ExxonMobil’s Electrofrac Process for · 9.4 ft3 HC vapor 6.6 ft3 HC liquid ... % v a p o r 5 0 % va p o r 7 5 % va p o r ... ExxonMobil’s Electrofrac research has focused on critical

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Upstream Research

ExxonMobil’s Electrofrac™ Process forIn Situ Oil Shale Conversion

William A. Symington, David L. Olgaard, Glenn A. Otten, Tom C. Phillips,

Michele M. Thomas, Jesse D. Yeakel

26th Oil Shale SymposiumColorado School of Mines

October 17, 2006

Colorado Energy Research Institute26th Oil Shale SymposiumOctober 16-18, 2006

ElectrofracOil Shale Conversion via Electrically Conductive Fractures

Upstream Research

+ +- V

Hydraulic fractures containingelectrical conductant

Conductive heating andoil shale conversion

Production wells• Early screening research indicated:In situ methods preferred.Heat conduction is the best way to “reach into” oil shale.Linear conduction from a planar heat source is more effective than radial conduction from a wellbore.

• Electrofrac concept is applicable with either vertical or horizontal fractures.

• Conductant electrical resistivity:high enough for resistive heating.low enough to conduct sufficient current.

• Electrofrac research has focused on critical technical issues:

Identification of conductant.Maintaining electrical continuity.Expulsion under in situ stress.Completion strategy for effective heating.


#170 Cast steel shot

Core-plug-scale experiments demonstrate:• Calcined coke is a candidate conductant.• Electrical continuity is not disrupted by

kerogen conversion.• Hydrocarbon expulsion under in situ stress.

Modeling indicates:• Volume expansion is a large potential drive

mechanism.• Fractures will generally be vertical.• Longitudinal fractures heat effectively.

5 years 10 years

20/40 Mesh Proppant

Calcined Coke

Hydrocarbons Expelledunder Stress

Effective Heating from Longitudinal Fractures

Electrical Continuity Undisrupted by Kerogen Conversion


Electrofrac Laboratory Research has Focused on Critical Technical Issues

Upstream Research

Temp, ºF

Coke Resistivity is Temperature Insensitive

00.20.40.60.8

11.21.41.61.8

2

0 100 200 300 400 500 600 700 800Temperature, degC

Nor

mal

ized

Res

istiv

ity EF135

EF136

• Physical properties (density, particle size) similar to fracture proppants.

• Electrical resistivity in desired range and temperature-insensitive. Resisitivity should be controllable by calcining process.

• Chemical stability up to calciningtemperature.

• Readily available. Current uses are -– Carbon anodes for aluminum smelting.– Anode beds for cathodic protection.– Packing for industrial electrical grounding.

Calcining Temperature Controls Resistivity

Identification of Conductant

Calcined Coke20/40 Mesh Frac Proppant

Data from Hardin, et al, 1992


Calcined Petroleum Coke is a Candidate Electrofrac Conductant

Upstream Research

0.06

0.07

0.08

0.09

0.10

0.11

0.12

0.13

1150 1200 1250 1300 1350 1400 1450 1500 1550

Calcining Temperature, degC

Res

istiv

ity, o

hm-c

m

TE-315TE-315TE-316TE-317TE-431

Nor

mal

ized

Res

istiv

ity(ρ

/ρ25

ºC)

• Stress applied with hose clamps to achieve electrical continuity.

• Sample heated externally to 360ºC for 24 hours – 90% uniform conversion achieved.

• Although embedment occurred to a minor degree, electrical continuity was not disrupted.

Particle Embedment Does Not Disrupt Continuity in Core-Scale Experiments

Maintaining Electrical ContinuityUpstream Research


1 inch

1/16 inch

Calcined petroleum coke or #170 cast steel shot (0.02 inch) packed in 1/16 inch deep tray

• Heated with 20 amps for 5 hours (~60 W). Circuit not disrupted by rock alteration.

• Internal measured temperature reached 268ºC. Estimated fracture temperature of 350-400ºC.

• Thermal expansion caused fractures in the sample.

