exploitation shale hydration for leachate disposed oil shale · 2013-10-26 · 82...

82 COLORADO SCHOOL OF MINES QUARTERLY

Exploitation of Shale Hydration for

Leachate Control in Disposed Oil Shale

George H. Watson

Amoco Corporation

Naperville, Illinois

David B. McWhorter

Colorado State UniversityFort Collins, Colorado

Juergen Braun

Colorado StateUniversityFort Collins, Colorado

Earl D. York

Rio Blanco Oil Shale Co.

Denver, Colorado

INTRODUCTION

Combusted oil-shale wastes undergo cement-like

reactions that enhance the retention of water and reduce

thehydraulic conductivity ofembankments formed by the

disposal of such materials. This paper describes the

measurement of thechanginghydraulicproperties and the

manner in which those changes function to minimize

water movement and consequent generation of leachate

from embankments of disposed shale.

Production of liquid petroleum from oil shale at a

commercial scale will result in large volumes of retorted

shale. At least a portion of the waste will be disposed in

embankments that will cover large areas and may reach

thicknesses of several hundred meters. Important

environmental considerations are the rates, volumes, and

chemical characteristics of any leachate that may occur

from such embankments.

The potential for leachate generation derives from the

redistribution (drainage) ofwateremplacedwith the solids

and from infiltration that is in excess ofwhat canbe return

ed to theatmospherebyevapotranspiration. It isanticipated

thatwater will be added to the solidwaste for cooling and

dust control. In addition, process waste water may be

codisposedwith the solids.Anyquantityof thesewaters in

excess of what can be retained by physical and chemical

processeseventuallywill drain from theembankment.Net

infiltration (recharge) will slowly penetrate the em

bankment and eventually become leachate, also.

The studies reported herein show that the volume of

emplaced watermat can be retained in an embankment of

combusted shale is greatly increased by cementatious

reactions that occur. In addition, these reactions can be ex

ploited to create one or more layers with small hydraulic

conductivity that act to minimize net infiltration. These

features are quantitatively evaluated for the LURGI-

combusted shale.However, theexperimental and analytical

methodologies are applicable to other combusted shales

that undergo cementatious reactions.

MATERIAL PROPERTIES

Allexperimentalmeasurementswereperformedon fine-

textured,granular solid from theLURGIprocess followed

by combustion at about 700C to remove residual carbon.

The feed oil shale was from the Green River Formation in

thePiceanceBasin inwesternColorado. Figure 1 shows the

grain-size distribution as determined byMcWhorter and

Brown (1985).Thedominantconstituentsof thecombusted

shale are Si02 (34%),CaO (24%),MgO (8%), AI2O3 (7.5%),

Fe203 (3.3%), K2O (3.0%), andNa20 (2.3%), as reported byMarcus and others (1984). McWhorter and Brown (1985)

determined the specific gravity of particles less than 0.045

mm diameter to be 2.743. Particles larger than 2 mm have

a specific gravity of 2.728.

Upon formationofa shale-waterpaste, hydrationbegins

almost immediately as evidenced by the generation of

heat. During curing under saturated conditions, the paste

solidifies and takes on the appearance of a cement block.

McWhorterandBrown (1985) andMcWhorter (1987) found

that the solidified material retained only 3.6% water byweightafterdrying in a vacuumoven at 105C for 24hours.

In addition, the quantity of retained water was found to

depend on the water content at which curing took place.

Water retained at 105C decreased as curingwater content

decreased. The maximum of 3.6% was achieved only for

samples cured at saturation or very near saturation

(McWhorter, 1987).

SATURATED HYDRAULIC

CONDUCTIVITY

The observation that variable amounts of water were

retained, depending on the water content at which the

LEACHATE CONTROL IN DISPOSED OIL SHALE 83

PERCENT FINER BYWEIGHT

0.1 1.0

GRAIN SIZE (mm)

10.0

Figure 1.Grain-size distribution forLURGI-combusted shale

(McWhorter and Brown, 1985).

