Outline. Introductory Resources. Gas hydrates an introduction

Transcription

1 Gas hydrates an introduction R. Gerhard Pratt 1 Introductory Resources Geological Survey of Canada: (Home page for the Arctic Gas Hydrate project) Woods Hole Field Center, Mass: 1 Queen s University in Kingston, Canada US National Energy Technology Lab (Dept of Energy) Clathrate hydrates of natural gases E. Dendy Sloan, 1997, Marcel Dekker Inc. Outline What are gas hydrates? Where are natural gas hydrates found? Why are they important? Deposition of natural gas hydrates Physical properties Geophysical site investigation Arctic gas hydrates 1

2 Gas Hydrates 1 m 3 in-situ hydrate contains 160 m 3 gas Arctic gas hydrates recovered from drill core several crystal structures are known to form clathrates of natural gas (si, sii and sh); almost all natural hydrates are si (shown here) hydrates of methane are the most abundant by far, but there are 8 natural gas components that form clathrates Chunks of marine gas hydrate recovered from a giant piston core. Methane ice worms Hessiocaeca methanicola inhabit the surface of shallow deposits of gas hydrates 2

3 Gas Hydrate stability regimes A melting lump of methane hydrate. As the hydrate dissociates, methane is released to feed a constant flame ("burning ice") while the escaping water drops off. Styles of deposition Occurrences of Gas Hydrates Known and inferred natural gas hydrate occurrences in marine (red circles) and permafrost (black diamonds) environments. USGS, 1999, Fact sheet FS (Author, Tim Collett) 3

4 Hydrate Ridge (off the coast of Oregon, Cascadia Margin, USA) Gas Hydrates and global climate change Typically hydrates exist at or close to stability threshold Quantities of carbon sequestered as natural hydrates are enormous Potential for very large releases of methane in short (geological) time Methane is an order of magnitude more effective as a greenhouse gas than CO2 Late Paleocene thermal maximum (LPTM) 55.5 Ma global temperature warmed by more than 4 C in less than 10,000 years Accompanied by: rapid influx of an immense quantity of inorganic carbon prominent extinctions, increase in global humidity, mammalian migrations Widely accepted to have been related to a massive release of methane by dissociation of methane hydrates Dickens, Castillo, Walker, 1997: A blast of gas in the latest Paleocene, Geology, 25,

5 Hornback, Saffer and Holbrook, 2004: Critically pressured free-gas reservoirs below gas-hydrate provinces (Nature, 427, 8 Jan 2004) Increased gas column thickness produces higher gas pressure at the BSR effective normal stress is is reduced, Mohr circle moves to the left overpressure causes fault reactivation and gas escape faults act as pressure relief valves and provide a mechanism for rapid release of large volumes of methane model is consistent with a wide range of offshore sites Gas Hydrates and geohazards Gas hydrates are associated with occurances of marine sediment slumping Gas hydrates are a hazard for petroleum drilling (risk of blowout, sea floor stability) Near surface hydrate dissociation can lead to weakening of of foundations 5

6 Gas hydrates as an energy resource Natural gas hydrate is thought to be the single most abundant hydrocarbon source known on Earth Dallimore, Uchida and Collett, GSC Bulletin 544, Gas hydrates as an energy resource Global resource estimates range from 3,000 to 10,000,000 tcm Conventional remaining natural gas resources are about 360 tcm Considerable uncertainty requires a probability based approach Mean estimate for USA is 9,000 tcm This is approximately 300 times that remaining in conventional resources Half of the USA methane hydrates occur offshore Alaska Cumulative probabilty curve methane hydrate resources for USA (USGS Fact sheet FS , 2001 Timothy Collett) 6

