Anatomy of a Coal Bed Fire

Transcription

1 Stanford University Global Climate & Energy Project GCEP Symposium Stanford October 3, 2008 Anatomy of a Coal Bed Fire Taku Ide and Lynn Orr

2 Geologic Storage of CO 2 in Coal Beds Deep unmineable coal beds offer potential for storing CO 2 by adsorbing it on coal surfaces. Key questions: How much CO 2 adsorbs? How does adsorbed CO 2 affect flow properties? How do mechanical properties of coal change when CO 2 adsorbs? How does seismic wave propagation change when CO 2 adsorbs? Can changes in seismic wave propagation be used to monitor where the injected CO 2 goes? Can enhanced methane recovery offset some of the cost? Image source: Dan McGee, Alberta Geological Survey

3 Overview of Lab Efforts Multidisciplinary approach based on complementary measurements Flow/Transport Lab Tony Kovscek, ERE Seismic/DARS Lab Jerry Harris, GP Geomechanics Lab Mark Zoback, GP

4 Flow, Geomechanics, Acoustics of Coals Flow experiments (Kovscek Lab) Geomechanics measurements (Zoback Lab)

5 Subsurface Monitoring (Harris Group) Baseline (t 0 ) Traditional Time-lapse from Full Surveys True 4D Images from Sparse Surveys Use Differential Acoustic Resonance Spectroscopy to measure acoustic properties CO 2 injection causes changes in the acoustic properties of coal Seismic monitoring detects the acoustic property changes Coal samples are often fragile and irregular Compressibility, attenuation & permeability measured with CH4, CO 2 present Use continuous measurements from sparse seismic surveys to track fluid movements and detect leaks

6 But coal beds can be a source of CO 2 emissions as well Coal bed fires are widespread on the planet: Spontaneous combustion (oxidation of pyrites) Lightning strikes at coal outcrops Fires lit by humans Once started, they are very difficult to extinguish. Fires in the US, China (200 Mt coal/yr burned), Indonesia (700 outcrops ignited in forest fires) have been burning for decades. Annual CO 2 emissions: approaching a billion tonnes of CO 2 /yr. Can we understand how coal bed fires work well enough to figure out how to extinguish them?

7 San Juan Basin San Juan Basin coal beds are like a pie-plate, tipped up at the edges. Removal of water from coal beds in the center of the basin causes updip migration of gas. There is lots of evidence of current fires and fossil fires. We are working to understand a fire on land owned by the Southern Ute Indian Tribe. coal fire and gas seeps Source: Fasset, USGS Prof. Paper 1625-B

8 The setting

9 Surface Expression: fissures release hot combustion gases. Gases include: CO 2, CO, CH 4, H 2 S, N 2, little O 2 : oxygen limited combustion.

10 Thermocouple temperature, ~50 ft depth, ºF Precipitated elemental sulfur Precipitated ammonium chloride

11 Effluent combustion products are hot!

12 So what have we done? Mapped surface topography and fissures created by the fire. Drilled multiple holes through the coal zone. Logged selected holes. Installed thermocouples in most holes. Some temperatures measured since 2001, better coverage now. Collected cuttings for analysis. Mapped an outcrop of an exposed fossil fire. Visited other active fires and outcrops. Measured gas compositions, isotope signatures Modeled stress distributions and displacements associated with subsidence in the vicinity of a burning coal layer.

13 Surface Anomalies

14 Drilling

15 Cuttings (ash and coal)

16 Core samples

17 Logs available for some holes Clinker Ash Coal Coal Cored hole (unburned) Hole in burned zone

18 Cross section with hole locations Estimated subsidence Ash Coal

19 Fossil Coal Bed Fire Observed dimensions of subsidence, fissures in fossil fire consistent with calculated vertical and horizontal displacements.

20 Temperature 2001 Temperature ( o C)

21 Temperature 2005 Temperature ( o C)

22 Temperature 2007 Temperature ( o C)

23 Gas compositions from nearby USGS holes show CO 2 and CH 4, no N 2. Gas compositions in the combusion area show lots of N 2, CO 2, some CH 4, traces of O 2. Isotope data: CO 2 from burning coal easily distinguished from CO 2 in place in coal beds or from burning CH 4. Gas Compositions

24 Isotope Data CO 2 from burned coal has a very different δ 13 C signature from CO 2 present in the coal bed methane. CH 4 present in effluent gas from the combustion zone also differs from the coal bed methane (some CH 4 can be created from partial oxidation of coal). May be possible to use carbon isotopes as a tracer to detect injected CO 2 vs combustion CO 2.

25 Fire Evolution? Fire ignited by lightning strike at the coal outcrop Combustion fed by O 2 from outcrop consumes coal Subsidence induces stresses that open existing fractures in the sandstone above the coal Resulting fissures (chimneys) create buoyancy-driven flow that sustains the fire

26 Possible location of combustion front? Possible ignition areas? Current hot zones Combustion front?

27 What s next? Shallow seismic observations of burned, unburned zones objective is to understand how O 2 reaches the combustion zone. Estimates of mass flows, pressure gradients in the neighborhood of the combustion zones Consider feasibility of CO 2 injection to disrupt O 2 flow into the combustion zone (there is a coal bed methane gas processing plant nearby that could be a source of CO 2 ).

28 Conclusions Coal bed fires are a significant worldwide source of CO 2. There are numerous active fires in the San Juan basin, along with fossil fires, that provide an opportunity to understand how these fires work. No method of extinguishing a fire has been demonstrated (but methods that fail have been demonstrated). Subsidence of formations above coal consumed by combustion creates fissures that release combustion product gases and the resulting buoyancy-driven flow provides O 2 to the combustion zone. It may be possible to design a CO 2 injection scheme that would disrupt O 2 flow required to sustain the fire.

29 See posters for much more detail! Structure, Surface Functional Groups, and Multicomponent Gas Adsorption - Diffusion Dynamics of Bituminous Coal, A. Dutta, Y. Liu, C.M. Ross, J., Wilcox, and A.R. Kovscek Gas Adsorption and Permeability Evolution in Coal, W. Lin and A.R. Kovscek True 4D Subsurface Monitoring of CO 2 Storage, J.M. Harris and Y. Quan Monitoring Stored CO 2 Using Seismic Data Evolution, Y. Arogunmati, and J.M. Harris Sensitivity Study of Time-Domain Controlled-Source Electromagnetic, Methods for Monitoring Geological CO 2 Storage, E. Um and J.M. Harris Physical Properties of Low-Rank Coal Samples from the Powder River Basin, WY, P. Hagin and M. Zoback Feasibility Assessment of CO 2 Sequestration and Enhanced Recovery in Organic-Rich Gas Shale Reservoirs, J. Vermylen, P. Hagin and M. Zoback. Geomechanical Characterization and Reservoir Simulation of a CO2 Sequestration Project in a Mature Oil Field, Teapot Dome, WY, L. Chiaramonte and M. Zoback Geology, Geomechanics of Subsidence, and Combustion in a Coal Bed Fire, T. Ide, D. Pollard, R. Mitchell, and F.M. Orr, Jr.

30 Acknowledgements GCEP support for Taku Ide made the Stanford work possible. Generous support from the Southern Ute Indian Tribe for field work made many of the detailed measurements possible. Continued interest, support, and many helpful conversations with Bill Flint, SUIT Department of Energy is gratefully acknowledged. Field assistance by Jonathan Begay and Ashley Neckowitz was invaluable.