shale was cured, suggested that relevant hydraulic prop

erties might similarly be dependent on thewater content

during curing. Seven sets of duplicate samples were pre

pared and curedwith the aimofdetermining the influence

ofwater content during curing on subsequentlymeasured

hydraulic conductivity at saturation. Table 1 indicates the

manner in which the various groups of samples were

prepared and cured.Water contents of 10%, 15%, and 20%

byweightwere targeted for thegroup II samples.However,vapor adsorption proved to be much slower than

anticipated, and a final water content of about 1.5% was

achieved for all group II samples. The permeameter cells

containing group I and group II samples were stored in

containers in which relative humidity was controlled to

values corresponding to 0, 10%, 15%, and 21% water con

tents. Sri Ranjan (1989) explained how the appropriate

relativehumiditieswere estimated.Allgroup I and II sam

ples were weighed weekly to assure that water contents

remained unchanged during the 90-day curing period.

Following the 90-day curing period, all the samples

listed in Table 1 were vacuum saturated. Thewell known

falling-head procedure was used to measure hydraulic

conductivities at various times over a 216-day period.

Between measurements, the permeameter cells were im

mersed in water to maintain the materials in a saturated

condition. These cells were constructed with a rubber

sleeve in contact with the test material. External pressure

Table 1. Distinguishing features of samples cured for subsequent hydraulic conductivitymeasurements.1

Sample

Group Designation Sample Preparation and Curing Process

0A

0B

No water added. Samples exposed to dry atmosphere for 90 days.

II 1040A Water added by vapor adsorption from vapor-saturated nitrogen

1040B continuously flowing through sample. Target water content = 10%.

1540A Water added by vapor adsorption from vapor-saturated nitrogen


2040A Water added by vapor adsorption from vapor-saturated nitrogen


Ill 10A Water added"instantaneously"

by spraying and mixing to achieve 10%

10B water content.

15A Water added"instantaneously"


15B water content.

20A Water added"instantaneously"


20B water content.

1. All samples 1,400kg/m3

dry bulk density and cured in rubber-sleeved permeameter cells for 90 days.


was exerted on the sleeve to assure that no annular space

formed between the permeameter wall and the sample.

Figure2 shows the time trend forhydraulicconductivities

measured for the three groups of samples. Because data

from group II sampleswere indistinguishable from group

I data, the measurements for both groupswere plotted as

filled circles.The salient feature of thedata forgroups I and

II is the rapid decline of hydraulic conductivity, which at

216 days had declined to about 3 xIO-8

cm/s. Most of the

reduction occurred in the first 50 days of the test period. In

marked contrast to the group I and II samples, group III

samples showed only a slight decline in hydraulic con

ductivity. Immobilization of water resulting from

cementatious reactions are believed responsible for the

dramatic decline of hydraulic conductivity in the group I

and II samples.An armoring effect promoted by thecuringconditions is offered as an explanation for the markedly

di fferentbehaviorof thegroup III samples.The concepts of

immobilization and armoring effect are further discussed

below.

SATURATED HYDRAULIC CONDUCTIVITY (cm/s)10"

10_

10_

10'

10"

10"

""*K

20%

Group III

15%

^b-H-n 10%

MD

X

l\f\

Groups I & II

1

I>,,,,

50 100 150

TIME (days)

200 250

Figure 2. Saturated hydraulic conductivity of LURGI-

combusted shale.

IMMOBILIZATION OF WATER BY

CEMENTATIOUS REACTIONS

The observed three-order-of-magnitude reduction in

hydraulic conductivity suggests thatan increasingly largerfractionof theporespacebecomes incapableofcontributingto water flow as curing proceeds. However, practically all

thewater present during curing can be driven off at 105C(Brown andMcWhorter, 1985;McWhorter, 1987). Further

more,no significant changes inbulkvolumewereobserved

to occur during curing. These observations indicate that

the porosity of thematerial remained essentially constant,

and the large reduction of hydraulic conductivitymust be

attributable to immobilization of water.

A great deal of attention has been given to the various

states inwhichwatermay exist in cement pastes.McCarter

(1987)consideredwater incementpastes toexistascapillarywater and asgelwater, the latterbeingpartof thegel phase

responsible for the development of strength of the paste.

McCarter argued thatwater in the capillary stateprimarilyis responsible for the conductance of electrical current at

low frequencies. At high frequency, gel water contributes

to electrical conductance, andMcCarter exploited this fre

quency dependence toestimate the relative volume of gel

in curing cement pastes.

