I.

SPE 6173

Characterization of Crude Oil for Fireflooding UsingThermal Analysis Methods

J. H. BAE

I

GULF RESEARCH & DEVELOPMENT CO.PITTSBUR12H

ABSTF.ACT

To study the tbermo-oxidative behavior o/ crudeoils, di//ererztial tbermui analysis and tberrnogravi-rnetric instruments were developed that could be usedat 1,000 ‘F and 1,000 psig in a flowing atmosphere.Subsequently, 15 crude oils, ranging from 6 to 38‘API gravity, were used at pressures of 50, 500,and 1,000 psig. Both nitrogen and air atmosphereswere used in the experiments.

The results show that crude oils can be groupedinto three types according to their tbermo-oxidativecharacteristics, The gravity of the crude oils doesnot correlate well with these patterns. lt is alsoshown that the dependence oi /uel availability o>:temperature and pressure varies with different crud(;oils. Furthermore, crude oils generally gain weightin an air atmosphere in relation to the evaporationcurve obtained in a nitrogen atmosphere at bothlow and high temperatures. This shows that theavailability of oxygen at !OW temperatures changesdrastically the quality and quantity o/ availablefuel, The beat generated by Iow-temperanceoxidation might be significant in iireflooding.Finally, a qualitative correlation of the results ofthermal analysis with those o! combustion-tubetests is indicated,

INTRODUCTION

A substantial investigative effort has been madeover the years, both in the laboratory and in thefield, to understand the mechanisms of fireflooding,and a general understanding of the procesci riowexists. However, the many factors that affect theprocess and the interrelstionships of these factorsmake the process a complicated one. This alsomakes it difficult to predict the behavior of combustionby simple means. The linear Iabotatory combustion-tube test appears to be fairly standard in theindustry. Even in this type of experimentalapproach, translation of the linear tube-test resultsto the field is not always pssible.

Two of the most important factors in the

Original manuscript received in Society of Petroleum Engineersoffice Aug. 9, 1976. Paper accepted for publication Maroh 10,1977. Revised manuscript received March 2S, 1977. Paper(SPE 6173) was first presented at the SPE-AIME 51 $t AnnualFall ‘i’echnicai Conference and Exhibition, heId in New Oriearm,Oct. 3-6, 1976. ~Copyrlght 1977 American Institute of Mining,Metaliurgicai, an Petroleum Engineers, Inc.

JUNE, IW

combustion process are fuel deposition and oxida-tion. Unfortunately, these presently are also thefactors about which the least is known. Fuel forthe process is usually thought to be the heavyfraction of crude oil held in the pores afrer the fluiddisplacement. The rate of advance and the peaktemperature of the combustion front depend on theamount of fuel, availability of oxygen, and the rateof fuel oxidation. In fact, fuel deposition andoxidation govern the ability to sustain forwardcombustion and strongly influence the economics ofa combustion project.

Attempts have been made to use the thermalanalysis methods in connection with forwardcombustion. In particular, differential thermalanalysis (DTA) was used to study the oxidationof crude oil in porous media.llz DTA is a techniquewherein energy changes in a substance are detectedand measured as a function of time or temperature+3In practice, the temperature of the sample iscompared continuously with a reference materialtemperature. The difference in temperature isrecorded. Another thermal analysis method isthtxrnogravimetricanalysis t,TGA). In this technique,a sample is weighed continuously as it is heated ata constant rate. The resulting curve of weightchange vs time or temperature gives the TGAthermogram.

The objective of this work was to study thethermo-oxidative behavior of crude oils using bothDTA and TGA techniques to gain some insight intothe combustion process, especially the iheldeposition and oxidation. At the same time, wehoped to obtain information useful for predictingthe thermal behavior of crude oil in the combustionprocess. Toward this goal, DTA and TGAequipment was developed that could be used at~,000 ‘F and 1,000 psig in a flowing atmosphere.Fifteen crude oils in a wide gravity range (6 to 38‘API) were analyzed, and the resuIts are reportedhere.

