reservoir characterization of coals in the powder river ... that on average one injection well was...

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Reservoir Characterization of Coals in the Powder River Basin, Wyoming, USA, to Test the Feasibility of CO2 SequestrationHannah E. Ross1* and Mark D. Zoback11Department of Geophysics, Stanford University, Stanford, CA 94305-2215, *[email protected]

The aim of this study is to investigate the feasibility of sequestering CO2 in unmineable coalbeds of the Powder River Basin (PRB), Wyoming, through geomechanical and geologic reservoir characterization and simulations. We are particularly interested in whether hydraulically fracturing the coal will increase injectivity and improve sequestration capacity, and whether enhanced coalbed methane recovery (ECBM) will offset the cost of sequestration.

We found that gravity and buoyancy were the major driving forces behind gas flow within the coal, and that coal matrix swelling resulted in a slight reduction in injectivity. However, hydraulically fracturing the coal close to its base helped mitigate the negative effect of permeability reduction on injection rate.

Our simulations show that after 6 years of CO2 injection, ~95% of the total CO2 injected into the Big George coal would be sequestered and that CH4 production would be ~7 times greater with CO2 injection than without. We found that on average one injection well was able to sequester ~23 kt of CO2 a year. Based on this injection rate, it would take ~2,300 injection wells (each with a lifetime of ~6 years) to sequester the current CO2 emissions for the State of Wyoming. Since there have already been ~15,000 CBM wells drilled in the PRB, and ~50,000 more projected to be drilled in the next decade, utilization of 2,300 wells for CO2 sequestration is quite feasible, especially in light of the potential for significant cost recovery through enhanced methane production.

Summary

Reservoir Characterization Preliminary Results for CO2 Injection and CH4 Production

2) 3D ModelOur 3D model was built in an area where Colmenares and Zoback (AAPG, in press) found horizontal hydraulic fractures. Vertical hydraulic fractures may penetrate the overlying strata creating potential leakage points for CO2. We used 5 PRB CBM wells to construct our model.

We used the Computer Modelling Group’s ECBM simulator GEM.Cleat spacing = 10 cm.Initial reservoir pressure gradient = 7.12 kPa/m (0.315 psi/ft) (ARI, 2002). All simulations were run with and without matrix shrinkage and swelling modeling. Base case: primary production for 11 years.Injection case: injection begins at 1800 days. Simulations were run with an injector BHP constraint of 4000 kPa (580 psi). This is set below S3 (6200 kPa (900 psi)) so no hydraulic fractures are created.Injection with hydraulic fracture case: a horizontal hydraulic fracture was placed at the base of the injection well. The hydraulic fracture has a radius of 60 m, permeability of 1000 mD, and porosity of 30%.

If S3 = Sv then horizontal hydraulic fractures form

Simulation Setup

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CO2 adsorptionCO2 desorptionCH4 adsorptionCH4 desorptionN2 adsorptionN2 desorption

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CO2, Swi=0 CO2, Swi=8.47%CH4, Swi=0 CH4, Swi=8.67%N2, Swi=0 N2, Swi=8.54%

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Adsorption Isotherms for Moist CoalClosed symbols = dry coalOpen symbols = moist coal

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nx dx 20 m ny dy 20 m nz 6 dz 3x4 m a n d 3x1.3 m

4241

CBM wells in the PRB are routinely hydraulically fractured through water enhancement tests. In some areas the hydraulic fractures propagate horizontally, whereas in other areas they grow vertically (Colmenares and Zoback, AAPG, in press).

4) Adsorption IsothermsWe used adsorption/desorption isotherms from lab experiments conducted on dry and moist PRB coal samples by the SUPRI-A group, Department of Petroleum Engineering, Stanford University (courtesy of A. Kovscek and T. Tang).