McCarter's (1987) work motivated the use of low-

frequency electrical conductance as a means ofmeasuringthemobilewater in thepresent study.Triplicate samples of

LURGI-combusted shale were prepared in electrical

conductance cells at a drybulk densityof 1,400kg/m3

and

a saturation of unity (Sa = 1 ). The cells consisted ofLucite

rings5.1 cm in diameter and 2.15 cm long. Aluminumdisks

were placed in contact with the shale on both ends of the

cell and held in place by a spring-loaded device designed

to maintain constant force on the disks. Annular spaces

formed by the walls of the rings, and the edge of the disks

were taped to suppress evaporation.Gases generated dur

ing curing could escape because the tape did not provide

a seal. A Hewlett Packard Model 4192A impedance ana

lyzer with a frequency range of 5 Hz to 13 MHz was used

tomeasure electrical conductance.Conductancemeasure

ments were made at frequencies of 100, 1,000, 10,000,

1 x IO5, and 3 x IO5 Hz (Sri Ranjan, 1989). Only the results

measured at 100 Hz are reported herein.

Themeasured electrical conductance (averageof triplicate

samples) is shown in Figure 3 as a function of time. Note

that electrical conductance declineswith time inmuch the

samemanner as for hydraulic conductivity. The decline of

electrical conductance is attributed to a decrease in the

volume of capillary (mobile) water as hydration proceeds.

It is expected that water contributing to electrical con

ductance is the same water that contributes to hydraulic

conductivity. The apparent similarity between the electric

and hydraulic results is not surprising, therefore.

Figure 4 depicts the volumes of air, water, and solid

before and after hydration. The cement-like reactions im

mobilize a portion of the total volume ofwater,but neither

the solid or air volumes significantly change. Immobilized

waterproperly cannotbeconsidered as solid because it can

bedrivenoffby the simpleprocessofraising the temperatureto 105C. Porosities measured prior and subsequent to

hydration are virtually identical.


0.08

0.06

ELECTRICAL CONDUCTANCE (S/m)

0.04 -

0.02 -

20 30 40

TIME (days)

Figure 3. Electrical conductance of saturated shale.

50

Before Hydration After Hydration

Air (Va

;>> Mobile Water (Vm)

1'

',

',

'

Immobile Water|

I (Mm) '

I i I I

Figure4.Definition sketch forcharacterization ofwatercontents.

We define the apparent saturation, Sa, as the fraction of

theporevolumeoccupiedbywater.Theapparent saturation

is changed only by the transfer ofmobilewater across the

boundaries of the bulk element shown. The fraction of the

pore volume occupied bymobile water is denoted by Sm.

Based on Archie's (1942) formula relating electrical

conductance to porosity, Sri Ranjan (1989) wrote,

/ct,(U)\0-5

a)

where Sm(l, t) is themobilewater saturation atany time for

Sfl = 1. The corresponding electrical conductance is oc/.(l, 0,

and at(l, 0) is the electrical conductance for Sa = 1 and t =

0. Equation 1 permits themeasured electrical conductance

data to be interpreted directly in terms of mobile water

saturation. Figure 5 shows mobile water saturation as a

function of time as calculated from equation 1 using the

electrical conductance data shown in Figure 3.

MOBILE WATER SATURATION

Estimated data

Fitted curve

-i i i i i_

10 40 5020 30

TIME (days)

Figure 5.Mobile water saturation as a function of time.

The relationship shown in Figure 5 permitsestimationof

the mobile water saturation as a function of time under

conditionsofcomplete saturation (Sfl = 1 ). SriRanjan (1989)

wrote,

Sm(Sa,t) = SaSm(l,t) (2)

from which the mobilewater saturation can be calculated

foranyvalueofapparent saturation.Equation 2 isbased on

a generalization of the work ofWyckoff and Botset (1936)

and extensivemeasurements of electrical conductanceas a

functionof timeatvariousconstantvaluesof Sa (SriRanjan,1989). This result, together with the function Sm(l, t), as

depicted in Figure 5, has direct practical relevance, as will

be demonstrated subsequently.