EXPERIMENTAL EQIJIPMENT

For our purposes, it was necessary in the DTAblock to have a porous matrix to hold the oil andprovisions for flowing gas thtough the sample atpressures up to 1,000 psi. The DTA block used isshown schematically in Fig, 1. The body had thedimensions of 1-1/2 x 1-1/2 x 2-3/4 in. and had

211

two cylindrical 3/8-in. -diameter.in, apart in the body. For

cells dri:led 1-1/4sealing purposes,. .

converted 3/8-in. tubing fittings were welded onthe top of the block. The PTA block was made ofNo. 304 stainless steel. Chromel-Alumel :hermo-coupies, butt-welded of 28-8auge wire, were placedin the center of the cells. These enter the cellsfrom below through 2- and 4-wire thermocouplegl:. rids, Two thermocouples were set in the referencecel and one in the sample cell. These measure thetemperature of the cell and the temperaturedifference between the sample and reference cells.

Entering gas is preheated by flowing aiound theblock through holes drilled just inside the perimeter.The gas first passes through the reference cell (onthe right in the diagram) and then throug~ thesample cell. Stainless steel screens are placedbefore and after each cell. The cells are filledwith 60 mesh Norton Alundum, 3/8 in. deep, and ontop of this is a close-fittipg plug to make the cell acylinder of equal height and diameter.

The furnace is made of two concentric cylinders.The inner cylinder is insulated with mica on whichresistance wire is wound and is held in place withAlundum cement. The annulus between the innerand outer cylinder is filled with vermiculite tominimize heat loss. The furnace and the block aremounted on a tran site base as shown in Fig. 2.

The DTA system is shown schematically in Fig.3. Air from the cylinder passes through a flowmeterinto the DTA block and then goes to the flowregulator. The rate is measured accurately througha soap bubble flowmeter and then is vented,

The TGA equipment is described in, Ref. 4. Thewhole system is essentially the same as shown inFig. 3 except that the DTA block is replaced by aTGA vessel. Also, there is a provision to evacuatethe TGA system for use in a nitrogen atmosphere.

EXPERIMENTAL PROCEDURES

A small drop of crude oil to be tested wasplaced in the center of the DTA sample cell. Theamount of sample was determined by weighing thedropper or syringe before and after the sampleinjection. The sample size was about 20 mg, but itcould not be controlled precisely. Time was givenfor the oil co soak into the Alundum before the plugwas inserted and the cell was sealed. The furnace

I!!!!!L

FIG. 1 — DETAIL OF DTA BLOCK.

212

was placed in position. Compressed zero-grade airfrom a commercial cylinder was introduced at adesired pressure, and the flow rate was regulatedand measured as described above. The flow ratewas 10 ml/min measured at room temperature andpressure. This roughly corresponds to 20 scf/sqft-hr in rhis system.

The temperature of the block was increasedlinearly at a rate of 6 ‘F/rein up to 1,000 ‘F. Thistemperature was maintained for a while to ensurethat all the carbon was burned off. In reality, thelast peak in the thermogram represented the burningof coke, and the sand was always clean when thecell was opened. The temperature of the referencecell was recorded. The differential temperature ofthe reference and sample cells was fed into adirect-current microvolt amplifier. The output fromthis unit was then recorded. When the experimentwas over, the pressure in the system was reducedgradually and the cell was allowed to cool to roomtemperature.

In the TGA experiment, a sample of about 20-mgcrude oil was used in both air and nitrogenatmospheres. The flow rate of gas was 10 ml/minmeasured at room conditions. In the case ofnitrogen runs, the TGA system was evacuatedbefore the introduction of the nitrogen. ‘The samplewas heated at 6 %F/min to 1,000 OF. With air,nothing remained on the sample holder at the endof the run. However, with nitrogen, the residue

—==

m— -1

FIG. 2- DTA BLOCK AND FURNACE ASSEMBLY.