The initial cleat permeability and porosity values came from the literature, and we used geostatistical techniques to populate our model with multiple, equiprobable cleat permeability distributions. We further constrained the cleat permeability and porosity values through history-matching the water production from CBM wells in our study area (WOGCC).

1) State of Stress

a) Horizontal face cleat permeability. b) Horizontal butt cleat permeability. c) Vertical face cleat permeability. This figure shows our 3D model populated with cleat permeability values for realization 1. The heterogeneity and anisotropy in coal cleat permeability is modeled using geostatistical techniques. The horizontal face cleat permeability is higher than in the butt cleat and vertical directions (Laubrach et al., 1998). Depth to the top of the coal varies from 315-360 m.

The PRB contains the fastest growing CBM play in the United States (there are currently ~15,000 CBM wells in the PRB and ~50,000 more to be drilled).Wyoming has a number of point sources for CO2 capture, which emitted 52 megatons of CO2 in 2000 (EPA, 2005). Wyoming has a CO2 pipeline network, with a proposed extension to the PRB (Nummedal et al., 2003).

We focused our study on the Big George Coal, part of the Wyodak-Anderson coal zone of the Paleocene Fort Union Formation.The average depth of the coal is 335 m and coal thickness varies from 14 to 62 m (Flores and Bader, 1999).

Powder River Basin

Big George Coal

Source for CO2 pipelines location: http://www.corporateir.net/ireye/ir_site.zhtml?ticker=apc&script=411&layout=0&item_id=439732

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Coal Bed MethaneDevelopment Area

Study Area

Powder River Basin

Location map of the Powder River Basin, Wyoming, and our study area. Our 3D model is located in the southern part of our study area.

Sensitivity Analysis

Injection well (inject pure CO2)

Production well

CH4/CO2 front

Five Spot Pattern

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Total volume of CO2 injected and total volume of CH4 produced after 11 years. Hydraulically fracturing the coal at the base of the injection well increased the total volume of injected CO2 by ~30%. With ECBM there was a ~7 fold increase in CH4 production. Hydrofrac stands for hydraulic fracture and S&S stands for matrix shrinkage and swelling.

ECBMPrimaryProduction

Total Volume of CH4 Produced

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Cleat spacing,cm

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Cleat compressibility,

1/kPa

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Exponent

Total Volumes Injected and Produced from Sensitivity Analysis

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CleatporosityCleat permeability

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Total volumes of CO2 injected and CH4 produced due to changes in the model and input parameters for the fluid flow simulations. Cleat compressibility, Young’s modulus, Poisson’s ratio, matrix volumetric strain and the exponent used to relate cleat porosity and permeability are all included in the Palmer and Mansoori (1996; 1998; GEM 2005) equation. The dark blue boxes correspond to the base case, and the values used for the parameters in the base case are listed in Table 1 of our GHGT-8 paper. All of these simulations incorporate matrix shrinkage and swelling, but no hydraulic fracture.



Total volumes of CO2 injected and CH4 produced if injection is stopped at the first sign of CO2 breakthrough (100% of the total CO2 injected is sequestered), compared to the total volumes at the end of 6 years of injection, where only 95% of the total CO2 injected is sequestered. In our simulations, the first sign of CO2 breakthrough is at ~3,000 days, after ~3 years of injection. ECBM would still be profitable after only 3 years of CO2 injection, with CH4 production increasing by a factor of 5 over primary production. The CO2 injection rate would drop to ~21 kt per year compared with ~23 kt. And the number of injection wells needed to sequester the current CO2 emissions for the state of Wyoming would increase to ~2,500 wells, each with a lifetime of 3 years, compared to ~2,300 wells, each with a lifetime of 6 years.

3) Geostatistical Characterization and History-match of Cleat Permeability and Porosity

Powder River Basin, Wyoming

CO2/CH4 front

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Horizontal hydraulic fracture

(Modified from Colmenares and Zoback, 2004)

reservoir characterization of coals in the powder river ... that on average one injection well was...

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