THE CONCEPT OF ARMORING FOR

Sa LESS THAN UNITY

Group I and II samples, forwhich a dramatic decrease of

hydraulic conductivity was observed, were maintained

nearly dry prior to initiation of the measurements (see

Table 1). Therefore, little or no opportunity was availablefor hydration prior to the time the samples were saturated

86 COLORADO SCHOOL OFMINES QUARTERLY

for themeasurementofhydraulic conductivity. Hydration

occurredduring thecourseof themeasurementsand causedthe immobilization ofwater and theobserved reduction of

hydraulic conductivity.

In contrast, group III samples were exposed to constant

water contents of 10%, 15%, and 20% byweight for 90daysprior to initiation of the hydraulic conductivity meas

urements. Recall that Figure 2 shows only a slight decline

of hydraulic conductivity throughout a 216-day periodsubsequent to being saturated. It is hypothesized that the

90-day curing process at less than full saturation created ashell or armor around the solids that prevented further

hydration upon complete wetting.

Thearmoringhypothesiscanbeunderstood by referencetoFigure6,which shows theextentofwater immobilization

in fully and partially saturated elements of the combustedshale. Cementatious reactions and the products thereof

necessarily are limited to that portion of the pore space

occupied by water during curing. Because group III ma

terialswere less than fully saturated, the largerpore spacesoccupied by air during curingwere not greatly affected byhydration. In effect, the large pores and channels occupied

by air were"frozen"

into place during the curing process.The bounding surfaces of the space occupied by air werearmored by the reactions, and further hydration upon

subsequent saturation of the materials was effectivelyprevented. Thus, the hydraulic conductivity of group III

samples was not greatly affected during curing or sub

sequent thereto.

Cured at Sa = 1 .0 Cured at Sa = 0.5

Figure 6. Fraction of pore space occupied by gel phase after

curing at two different saturations. Fraction of void volume

available for flowupon subsequent saturation is (a) 0.1 in. and

(b) 0.55 in.

The tendency for combusted shale cured at less than full

saturation to become armored and to freeze into place the

pore space contributing to large hydraulic conductivityhas important practical implications. Combusted shale

placed in an embankment at water contents less than full

saturation cannot be expected to undergo the large re

duction ofhydraulic conductivity observed in group I and

II samples.Apparently, a criticaldegreeofsaturation exists

thatmustbeexceeded toachievefull reductionofhydraulic

conductivity. It is to be noted that about 65% of the pore

space of samples 20A and 20B was occupied by water

during curing. Evidently, this degree of saturation is not

adequate toachievefull reductionofhydraulicconductivity.

Subsequent studies showed that the critical saturation is

about85%.At saturationsgreater than85%, little likelihood

exists thatcontinuous, interconnected air-fil led poresexist.

Ways in which the critical saturation and associated re

duction ofhydraulic conductivity can be achieved to form

low-permeability layers in shale embankments are dis

cussed in a subsequent part of this paper.

DRAINAGE OF EMPLACED WATERS

As mentioned previously, one mechanism by which

leachatepotentiallycanbegenerated isby internaldrainageofwatersplaced in theembankment at the time ofdisposal.

Thewatercontentsatwhichdisposal embankmentswillbe

constructed under commercial operations remain to be

established. However, it is unlikely that emplaced water

contents will exceed 20% byweight and, probably, will bemuch less. Any emplaced water in excess of what can beretained by immobilization and by capillarity eventuallywill appear as leachate.

Waters placed in the embankment with the shale will

tend to slowly drain toward the bottom of the pile. Figure

5 and equation 2 indicate that 90% ofemplacedwaterswill

become immobilizedwithin 20days.Twentydays isa brieftime in relation to the time required for significantdrainage

to occur at water contents of 20% or less. Therefore, esti

mation of the quantity of drainable water can bemade byassuming that waters remaining mobile subsequent to

hydration are theonlywaters available fordrainage.All or

a portion of themobile waters will be retained within the

embankment by capillarity, as is demonstrated in the

following example.Suppose LURGI-combusted shale is emplaced in 5-m

lifts at an average drybulkdensityof 1 ,400kg/m3

and 15%

water content (dry weight basis). These figures convert toan emplaced saturation of Sa = 0.5. The total volume of

water emplaced in each lift is 1 .05m3/m2. After 20 days of

curing, the saturation of mobile water will be 0.05 (from

Figure 5 and equation 2). Hence, the volume of potentiallydrainable water in each lift 20 days after emplacement is

0.105 m3/m2. The potentially drainable water is capillarywater, water that is free to drain until mechanical equi

librium is achieved.