PRESSUREQAUQE

**RE”mR

u L-J p$fQAS

CYLINDER CONTROLLER VENTANO RECORDER

FIG, 3 — SCHEMATIC DIAGRAM OF DTA SYSTEM,

SOCIETYOF PETROLEUM ENGINEERS JOURNAL

Crude Location of01I Origin

A MieslsslpplB Venezuelac CfmadaD CaliforniaE M;jslssipPl

Arkansas: TexasH TexasI TexasJ WyomingK TexasL AfricaM TexasN Loulslanao Arkansas

TABLE 1 — CRUDE OIL PROPERTIES

viscosity “ Composition (weight percent)

~r~;; at 77 “F Hexane Polaro (Cp) Saturates Aromatics Insoluble Compound

6.3 70,000 2S.0 44.8 22.9 6.313.3 10,700 30.2 42.4 16.3 9.113.9 1,s00 16.6 49.2 25.9 8.316.0 741 39.0 40.3 1.7 11,016.5 513 31.3 49.3 13.6 5.8

19.5 166 59. ~ 31.7 ‘ 2.6 6.220.4 75 60.1 33.0 0.9 6.023.6 60 65,6 27.3 0.4 6.723.P 170 56.5 36.7 0.5 6.324.1 43 22.0 39.9 26.6 11.3

24.2 0.3 59.0 39.4 0.1 1.527.6 54 5s.1 29.6 1.2 10.934.8 6.0 56.9 31.9 0.6 5.935.7 9.5 78.1 17.1 0.5 4.337.8 5.3 70.8 23.9 0,s 4.5

remaining was burned off and the weight wasmeasured.

On the DTA thermograms, rhe base line wasdrawn as a straight line connecting the extremeends of the curve since several runs showed thatthe base-line drift was very gradual and smoothover the temperature range of the experiment.Since the sample size affects only the magnitudeor height of the DTA curve proportionally,s theDTA thermograms shown are based on a 20-mgsample.

In high-pressure TGA operation, the apparentweight change was corrected for buoyancy effects,based on the results of blank runs. The thermogramsare shown here as a weight percentage of the original.

RESULTS AND DISCUSSION

Fifteen crude oils ranging in gravity from 6 to 38“API were used at pressures of 50, 500, and 1,000psig. Air atmospheres were used in DTA experiments,and both air and nitrogen atmospheres were usedin TGA experiments. The results reported here arebased on more than 200 runs. The properties ofcrude oils are listed in Table 1.

GENERAL CHARACTERISTICS OF THERMOGRAMS

Each crude oil gave a characteristic thermogramthat was quite distinctive from the others. Fig. 4shows DTA curves for three crude oils at 1,000psig. The base line for each curve has beenshifted vertically for purposes of visualization. Itshould be recognized that the DTA curve is verydependent on the physical characteristics of theexperimental apparatus and the procedure used, aswell as sample parameters. Thus, DTA curves arenot reproducible from one apparatus to another.Since we kept the variables constant, the differencein DTA curves in Fig. 4 should reflect the differencein thermal behavior of the crude oils.

In general, the first peak started just below 400OF, signaling the onset of low-temperature oxidation.The second peak appeared around 700 OF, and bythe time the temperature reached 900 ?F, *ereaction was completed, except for Crude Oils Aand B. The DTA peaks were not of the shape tha-

JUNE.1P77

one would expect with a single component. In allcases, the two peaks were not separated completely,indicating a continuous heat generation in thistemperature range. As shown in Fig. 4, some crudeoil showed high heat generation at low temperatures,some at high temperatures, and some at both lowand high temperatures.

Figs. 5 through 7 show the TGA thermogramsobtained at 1,000 psig in both air and nitrogenatmospheres. As expected, the thermograms arevery di fieient. The nitrogen curves represent thedistillation and cracking characteristics of the oil.The major differences appear in the oxidationcurves. There were three distinctive behaviors asshown in the figures, designated Types I throughIII. Each crude oil used in this work could beclassified as one of these types.