7 Distribution Canadian Gas Hydrates hydrate reserves in the ocean are thought to be two orders of magnitude more abundant than subpermafrost reserves Sloan, 1999 Conventional in-place natural gas resources estimates are about 27 tcm this is approximately 150 times annual production In-place gas hydrate gas volume is approximately times volume from conventional resources (Majorowicz and Osadetz,, AAPG 2001) Property Physical properties of gas hydates Young s modulus (GPa( GPa) Poisson s ratio Bulk modulus (GPa( GPa) Shear modulus (GPa( GPa) Vp/Vs Vp (km/s) Density (g/cc) Dialectric constant Resistivity (Ω m) Thermal conductivity (W/m-K) Ice Structure I 8.4 ~ ~0.91 ~ Structure II 8.2 ~0.33 NA NA NA 3.6 ~0.91 ~ Water Variable ~0.59 Gas hydrates are distributed within sedimentary hosts rocks in a variety of different styles, including nodular, disseminated, layered and/or massive. 7

8 Porosity vs compressional wave velocity (Arctic Gas Hydrates at Mallik) Downhole geophysical logs, ODP drilling, Peru margin, Pacific ocean Lee and Collett, 1998, GSC Bulletin Direct seismic detection of methane hydrate on the Blake Ridge Hornbach et al, Geophysics, 68, P92 Hornbach et al, Geophysics, 68, P92 8

9 Schematic representation showing the gas hydrate-related features along (A) passive continental margins. (B) Active continental margins. Both cartoons have a vertical exaggeration of approximately 50. The Bottom Simulating Reflector (BSR) (Images from Hyndman et al, Natural Resources Canada) Chand, S. & Minshull, T. A. (2003) Seismic constraints on the effects of gas hydrate on sediment physical properties and fluid flow: a review. Geofluids 3 (4), doi: /j x Cartoons showing how a paleo-bsr consisting of hydrate might form. (a) Initial situation: the white reflector crosscutting strata represents the BSR. (b) After erosion occurs on the ridge, the thermal gradient equilibrates, forcing the BSR downward. The localized high concentration of methane hydrate at the former BSR creates a high-velocity hydrate lens, crosscutting strata. 9

10 Sea floor resitivity sounding Marine Gas Hydrates vs Arctic Gas Hydrates Marine Hydrates Clear BSR Low saturation (<20%) Free gas Massive deposition in near-fluid sediments Predominantly biogenic Arctic Hydrates No clear BSR High saturation ~40-80% No obvious free gas zone Irregular distribution in sandier sections Mixed biogenic and thermogenic Edwards, 1997, Geophysics, 62, P63 Arctic Gas Hydrates Map of Mallik hydrate deposits 110,000 x 10 6 m 3 (gas volume trapped as gas hydrate) 60% of Canada s annual gas production (Collett et al, 1999) 10

11 Arctic Gas Hydrates Mallik 2002 Program Production plan Expanded consortium (seven international partners) 1200 m deep main well, plus two observation wells Full scale field experiments in depressurization and thermal stimulation Objectives: I) assessment of production potential 2) stability under climate change Science Operator: Geological Survey of Canada (Scott Dallimore) Well Operator: Japan Petroleum Company Canada (JAPEX) 11

12 Drilling plan Mallik 3L-5L 5L-2L (2002) Well log section Mallik 2L-38 (1998) Seismic methods Surface seismic (time lapse) (D. Schmidt, University of Alberta; M. Reidel, Pacific Geoscience Centre) 12

13 Surface seismic (time lapse) Surface seismic (time lapse) (D. Schmidt, University of Alberta; M. Reidel, Pacific Geoscience Centre) (D. Schmidt, University of Alberta; M. Reidel, Pacific Geoscience Centre) VSP VSP (B. Milkereit, U of T; Contractor: Schlumberger) (B. Milkereit, U of T; Contractor: Schlumberger) 13

14 Crosshole method Fast Acquisition - Processing/Interpretation Source Well Receiver Well One File for each Fan Properties Structure Advanced Interpretation M. Weber & K. Bauer (GFZ, Potsdam), G. Pratt (Queen s), S. Shimizu (JNOC); Contractor: Tomoseis Crosshole method (Tomoseis( Tomoseis) Crosshole method (Tomoseis( Tomoseis) 14

15 Crosshole method (Tomoseis( Tomoseis) Waveform tomography final results Results at 5L-38 Velocity Attenuation Attenuation Production plan Gas flaring at Mallik (Posted on the GSC web site, March 12, 2002) 15