Because the shale will be cured at Sa = 0.5, the armoringphenomenon previously described will occur. Capillarywater will tend to drain over a very long time period.


During this period of redistribution, relevant hydraulic

properties will be time invariant because of the armoringthatoccurred during curing. In particular, the relationshipbetween capillary pressure and saturation of capillarywaterwill be approximately that of the unhydrated shale.

A popular equation relating the saturation of capillarywater to capillary pressure is the Brooks-Corey (1966)

relation:

c V(3)

where Sc is the saturation of capillarywater, hc is capillarypressure, hd is displacement pressure, and X is the pore-

sizedistribution index. For theLURGI-combusted shale,

hd = 276 cm of water, and X = 0.44.

Redistribution of capillary water will continue until

mechanical equilibrium prevails. There exists amaximum

volume of capillary water that can be retained atmechan

ical equilibrium. If the total volume of capillary water

(mobilewater after hydration is complete) is less than the

maximum volume retained at mechanical equilibrium, no

leachate will result from internal drainage.

At mechanical equilibrium, the capillary pressure head

is distributed linearly with elevation according to,

hc = Z + hc(0) (4)

where Z is measured upward from the base of the em

bankment.Thevalue ofhc(0) is taken tobe zero, a condition

corresponding to a coarse-grained foundation material

upon which the embankment rests. Then the maximum

volume ofcapillarywater that canbe retained perunit area

at mechanical equilibrium is,L L

Vc= (?jScdZ = ^Scdhc + (?hd (5)

0 hf

in which L is total height of the embankment, and (j> is

porosity. Evaluation of the integral in equation 5 using

equation 3 gives,

V =^Jt[(t-) -*]. k*1 (6)c 1-a.L

h/J

To determine the volume of capillary water that

eventually will drain from the pile (if any), the volume of

mobilewater remaining after hydration iscompared with

the volume computed from equation 6. If the volume of

mobile (capillary) water is greater than Vc, the difference

eventually will appear as leachate. Otherwise, no leachate

resulting from internal drainage will be generated.

Suppose the embankment in the example consists of 40

lifts of 5 m, for a total height of 200 m. Then equation 6

indicates that amaximum 22m3/m2 ofcapillarywater can

be retained at mechanical equilibrium. The volume of

mobilewater remaining in each lift after 20 days of curing

is 0.105m3/m2, for a total of 4.2m3/m2 in 40 lifts. Because

the volume of mobile water is less than what can be re

tained at mechanical equilibrium, it is concluded that no

leachatewillbegenerated from thisexample embankment

by themechanismof internal drainage. Furthermore, there

isa "factorofsafety"

of5, avalue sufficient toaccommoda te

a great deal of uncertainty in the parameters used in the

computations.

The importanceof immobilizationofwaterbyhydration

warrants emphasis. If immobilization did not occur in the

example embankment, the total volume of capillary water

would be 42m3/m2. Only 22m3/m2

would be retained at

mechanical equilibrium, and 20m3/m2

eventually would

drain from the embankment as leachate.Thus,hydration is

instrumental in preventing the generation of leachate in

this example. It is expected to be similarly important in

actual field disposal operations.

In closing this section, an important difference is noted

between those waters retained in the shale by immobil

ization and those retained by a balance between pressure

gradient and gravity (i.e., mechanical equilibrium). The

formerare incapableofbeingdisplaced, forexample, as the

result of net infiltration into the embankment. In contrast,

the latterwaters are free tomove shouldmechanical equi

libriumbe disturbed or fail to develop as a result of inflowof net infiltration. Therefore, it is important that net in

filtration beminimized so that any capillarywater will be

retained within the embankment.

CREATION OF

LOW-PERMEABILITY LAYERS

It isclear from the experimental results that the armoringeffectwill prevent full reduction ofhydraulic conductivityif curing takes place at a saturation below some critical

value. Sri Ranjan (1989) speculated that the critical value

should be about Sa = 0.85. It is likely that the bulk of thedisposal pile will be emplaced with water contents in the

range of 5% to 20% by dry weight. Thus, the desirable

effects of hydration on hydraulic conductivity are not

likely to be realized in the bulk of the pile.