The distinguishing feature of the crude oils ofType I is that there is a large relative weight gainstarting at about 400 OFin relation to the evaporationcurve obtained with a nitrogen atmosphere. Also,there is a sudden and very rapid oxidatio~ at about65o ‘F that consumes all the hydrocarbon remaining.In Type H, crude oil loses weight almost immediately,and there is no apparent weight gain in relation tothe nitrogen curve. All the hydrocarbons are goneat about “400 OF with a very rapid oxidation. On theother hand, the oxidation curve for Type III showsa rather gradual change, and this curve closely

~400s00s00

TEMPERATURE,“F

FIG. 4 — DTA THERMOGRAiIi:s AT 1,000 PSIG.

213

TABLE 2 — EXPERIMENTAL RESULTS AT 1,000 PSIG

Combustion-Tube Test*TGA* Flllal

Type IABcE

Ty: II

J

:N

Type IIIDGHI

:

. -., .DTA* I’42 ppue ~nsumptton

~$$’1~ ~1 (“F) 72 rF) To ~~ (perce%)(lb psr 100 lb

of rock) TP ~ F)—— .

6.3 520 8KI 654 36 1.75 1,24513.3 560 860 650 38 1.64 97713.9 540 720 650 32 1.29 90316.5 640 750 600 32 1.30 773

19.5 550 800 425 6524.1 640 750 405 6227.6 4~ 750 415 6334.6 * 630 760 415 4535.7 430 730 390 52

16.0 520 750 640 2020.4 410 740 660 1123.4 420 750 700 1123.0 470 770 700 1124.2 416 740 660 537.8 460 780 680 13

0.980,880.900.700.76

1.231.020.921.090.480.62

928S787*

& Ylo)565

94575s746603

& 500)500

* TI Is the temperature at the maxlmurn of the f}ret DTA peak.T2 Is the temperature at the maximum of the second DTA peak.T Is the temperature at which there Is no residue remalnlng In an alr atmosphere,~ Ie the combustion peak ta$’nperature.

parallels the nitrogen curve. However, one canobserve a low- and high-temperature oxidationwhere the crude oil gai..s “weight in relation to thenitro~en curve. All the crude oil is oxidized atabout 700 W. In this group, the oxidation e~rvesometimes crosses the nitrogen curve.

The very rapid oxidations that occurred in someTGA experiments were not noticeable in the DTAruns. In the TGA experiment, the small amount ofoil sample almost coats the sample pan and theoxygen partial pressure should remain constantbecause of the large volume of the TGA vessel. Itmay be possible that a rapid temperature rise,localized on the sample pan, caused this rapidoxidatio.1. In fact, we noticed a small spike on thetemperature recorder from the thermocouple justbelow the sample pan. This may not be the case inthe DTA apparatus, resulting in slower oxidation ofhydrocarbon or residue. In any case, the TGA runclearly shows the thermo-oxidative character stiesof crude oils.

In Table 2, the crude oils used are groupedaccording to their oxidative behavior in TGAexperiments, along with some results of DTA and

AIR

o , r , , ,

-IO0600W0400600600 mm~l~TEMPERATURE , ‘F

FIG. 5 — TYPICAL TGA THERMOGRAMS FOR TYPE ICRUDE.

214

TGA runs. One observation from this table is thatthe API gravity cannot be used as an all-descriptivemeasure of the thermal behavior of crude oil. Also,as shown in Table 1, there is no completecorrelation between viscosity or composition ofcrude and the thermo-oxidative characteristics ofcrude oils. However, it is generally true that thelow-gravity, high-viscosity crudes tend to fall inType I.

100

*SO

e“ h“”%CRUDE L

z 10000$1$

~ 60z~

; 40 TYPE Z

g

* 20 N!TRO13EN

AIR

01 IIoorwmwowoeoo 7m 600600

TEMPERATURE, ●F

FIG. 6 — TYPICAL TGA THERMOGRAMSFOR TYPE 11CRUDE.

\\1000psig

0 ‘y’:”,,\y>100 2006004m Wo 600700 6006001

TEMPERATURE, ●F

o

FIG. 7 — TYPICAL TGA THERMOGRAMS FOR TYPE IIICRUDE.

