An evaluation of porosity and permeability changes in oil shale

due to thermal stressesJacob Bauman, Ravindra Bhide, Milind

DeoUniversity of Utah Department of

Chemical Engineering

History

• An Assessment of Oil Shale Technologies(1980), Office of Technology Assessment

– In-situ technologies

• TIS (True In-situ)– Explosives were used to fracture/rubbalize in-situ resources

• MIS (Modified In-situ)– A portion of the resource was mined, and the remaining

underground portion was rubbalized with explosives. The mined portion could be retorted at the surface if reasonable.

History

Industrial Processes

• A few current active technologies

– Shell ICP (In-situ conversion process)

• Electric heating with conductive heat transfer

– ExxonMobil Electrofrac

• Fractures are injected with heating material

– AMSO CCR (Conduction, convection, and reflux)

• Horizontal heating and producing wells

http://www-static.shell.com/static/usa/downloads/about_shell/upstream/icp_factsheet.pdf

http://www.nevtahoilsands.com/pdf/Oil-Shale-and-Tar-Sands-Company-Profiles.pdf

http://www.amso.net/Our-Concept/Our-Process.aspx

Geomechanics

• Understanding of permeability dynamics will be crucial for successful process design.

Experiments in the Literature

• Tisot, P. R. and Sohns, H. W. “Structural Response of Rich Green River Oil Shales to Heat and Stress and Its Relationship to Induced Permeability.” Journal of Chemical and Engineering Data, Vol. 15, No. 3, 1970. pp 425-434.

Experiments in the Literature

Experiments in the Literature• Tisot, P. R.; Sohns, H. W. “Structural Response of Rich

Green River Oil Shales to Heat and Stress and Its Relationship to Induced Permeability.” Journal of Chemical and Engineering Data, Vol. 15, No. 3, 1970. pp 425-434. • “...kerogen ... is the predominant contributor to

[rich oil shales] properties and to their response to heat and stress.”

• “In most instances the induced permeability in the column of fragments was reduced to zero.”

• “This investigation shows that structural deformation in rich oil shales can be expected to occur ahead of the retorting zone.”

Experiments in Literature

• Thomas, G. W. “Some Effects of Overburden Pressure on Oil Shale During Underground Retorting.” Society of Petroleum Engineers Journal, Vol. 6, No. 1, 1966. pp 1-8.

Experiments in Literature

Experiments in Literature

• Thomas, G. W. “Some Effects of Overburden Pressure on Oil Shale During Underground Retorting.” Society of Petroleum Engineers Journal, Vol. 6, No. 1, 1966. pp 1-8.• “Massive thermal fracturing and exfoliation, whereby

the raw shale is reduced to a friable matrix, do not occur while retorting oil shale in an overburden environment.”

• “Induced permeability and porosity at a given overburden pressure increase with the hydrocarbon yield.”

• “Pore structure is created by removal of oil and water, decomposition of carbonates and microscopic expansion cracks, the last being of minor importance.”

INL Experiments and Modeling

• Mattson, E. D. et al. “Permeability Changes of Fractured Oil Shale Cores During Retorting.” Presented at 29th Oil Shale Symposium. October 20, 2009.

• Huang, Hai et al. “Massively Parallel Modeling of Coupled Thermal-Hydro-Mechanical Processes During In-situ Oil Shale Retorting.” Presented at 29th Oil Shale Symposium. October 2009.

INL Experiments and Modeling

Field Tests in Literature

• Prats, M.; Closmann, P. J.; Ireson, A. T.; Drinkard, G. “Soluble-Salt Processes for In-Situ Recovery of Hydrocarbons from Oil Shale.” Journal of Petroleum Technology. 1977, 29, 1078-1088.

– Solution mining of nahcolite created free surfaces for oil shale rock to fail by “stress release at open faces, thermally induced stresses, and thermally induced pressures.”

Field Tests in Literature

Quotes from Industry Chapters in ACS Symposium Series 1032

• AMSO– “The shale ... will want to expand as

it is heated, but since it is confined by the cool shale, it undergoes compressive failure and fills the high permeability conduit with rubble.”

– “... the thermomechanicalfragmentation process is expected to propagate out to retort diameters of 100 or more feet ...”

Quotes from Industry Chapters in ACS Symposium Series 1032

• Shell– “... injection of hot water to leach the

nahcolite and other salts ... was successful ... in generating the required permeability and porosity.”

– “... it was hypothesized that bulk heating with thermal conduction would generate permeability and that the gases generated during retorting will drive liquid oil from the pores of the shale.”

Quotes from Industry Chapters in ACS Symposium Series 1032

• ExxonMobil– “... hydrocarbons will escape from

heated oil shale even under in situ stress.”

– “[Our] set of experiments clearly indicates that, even under conditions of overburden stress, the kerogen conversion and expulsion process creates porosity and permeability that was not present in the original oil shale.”

Thermal Stress and Solid Mechanics• Brittle materials may fracture when heated due to anisotropy

and non uniform dimensional changes.

• Polymers can expand or deform a great deal during heating and have low thermal conductivity.

• Increased porosity reduces heat conduction efficiency (convection within pores is ineffective).

• Free expansion is stress free. Constrained expansion leads to compressive stress.

• Temperature gradients within a solid cause differential dimensional changes.

• Rocks are generally stronger under compression than under tension.

• Pores, or a ductile phase, impede propagation of thermally induced cracks.

Uintah-MPM

TE vthermal

• This model assumes isotropy, and stress varies linearly with strain. All deformation is elastic. No failure criteria has been added in the results presented here.

Uintah-MPM

Uintah-MPM

STARS Results - Domain

STARS Results – Temperature

STARS Results - Deformation

STARS Results - Permeability

0

00

1exp mulkkk

Oil Production Comparison

0

20000

40000

60000

80000

100000

120000

140000

0

5

10

15

20

25

30

35

40

45

0 1000 2000 3000 4000 5000 6000 7000 8000

Cu

mu

lati

ve O

il SC

(b

bl)

Oil

Rat

e S

C (

bb

l/d

ay)

Time (days)

Oil Rate kmul = 1

Oil Rate kmul = 5

Oil Rate kmul = 5 + GeomechanicsOil Rate kmul = 8

Cum Oil kmul = 1

Cum Oil kmul = 5

Cum Oil kmul = 5 + GeomechanicsCum Oil kmul = 8

Conclusions

• Understanding geomechanics in oil shale should be crucial for most in situ heating strategies.

• Permeability pathways may develop due to mechanical failure, or by some other mechanism in an in situ environment.

• The material point method implemented in the Uintah computational framework can give qualitative understanding of geomechanicalbehavior.

• Permeability dynamics have a significant impact on simulated results.