On theotherhand, low-permeability layerscanbecreated

by intentionally effecting saturated or near-saturated

conditions at strategic locations (for example, layers near

the bottom and top of the embankment). To prevent

armoring, the saturated or near-saturated condition must

be created before the shale is exposed to saturations less

than the critical value for any significant time. This was

easily accomplished in the experiments described above

simplybymixingdry shalewithwater toachieveconditionsfavorable for maximum hydration. However, premixingshaleandwater toessentially saturatedconditions followed

88 COLORADO SCHOOLOFMINES QUARTERLY

by placement on the embankment likely seems infeasiblefrom the standpoint of materials handling. A possible al

ternative is to placeoneormore liftsofpracticallydry shaleand then create saturated or near-saturated conditions byapplication of water.

Application of water, for example by irrigation, to thesurface of a lift of dry shale will result in saturation valuesthat range from zero below the advancingwetting front to

near unity at the surface. Shale at any arbitrary point in the

irrigated lift gradually will wet up as the wetting front

passes the point in question. Thus, the shale at this point

will beexposed to saturations less than thecritical value for

some period of time. We define the exposure time as the

time interval between first arrival of water at a point and

the time at which critical saturation is achieved. Exposure

time is zero for shale on the surface of the irrigated lift and

increaseswith depth. Shale at depthswith exposure times

greater than about two or three days are expected to

experience significant armoring and less than complete

reduction of hydraulic conductivity.

Sri Ranjan (1989) solved the differential equation for

infiltration using hismeasured time-dependent hydraulic

conductivity function. On this basis he predicted that all

pointswithin 70 cm of the shale surface would reach a sat

uration of 0.85 ormore within two days after initiation of

irrigation. Thus, a 70-cm layer of fully reduced hydraulic

conductivity (3 x 10-8cm/s) was predicted. Due to the

armoring effect, shale at depths greater than 70 cm would

not experience full reduction in hydraulic conductivity.

Figure 7 shows Sri Ranjan's predicted results.

140DEPTH OF PENETRATION (cm)

S - 0i

120

100

X q 0 P5

80

f* S*?'

txposure time = z daysm

/1 cm

60

/ y^ X. = 0.30

40 /y h^

= 331 cm

20

, , i i i i . i _i 1 1

"0 1 2 3 4 5 6

TIME (days)

Figure 7. Depths to Sa = 0 and Sa = 0.85 as function of time.

Braun (1990) conducted experiments to measure the

thickness of the layer of fully reduced hydraulic con

ductivity.Vertical columnsofair-dry shalewere infiltrated

by water until the infiltration rate declined to a small,

approximately constant value. Water was introduced at

the top of the column at practically zero pressure. Figure 8

shows the rapid decline of the infiltration rate into three

0.001

1E-07

600 800

timt. hours

1000 1200 MOO

-*-

column 1 ? column 2 HB-column 3

Figure 8. Infiltration into columns at room temperature.


replicate columns. The decline is attributable both to re

duction inhydraulicgradientand to reduction inhydraulic

conductivitydue to hydration.

Braun's columns were constructed of stacked, 5-cm

sections.Following the infiltrationexperiments, the sectionswere separated, and saturated hydraulic conductivity of

thematerial in each sectionwasmeasured. Themeasured

distributionofhydraulic conductivity along thecolumns is

shown inFigure 9. Indeed, thehydraulic conductivity pro

file shows an interval of low hydraulic conductivity,

followed by a sharp increase. These data indicate that a

layerabout 90 cm thickwas formed inwhich the hydraulic

conductivitywas fully reduced.

water and, therefore, the thickness of the low-permeabilitylayer. To investigate this possibility, Braun (1990) again

conducted infiltration experiments, this time into columns

maintained at about 80C. Figures 11 and 12 show the dis

tributions ofhydraulic conductivity and volumetricwater

content, respectively, observed in those experiments.

Indeed, the thickness of the interval of low hydraulic con

ductivity was reduced to about 20 to 30 cm.In summary,Braun (1990)observed in room-temperature

experiments the creation of a 100-cm interval of fullyreduced hydraulic conductivity,belowwhich existed a 40-

cm transition zone. The average hydraulic conductivities

in these two intervalswas found to be 3.3 x 10-8 cm/s and

3* column 1 t column 2 ? column!