SOCIETYOF PETROLEUM ENGINEERSJOURNAL

AVAILABLE FUEL

Firefi.~oding is generally recognized as a fuel-dominate~ process. Much effort has been devotedto finding ways to predict the amount of fuelavailable in the combustion process. The c~mplexinteractions between reservoir rock and oil makethis almost impossible.

Downstream of the combustion front, crude oilwill go through & stillation and thermal cracking.The TGA thermograms obtained in a nitrogenatmosphere represent these processes, even thoughin an actual process crude oil properties mighthave changed because of solvent displacementahead of the combustion front. To demonstrate thedifference in distillation characteristics, some ofthe TGA test results are shown in Figs. 8 and 9.Notice that in Fig. 8, all four crude oils havevirtually the same 24-OAPI gravity. In these figures,whatever is remaining at a particular temperaturewould be availabie as fitel. The difference inweight remaining at a particular temperature fordifferent crude oils is shown rather strikingly. InFig. 9, the crude oils have widely differinggravities, but at high temperatures the weightremaining varies less than that shown in Fig. 8.

From these figures, it could be concluded thatthe amount of fuel available will change with thecombustion peak temperature. The degree oftemperature dependency is characteristic of thecrude oil. For most crude oils, the weight changeabove 700 ‘F is much smaller than that in the low-temperature range and is almost linear. Furthermore,the weight change most probably will accompany achange in fuel composition.

This implies that in a laboratory study where

100

* @o- ~ PDIS NITROGEN

g

z60

~ ~.

=g

CRUOE K~ 20

0no2003004wo Em6W?09 Wo 9001000

TEWERATURE, ‘P

FIG. 8 — TGA THE~MOGRAMS,

1000 PAS MITROOEN

CRUOE

coking and combustion runs are conductedseparately, the coking temperature should be thesame as the combustion temperature to be realistic.If the coking temperature is lower than thecombustion temperature, both a weight and acomposition change will occur upon heating frorrthe coking temperature to the combustion tempera-ture. This has been repotted in Ref. 6.

In the combustion process, fuel availability isnot necessarily the same as fuel consumed.Consequently, one interesting question is at what

temperature would all the fuel available be utilizedcompletely. This type of information can be obtainedeasily with DTA runs; an example is shown inFig. 10. The procedure was simply to start theDTA experiment as usual. Then, when the samplewas heated to a desired temperature, the temperaturewas held constant for about 2 hours. The uc marksin Fig. 10 indicate the constant-temperature period.The heating was then resumed at the usual rate.In this figure, the curves for 500 and 600 ‘T showthat an apparent slow oxidation is taking placebetween the tic marks, as indicated by continuousheat generation. We do not know the reason for thedip on these curves. However, at 700 ‘F, all thefuel is completely oxidized for this crude oil. Froma viewpoint of fuel utilization, th combustion-front temperature does not have to exceed thistemperature for a complete consumption of availablefieL Of course, this temperature wilI vary withthe crude oil used.

If oxygen is available downstream of. thecombustion front, then low-temperature oxidationwould take place. The effect of this on fuelavailability is shown in Figs. 5 through 7 in curvesobtained in an air atmosphere. For example, inFig. 5, the low-temperature oxidction significantlychanges the amount of iiael available in thetemperature range of 400 to 600 ~. Moreover, thereaction at 650 OF may suggest changes in fuelcomposition and/or the oxidation characteristicsof the fue~.

OXIDATION OF FUEL

As mentioned previously, oxidation of fueI infireflooding is one of the most important, and yetleast understood, factors. In general, the oxidationof fueI is divided into Iow- and high-temperature

0

CRU9E J

1000 Pdg AIR

Oo.v$: ,%

00 “f

L

tlMETSWERATURE, W

FIG. 9 — TGA THERMOGRAM.

