their SARA analysis.

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In this paper, the SARA-based approach is used to builda plausible and comprehensive reaction scheme thatdescribes the chemistry of in-situ combustion;

PSEUDOCOMPONENT SELECTIONDistillation CutsThe conventionalapproach to pseudocomponentselection

has been to separate oil into boiling-point r a n g e s . ' , 4 ~ 6 Thisallows reasonable variations in the physical properties of theoil, and can effectively portray distillation effects i f at leastthree distillation cuts are used.' In addition, gas and a uonvolatile pyrolysis (thermal cracking) product called "coke"are added to the pseudocomponent list. Coke and gas areusually formed by pyrolysis of one or more of the higherboiling cuts.

This approach implicitly assumes that all the material ina selected boiling point range will participate equally inchemical reactions. It also assumes that the high-boiling ornon-volatile products of both pyrolysis and low-temperatureoxidation reactions are the same. As pointed out below, thisis not the case.

SARA FractionsSARA analysis separates oil into fractions according totheir solubility in solvents of differing polarity and theiraffinity for absorption on a solid granular packing (natural

clays, silica gel and alumina have all been used).Asphaltenes are separated first by collecting the precipitateformed by adding a specified quantity of a paraffmicsolvent, usually either n-pentane or n-heptane.The portion that remains dissolved, the maltenes, isfurther separated by elution through the solid packing by

solvents with increasing polarity. Ciajolo and Barbella, forexample, precipitated asphaltenes with n-heptane and thenseparated the maltenes on a silica gel column. The saturates(which they called "paraffms") were eluted with hexane, thearomatics were eluted with 1% diethylether in hexane, andthe resins (which they called "polars") were eluted withacetone and methanol in equal volumes.

A simple examination of available tables shows that thedipole moments of different functional groups show littlevariation with chemical environment Consequently, anyseparation according to polarity will lump together thechemical species with the same functional groups. It is wellknown that the ease of oxidation varies with the functionalgroup. The individual SARA fractions can therefore beexpected to show distinct behaviour with respect tooxidation.

While more factors are involved in thermal cracking,functional groups can influence these reactions as well.Thus it is logical that the different SARA fractions should

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behave differently under heating.As summarized above, chemical principles indicate that aSARA-based reaction modal could combine combustion, lowtemperature oxidation and thermal cracking, -- the three types

of reactions that dominate in-situ combustion -- into a unifiedmodel. As shown in the following =tion, there is alreadysubstantial evidence in the literature to support this claim.E vi de nc e f or a S ARA BasisCiajolo and Barbella examined the behaviour of fourdifferent fuel oils by studying the weight loss of each fractionduring thermogravimetric analysis (TGA). The weight losseswere measured under both nitrogen and air as the temperature

was raised at a constant rate.Except for uniformly low asphaltenes contents, the four oilsdiffered substantially in their SARA analysis and viscosity. Inspite of this, the results showed a strikingly consistent pauem.While each fraction showed a distinctive behaviour, the resultsfor each fraction were similar regardless of its source. Forexample, polars produced 4 to 5 wt % coke under nitrogen,and 23 to 25 wt. % under air. The aromatics and asphalteneswere much less affected by whether nitrogen or air was present,but the aromatics produced no coke under nitrogen, while theasphaltenes produced 51 to 60 wt % coke. Evidently, thecontribution from the maltenes to coke production arose mainlyfrom the polars.The thermograrns revealed a distinctive oxidation behaviourfor each fraction. Paraffins burned in the range of 300 to

350C, about 50C lower than aromatics. Polars began tooxidize above 400C, but the extent to which they burnedrather than pyrolyzed was not evident. The asphaltenesappeared not to burn at all. Instead, they probably pyrolyzedat about 500C to form coke, which burned at a highertemperature.

The ouly feature that varied substantially with the source oilwas the distillation properties. The aromatics distilled as twooverlapping sub-fractions, that were present in different relativeamounts. The paraffms distilled as either one or two fractions,with boiling ranges that overlapped strongly with the aromatics.Polars were generally much less volatile, and their distillationblended into the pyrolysis and oxidation temperature rangesnear and above 400C.

Other sources were examined to determine whether thisconsistent behaviour might be co-incidental. Corroborationwas found in the earlier data presented by Bae. Bae observedthat the TGA behaviour of widely differing oils fell into threegroups. Although Bae provided SARA analyses for these oils,a comparison of these analyses with the TGA behaviour showsthat, with one exception, the three groups each corresponded toa different group of oils with similar SARA composition.

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