Figure9. Saturated hydraulic conductivity as obtainedby infiltration at room temperature.

The volumetric water content in each 5-cm segment of

thecolumnwasmeasuredprior to conducting thehydraulic

conductivitymeasurements.Figure 10 showswater-content

distribution measured at the cessation of the infiltration

experiments.Note that thewater content from 0 to 90 cm is

high (above 0.45). Awater content of 0.45 corresponds to a

saturation of 0.85. Thus, the interval of fully reduced saturation corresponds to the interval in which the water

saturation exceeds 0.85, as predicted by Sri Ranjan (1989).The thickness of the layeroffully reducedhydraulic

conduc-

tivity observed by Braun (1990) is somewhat greater thanthe 70 cm predicted by Sri Ranjan.Retorted oil shale likely will be emplaced in a disposal

pile at a temperature well above the room temperature at

which the above experiments were conducted. Should a

higher temperature cause hydration to proceed more

quickly, one would anticipate that the rapid reduction of

hydraulic conductivity might reduce the penetration of

56 x IO-8cm/s, respectively. Similar experiments at 80C

(approximately) resulted in a 24-cm interval with average

hydraulic conductivity of 5.0 x IO-8cm/s. In the 17-cm

transition zonebelow, averagehydraulic conductivitywas

found to be 22 x IO"8cm/s.

ASSESSMENT OF PILE HYDROLOGY

The pressure of one or more low-permeability layers

within a pile of retorted shale is expected to influence the

rate of water movement within the pile. In particular, a

low-permeability layer located immediately below a layer

of topsoil is expected to significantly decrease the quantityof infiltrating waters passing into the interior of the pile.

This expectation is predicted on the fact that potential

evapotranspiration exceeds average annual precipitation

in the oil shale regions of Colorado, Wyoming, and Utah.

A low-permeability layer would function to impede


140

column 1 4 column 2 -S- cokjmni

Figure 10.Water content after infiltration at room temperature.

j-**-

column 1 * eolimn 2

Figure 11. Saturated hydraulic conductivity as obtained by infiltration at elevated

temperature.

downward percolation from the root zone during wet

periods, thus making more water available for evapo-

transpiration during subsequent periods of water

deficiency.

Braun (1990) used theHydrologic Evaluation ofLandfill

Performance (HELP II) model to evaluate the degree to

which a low-permeability layer functions to minimize

deep percolation into the pile interior. It is beyond the

scope of this paper to describe themodel in detail. Briefly,

themodel iscapableofcomputingwatercontent andwater

flux in a waste pile composed of several layers, each with

different physical properties. Themodel receives as input

precipitation on a daily basis. Surface runoff andevapo-

transpiration are computed on a daily basis. Basicallytrw


X< column 1 column 2

Figure 12.Water content after infiltration at elevated temperature.

model is a water-budget routine with a mechanism for

calculating flow from layer to layer. Such flow iscalculated

fromknowledgeof thehydraulicconductivityof the layers

as a function ofwater content.

Braun (1990) simulated disposal-pile hydrology on a

dailybasis for a simulation period of20years. Theprimaryoutput of interestwas the long-term average rate ofwater

flux into the pile interior. Precipitation data from Grand

Junction and LittleHills,Colorado,were used to establish

the statistical parametersofprecipitation in thearea. These

statistics then were used by the synthetic precipitation

generator to calculate daily precipitation for the Tract C-asite. Long-term mean precipitation values of 360 mm/yr

and450mm/yrwereused.Evapotranspiration iscalculated

in the model based on water availability in the root zone.

The degree to which a low-permeability layer affects

deep percolation into the pile is shown in Figure 13. The

uppermostcurve representsdeeppercolationasa function

of topsoil depth without a low-permeability layer. The

intermediate curve was calculated for a low-permeabilitylayer only 25 cm thick with a saturated hydraulic

conductivityof6.0 x10-8

cm/s. The lowermost curve is for

a 100-cm thick layer with a hydraulic conductivity of

3.3 xlO"8 cm/s.

It is observed that the presence of the low-permeabilitylayer has the greatest effect when topsoil depth is small.