J~E, 1~

oxidation. The temperature that distinguishes theseis arbitrary and ranges from 500 to 650 ‘F in theliterature. This temperature range corresponds tothe first DTA peak temperature or the temperaturein between the two DTA peaks, depending on thecrude used.

Low-temperature oxidation is important inreverse combustion or in fire-water flooding wherethe peak temperature approximates the steam-plateautemperature. It is also recognized that even in drycombustion, some oxygen is available in advanceof the combustion front. Thus, there are severalstudies in the literature on low-temperatureoxidation. However, the experimental conditionswere different. In one experimental app:~ach, cmdeoil was subjected co oxygen continuously as thedesired temperature was reached.6~ 7 In anotherapproach, crude oil was subjected to distillationor coking in an inert atmosphere before oxidation.8-l 1The DTA and TGA tests belong to the formercategory.

The onset of low-temperature oxidation wassignaled by the start of the first DTA peak. Asshown in Fig. 4, the amount of heat generated bythe low-temperature oxidation depends on the crudeoil. This is also the case with the ratio of the heatgenerated at high and low oxidation temperatures.From Figs. 5 through 7, one can see that crude oilsof Type 11 are more reactive at low temperaturesthan orhers, and those of Type I are more reactiveat high temperatures.

It generally has been believed that the heatgenerated by low-temperature oxidation is inconse-quential compared with that of high-temperatureoxidation. The DTA curves shown here indicatethat this may not be the case. In fact, most of thecrude oils showed a significantly greater heatgeneration at the low oxidation temperature at1,000 psig, Speaking qualitatively, crude oils ofType II might be good candidates for fire-waterflooding since they are highly reactive at lowtemperatures and enough heat could be generatedto sustain a low-temperature combustion front.

It has been concluded that under the sameconditions, high-API-gravity crudes are moresusceptible to low-temperature oxidation becauseof their relatively high hydrogen content. However,a close look at Table 2 shows that this conclusion

moroosooaoooo SwmomowotoooTEMPERATURE , ●F

FIG, 11 — TYPICAL TGA THERMOGRAMS FOR TYPE ICRUDE.

216

might not be justified.The study of oxidation kinetics of fue

simple. In’ the literature, the kinetiinvestigated on the basis of oxygen consumption asmeasured by the disappearance in the effluent gas.The difficulty with this approach is that there isalways excess oxygen present, which might not bethe case in the actual field process. Furthermore,in most cases, ~e coking temperature was not thesame as the oxidation temperature. Since thequantity and quality change with temperature, theuse of an Arrhenius-type equation may not bejustified. This holds even more for low-temperatureoxidation. In fact, it appears that too much emphasishas been given to the reaction order or activationenergy than is warranted. In any case, the oxidationkinetics also can be studied using the DTAequipment used here if we accept its approximatenature. 5

EFFECT Oi” PRESSURE

The discussion has been confined to the testresults obtained at 1,000 psig. However, pressurehas a significant effect on the fuel availability andthe oxidation characteristics. Figs. 11 through 16show the TGA thermograms at 500 and 50 psig forthree crude oils. Though there is a small change inthe nitrogen curve with pressure, the most importantchange occurs in the oxidation curve. This isexpected since the total pressure change alsomeans an oxygen partial-pressure change. It isalso expectkd that greater alteration of thethermograms would be observed as pressure changesfrom 500 to 50 psig than from 1,000 to 500 psig.

o~ I IIw 200 300 400 900 600 700 800 900 ICKW

TEMPERATuRE, ●F

FIG. 12 — ~YPICAL TGA THERMOGRAMS FOR TYPE ICRUDE,

soCRUDE L

500 poleso -

40 -

Zo -

AIR

0, ,

100 roo 3o04000WEo0mowo mlTEMFSRATURE, “F

FIG. 13—TYPICAL TGA THERMOGRAMS FOR TYPE 11CRUDE.

socmTY oF PETROUWM ENGINEERS J021RNAL

This was the case. The effect of pressure issomewhat different for each type.