• Recovered 0.15 mL of oil.Steel Balls

Frac

ture

Fac

e

Spent OilShale

Epoxy-Filled Fractures

ConversionZone

InitialConversion

Zone

UnalteredOil ShaleSpent Oil Shale Thin Section

Showing Porosity (blue)

3 mm

Experiment Summary Photomicrograph under Fluorescent Light

#170 Cast steel shot

Simulated Electrofrac Heating Circuit Undisturbed by Kerogen Conversion

Maintaining Electrical ContinuityUpstream Research

Photomicrograph section


Expulsion Under In Situ StressUpstream ResearchExperiments Demonstrate Expulsion

of Hydrocarbons Under Stress

• Stress is applied in axial direction, strain is inhibited in lateral directions.

• Experiments under stress recovered 21 to 34 gal/ton from 42 gal/ton samples.

Oil shale inside Berea cylinder - jacketed and clamped

High temperature (Inconel) springs provide axial load. Gold foil records maximum spring deflection.


• Product compositions derived from MSSV (micro-sealed spherical vessel) pyrolysis experiments.

• Equation-of-state model used to calculate expected density and phases at typical Electrofrac conditions.

8.1 ft3 kerogen

7.2 ft3 mineral

7.2 ft3 mineral

2.9 ft3 coke

9.4 ft3 HC vapor

6.6 ft3 HC liquid

15.3 ft3 total 26.1 ft3 total

Before Conversion After Conversion @ 2400 psi, 750ºF(without liquid cracking to gas)

1 Ton of Green River Oil Shale (22% TOC, 42 gal/ton)

Volume Expansion Provides a LargePotential Drive Mechanism

Upstream ResearchExpulsion Under In Situ Stress

Temperature, ºF0 200 400 600 800 1000

4000

3000

2000

1000

0

Pressure, psi

Electrofrac P/TBubble point

Dew

point

25% va

por

50%

vapo

r

75% va

por

Phase Diagram


Hig

her

L

ower

Present day surface

Oil shale

Wasatch

Mesa Verde

North

Oil shaleoutcrop

5 miles

In Piceance Basin, Electrofrac Heating Fractures will be Dominantly Vertical

Upstream ResearchCompletion Strategy for Effective Heating


Fracture Orientation Transition Elevation

Humble tests (1964) constrain horizontal/vertical transition

Borehole ellipticity/breakoutsconstrain stress difference

Stress →

Elev

atio

n →

Fracture jobs constrain minimum principle stress

σvertical

σeast-west

σnorth-south

Geomechanical Model CalibrationLocation A

Stra

tigra

phic

Mar

kers

Symington, W. A. and Yale, D. P, 2006, ARMA GoldenRocks Conference

+ +- V

Hydraulic fractures containingelectrical conductant

Conductive heating andoil shale conversion

Production wells• Heating wells drilled horizontally, perpendicular to direction of least principle in situ stress.

• Vertical longitudinal fractures filled with electrically conducting material.

• Electrical conduction from the heel to the toe of heating wells.

• For reasonably spaced fractures, induced stresses should not alter the least principle in situ stress direction.

• Multiple layers of heating wells may be stacked for increased heating efficiency.

Preferred Process Geometry Relies on Vertical Electrofrac Heating Fractures

Completion Strategy for Effective HeatingUpstream Research


Process Schematic

Modeling Optimizes Heating Efficiency

Simulation Case Selected as “Typical” – 150 foot fracture height,5-year heating sufficient to convert 325 feet of oil shale, 120-ft frac spacing, 74% heating efficiency

Completion Strategy for Effective Heating

Thermal model view direction

Upstream Research

2.5 years 5 years 7.5 years 10 years

Temp, ºF


300

ft

100 ft

100

ft

ElectrofracOil Shale Conversion via Electrically Conductive Fractures

Upstream Research

ExxonMobil’s Electrofrac research has focused on critical technical issues. Research highlights include:

• Laboratory experiments demonstrating –Calcined petroleum coke is a suitable Electrofrac conductant.Electrical continuity is unaffected by kerogen conversion.Hydrocarbons will be expelled from heated oil shale even under in situ stress.

• Modeling including the following –A phase behavior model showing volume expansion is a large potential drive mechanism for expulsion. In situ oil shale can expand by 70% upon kerogen conversion.A Piceance Basin geomechanical model showing the stress state of the Green River oil shale favors vertical fractures.Heat conduction models showing that several fracture designs can deliver heat effectively. A “typical” case requires one Electrofrac heating well every 1.5 acres.


Documents

ExxonMobil’s Electrofrac Process for · 9.4 ft3 HC vapor 6.6 ft3 HC liquid ... % v a p o r 5 0 % va p o r 7 5 % va p o r ... ExxonMobil’s Electrofrac research has focused on critical