With deeper topsoil, all three systems perform practically

the same. This is because the greater depth of topsoil pro

vides sufficient storage to accommodate essentially all

incoming water. Even during wet periods, deep topsoil

stores excess moisture for subsequent evapotranspiration

during a dry period.With thinner topsoil, the underlying

low-permeability layer is responsible for minimizingdrainageofroot-zonewaters inexcessof thenatural storage

capacity. Thus, the low-permeability layer functions to

hold up excesswater inwet periods so that it canbe subse

quentlyconsumedbyevapotranspiration.Atgreater topsoil

depths, very littlewaterexistsinexcessofroot-zone storage

capacity, and the low-permeability layer plays a more

minor role.

Of course, the greater the amount of precipitation, the

greater is the importance of the low-permeability layer.

The data in Figure 13 were calculated for average annual

precipitation of 360mm/yr. Figure 14 shows the compu

tations for average annual precipitation of 450mm/yr, all

other parameters being equal to the previous case. Notethat the low-permeability layer significantly reduces deeppercolation over a greater range of topsoil depths.

CONCLUSIONS

Cementatious reactions between water and combusted

oil shale immobilize as much as 90% of available water

within a period of 50 days. This phenomenon, coupled

with capillary storage, virtually precludes leachateproduction as a result of internal drainage of emplaced

waters (for example, dust-control water, water used for

cooling, process-waste water).

92 COLORADO SCHOOL OFMINES QUARTERLY

20 25

DEPTH OT TOPSOL (H)

cose 1 cose 4 cose 5

Figure 13. Percolation into the pile from the bottom of different barrier-layer types

at average precipitation.

20 25

DEPTH OP TOPSOL (N)

cose 2 case 3 case 6

Figure 14. Percolation into the pile from the bottom of different barrier-layer types

at increased precipitation.

The cementatious reactions also result in about a three-

order-of-magnitude reduction in hydraulic conductivity.

However, this reduction is achieved only if the combusted

shale is cured atwater saturations equal to or greater than

about 85%.

Conditions favorable for full reduction of hydraulic

conductivity can be achieved in a25- to 100-cm interval by

irrigating the surface of a lift of dry shale. The 25-cm

thicknesswill resultwhen the shale isabout80C.At room

temperature, thehydration process is slower, and a 100-cm

layer of reduced permeability is achieved.

A low-permeability layer located immediately below a

topsoil layer functions to hold upwater thatwouldother

wise drain from the root zone into the interior of the pile.

Waters thus prevented from rapid drainage are subse

quentlyconsumed byevapotranspiration.Theimportance

of the low-permeability layer in minimizing deep

percolation diminishes as topsoil depth increases.


REFERENCES

Archie, G.E., 1942, The electrical resistivity log as an aid in

determining somereservoircharacteristics:AIMETransactions,Petroleum Division, v. 146, p. 54-61.

Braun, J.M., 1990, Hydrology of LURGI-combusted oil shale

disposal piles: Ft. Collins, Colorado State University, M.S.

Thesis.

Brooks, R.H., and Corey, AT., 1966, Properties of porousmedia

affecting fluid flow:AmericanSocietyofCivilEngineers,Journalof Irrigation and Drainage Division, v. 92, no. IR2, p. 61-88.

Marcus, D., Sangrey, D.A., and Miller, S.A., 1984, Effects ofcementation process on spent shale stabilization: Paper pre

sented atSME-AIMEAnnualMeeting,LosAngeles,California.

McCarter, W.J., 1987, Gel formation during early hydration:

Cement and Concrete Research, v. 17, p. 55-64.

McWhorter, D.B., 1987, Retardation of flow in oil shale residues

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Mining and Minerals Research.

McWhorter,D.B., and Brown,CO., 1985,Adsorption and flowof

water in nearly dry LURGI retorted oil shale: Report to Rio

Blanco Oil Shale Co., 94 p.

Sri Ranjan, R., 1989, Water movement in LURGI-combusted oil

shale affected by time-dependent hydraulic properties: Ft.

Collins, Colorado State University, Ph.D. Dissertation 127 p.

Wyckoff, R.D., and Botset, N.G., 1936, The flow of gas-liquid

mixtures through unconsolidated sands: Physics, v. 7

(September), p. 325-345.

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