For the crude oils of Type I, as shown in Figs.5, 11, and 12, the oxidation behavior at 500 psig isessentially the same as at 1,000 psig. However, at50 psig, there is no abrupt oxidation at about 600OF. Instead, the oxidation is more gradual and iscomplete at slightiy above 800 ‘F. The crude oilsof Type II behave somewhat similarly with pressureto those of Type I as shown in Figs. 6, 13, and 14.On the other hand, the crude oils of Type III showa different behavior with pressure, as shown inFigs. 7,,15, and 16. In general, the change withpressure is rather gradual. Notice, however, that at600 .OF, for example, the oxidation curve shows thelargest amount of fuel remaining at 500 psig,compared with that at 1,000 and 50 psig. ~is wasnot the case for all the crude oils of Type 111.

It is clear from these figures that the amount andquality of fuel will vary with pressure, and that thepressure effect is different for different crude oils.When there is no oxygen available downstream ofthe combustion front, the change in the amount offuel with pressure is shown by the nitrogen curves.When there is oxygen available downstream, thecurves obtained in an air atmosphere will show thechange with pressure. It is hard to generalize theeffect of pressure on the amount of fuel available.Our experience shows that the pressure effectshould be tested for individual crude oils.

Fig. 17 shows the DTA thermograms for Crude Lat three different pressures. It is interesting tonote that as pressure decreases, the amount ofheat generated at low temperatures also dec:e: *&s.

100, .-, .—

MTROSEII

01R0200W0400W0600 7m 6006WIW9

TEMPERATURE , ●F

FIG. 14—TYPICAL TGA THERMOGRAMS FOR TYPE IICRUDE.

‘“1 -%

1200 aoo4m 600600 700 eooeoolmTEMPERATURE, “F

FIG. 15 — TYPICAL TGA THERMOGRAMS FOR TYPEIII CRUDE.

JuNs, 1977

However, the amount of heat generated at hightemperatures does not decrease as much. Also, themaximum peak temperature moves to highertemperatures as the pressure decreases. This iscaused by slow oxidation at low pressures and canbe explained from DTA theory. S

THERMAL ANALYSIS AS A SC, :EENING DEVICE

The thermal analysis methods used here providevaluable inform ation on the thermo-oxidativecharacteristics of crude oils, which in rum givesinsight into the behavior of crude oil in fireflooding.For example, most of the behavior observed byAlexander et al. G could be interpreted or predictedusing such results as reported here, at least in aqualitative manner. Furthermore, even moreinformation could be obtained by changing theexperimental conditions. As an example, the effectof coking temperature on the amount of heatgenerated could be obtained easily with DTA byincreasing the temperature in an inert atmosphereand then switching over to air. Moreover, thetemperature schedule could be changed as desired.

It is tempting to correlate the thermal behaviorof crude oil observed using thermal analysismethods and fireflooding results. In view of thecomplexities involved, this simplistic approach isuntenable. Even the results of controlled experimentssuch as combustion-tube tests cannot be predictedquantitatively from the thermal behavior of crudeoils themselves. It appears that only qualitativeinformation can be obtained.

$p - CRUOE I

i

$*

ii 20 - AIR

olooroo3W400000eoo 700 000 600 Iw

TEMPERATURE, “f

FIG, :6 — TYPICAL TGA THERMOGRAMS FOR TYPEIII CRUDE.

CRUDEL

100pv

~SO0400WOW7QQ-

TEMPERATURE,“F

FIG. 17 — EFFECT OF PRESSURE ON DTATHERMOGRAMS.

217

Table 2 lists some results of DTA and TGAexperiments as well as the peak temperatures andfuel consumption in linear combustion-tube tests,all obtained at 1,000 psig. The equipment andprocedures for rhese tests are described in Ref. 12.A careful examination of the table shows thatwithin each type, the fuel consumption agrees wellqualitatively with the residue percentage obtained

in nitrogen atmosphere at the temperature at whichall the hydrocarbons were oxidized in air atmosphere.More importantly, when one has considerableexperience in fireflooding with different crudeoils, then categorizing a crude oil using thermalanalysis methods and comparing rhe thermal behaviorin the same category can lead to a reasonableconclusion on the prospect of fireflooding withthat particular Irude.

CONCLUSIONS

Based on the data presented here, the followinggeneral observations and conclusions were drawn.

1. Differential thermal analysis and thermogravi-metric techniques are useful tools in studying theeffect of certain variables in fireflooding.

2. Thermo-oxidative characteristics of 15 crudeoils covering a wide gravi~ range showed threedistinctive patterns. The gravity of the crude oilsdoes not correlate well with these patterns.

3. The dependence of fuel availability ontemperature and pressure is different for differentcrude oils. This dependency can be determinedeasily using thermal analysis methods.

4. In an air atmosphere, Types I and 111 crudeoils gain weight in relation to the evaporationcurve obtained in a nitrogen atmosphere at both lowand high temperatures. This shows that theavailability of oxygen rat low temperatures changesdrastically tRe quality and quantity of fuel available.

5. All the fiels are completely oxidized at 700to 800 OF, depending on oxygen partial pressure.Available fuel is not necessarily consumed at alower temperature.

6. Heat generated by low-temperature oxidation”may be significant in fireflooding.

7. Qualitative prediction of combustion-tube test

behavior is possible from the results of thermalanalysis methods.

ACKNOWLEDGMENTS

The author is deeply indebted to D. W. Reed forhis contribution to the construction and operationof the apparatus used in this work, J. R. Shafferperformed part of the experimental work.

1.

2.

3.

4.

5.

6.

7.

8.

9.

10,

11.

12,

REFERENCES

Tadema, H, J,: ‘rMechaniam of Oil Production WUndergroun(f Combustion, ~‘ Proc., Fifth World Pet.Cong., New York (1959) 279.

Burger, J, G. and Sehuquet, B. C.: “ChemicalAspects of In-Situ Combustion — Heat of Combustionand Kinetics, ‘t Sot, Pet. &?/g. J. (octo 1972) 410-

422; Trans., AIME, Vol. 253.

Wendlandt, W. W.: Thermal Metboda of Analysis,Interscience Publishers, Inc., New York (1964).

Bae, J. H.: ~~A simple Thermogravimetrlc Apparatusfor Pressures up to 70 Atmospheres,’* Rev. *L inst.(1972) 983.Bae, J. H.: ~~Dete~ination of the Kinetic Param@teraFrom Differential Thermal Analysis,)) J. Therm, Anal.(1972) 261.

Alexander, J. D., Martin, W. L., and Dew, J. N,:~$Factors Affecting Fuel Availability and Composition

During In-Situ Combustion,sp 1. Pet. Tech (1962)1154-1 164; Trans., AIME, Vol. 225.

Weijdema, J.: ~~The Oxidation Kinetics of Liquid

Hydrocarbons in Porous Media With Regard to “Under-ground Combustion, ” Etd61 Kohle, Erdgas Pe/rocbem(196s) 520.

Smith, F. W. and Perkins, T. K,: “Experimental andNumerical Simulation Studies of the Wet CombustionRecovery Process,t) J. Cdn. Pet. Tech. (1973) 44.

Dabbous, M. K. and FuXton, P. F.: “Low-Temperature-Oxidation Reaction Kinetics and Effects on theIn-Situ Combustion Process,” SOc. Pet. Eng. ]. (June1974) 253-262,

Bousaid, 1, S. and Ramey, H. J., Jr.: “Oxidation ofCrude Oil in Porous Media,” Sot.Pet, Eng. J. (June1968) 137-148; Trans., AIME, Vol. 243.

Tadema, H, J. and Weijdema, J,: “Spontaneous Ignitionof Oil Sanda, ” Oil and Gaa j. (Dec. 14, 1970) 77.

Garon, A. M, and Wygal, R. J., Jr.: “A LaboratoryInvestigation of Fire-Water Flooding, ” Sot, Pet. EnsJ. (Dec. 1974) 537.544.

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