Analysing sand-dominated channel systems for potential gas-hydrate-reservoirs on the Southern Hikurangi Margin, New Zealand

Transcription

1 Analysing sand-dominated channel systems for potential gas-hydrate-reservoirs on the Southern Hikurangi Margin, New Zealand Miko Fohrmann 1 and Ingo A. Pecher 1,2 1, Lower Hutt, New Zealand 2 Heriot-Watt University, Edinburgh, UK IASS Hearing on Clathrate Hydrates 08.&

2 Outline Introduction Framework for gas hydrate research in New Zealand Location and Data Observations Potential reservoir analogues Seismic interpretation Velocities AVO attributes Simultaneous Inversion Results Lambda-mu-rho (LMR) Rock-physics modelling Conclusions

3 Framework for gas hydrate research in New Zealand New Zealand s Gas Hydrate Provinces Hikurangi Margin most promising because of proximity to major population centres, and favourable geologic setting (similar to Nankai Trough)

4 Framework: Motivation Energy: New Zealand s largest conventional gas field is being depleted No other known reserves of similar size LNG import is expensive One of the world s most promising gas hydrate provinces represents a significant medium- to long-term opportunity for New Zealand. Flare, burning gas from gas hydrates (Mallik, 2002, Mackenzie Delta, Canada T. Collett, USGS)

5 Framework: Hikurangi Margin Promising hydrate deposits within the ~ km 2 large Hikurangi gas hydrate province may contain ~20 tcf of gas (Pecher and Henrys, 2003) NZ s gas consumption ~0.15 tcf/yr. Source of gas for decades? Controlled-source electromagnetics data acquired at Opouawe Bank, indicated significant gas hydrate concentrations of up to 30% of the pore space in the upper 400 mbsf alone (Schwalenberg et al., 2010) Ice-like gas hydrate veins in sediment cores from the Hikurangi Margin (courtesy U. Washington)

6 Framework: Funding New Zealand Foundation for Research, Science, and Technology: Gas Hydrates Resources, collaboration between, NIWA, University of Otago, University of Auckland (currently: ). Energyrelated applied research (Royal Society of New Zealand s Marsden Fund: Pockmarks on Chatham Rise linked to gas hydrates, : Climate-related basic science) Funding (and no. of scientists) scaled to 4.3 Mio people... (international collaboration!)

7 Developing New Zealand s Gas Hydrates Resources Our Vision Appraisal of gas hydrate reservoirs ($9 m) Research funding (costs include international leverage) Government and/or industry consortium (costs include site surveys) Scientific drilling ($20-$80 m) Mostly industry Production testing (>$100 m) Develop commercial prod. facilities ($4.5 b) Figures in NZ$ 1NZ$ = 0.75 US$ Industry Prod. 150 PJ/yr ($1 b/yr) Industry exploration for gas hydrate reservoirs ($100s m) Gas hydrate regulation Funded

8 Framework: Production Tests and Models Production tests for onshore Mallik wells, Canada (e.g., Kurihara et al., 2008) Results used for calibrating production models Preferred production method: Dissociation by depressurization Need permeability Reservoir: Sand Gas-hydrate-bearing sands close to the base of gas hydrate stability D/V JOIDES Resolution for explorationstyle drilling offshore India. No offshore production tests have been conducted yet but they are planned for Nankai Trough in 2012

9 Framework: Goals Beside all technical issues of recovering gas hydrates, the challenge lies in: 1) delineating potential high-quality gas hydrate reservoirs and 2) estimating the gas hydrate concentration stored in these reservoirs. A potential gas hydrate system may be located in the East Coast Basin, in which numerous bottom-simulating reflections (BSRs) have been identified. The aim of this study is to reveal whether individual gas-hydrate-bearing layers can be identified and whether they are likely to be sand-dominated. High-resolution velocity analysis to indicate the possible extent of GH AVO attribute analysis AVO inversion on inferred gas hydrate filled sand channels Rock-physics modelling

10 Location and data The multi-channel seismic reflection Bruin survey acquired in 2005 on allowed for detailed amplitude-versus-offset (AVO) analysis.

11 Processing sequence Aim: To conserve the AVO information Coherent and incoherent noise removal (e.g., swell noise attenuation and Radon demultiple) Consistent amplitude recovery (e.g., directivity and geometrical spreading) Pre-stack (partial) migration, i.e., Kirchhoff DMO Velocity analysis (semblance and constant velocity gathers)

12 Tectonic setting

13 Possible analogues for gas hydrate bearing sediments Turbidites of the Miocene Whakataki Formation (Field, 2005)

14 Seismic analogue Seismic line IAE1-28, flattened on a base of a channel-levee set Borehole image from Titihaoa- 1 taken by Schlumberger s FMI tool. Sandstone beds appear dark (Field, 2005)

15 Bathymetry

16 Observations

17 Observations Depth converted section

18 Velocity analysis

19 Angle gather

20 AVO attributes Intercept and gradient

21 AVO attributes - Intercept & gradient

22 AVO attributes - Intercept & gradient

23 Scaled Poisson s Ratio (A+B) We see the expected response for gas sands a negative (orange) change in Poisson s Ratio at the top and a positive (yellow) change at the base of the layer.

24 Independent AVO Inversion and Lambda-Mu-Rho To constrain the inversion results in order to address the non-uniqueness issue, AVO inversion uses the fact that the basic variables, Z P, Z S, and ρ are related. The model is derived from the velocity model due to the lack of well data We start with two relationships which should hold for the background wet trend: - The S-wave background model is computed using Castagna s equation (S wave =C1*P wave - C2) - Density is calculated using Gardner s relationship Prior to inversion, three wavelets (near angle or 0-15 degree, degree, and far angle or degree) were statistically derived from the seismic data.

25 Result at pseudo well location

26 P-impedance volume Increase in P-impedance either due to: - substantial compaction - presence of gas hydrates

27 Lambda-Mu-Rho (LMR) analysis Impedance reflectivities (I p and I s ) are related to Lamé parameters of incompressibility (λ) and rigidity (μ or shear modulus) by the relationships: λρ = I 2 p - 2I 2 s and μρ = Is 2 where: v p = ((λ+2μ) / ρ) 0.5 v s = (λ / ρ) 0.5 (Goodway, 1997)

28 Distribution of sands

29 Velocity distribution

30 Rock-physics modelling

31 Potential gas hydrate reservoir? The inferred gas hydrate saturation of approximately 25% makes this location a candidate for further more detailed gas hydrate exploration. Based on our seismic interpretation and porosity estimates of 35%, an average net/gross of 60%, and a saturation of 25-40% we postulate the presence of ~0.05 km 3 of pure methane hydrate or 7.75 km 3 (~ 0.27 tcf) methane at standard ambient temperature and pressure (SATP) at this location. The location of this potential high quality reservoir at the BGHS, would make it a suitable candidate for potential exploration as results from numerical studies indicate. Lower porous and permeable sands on top of the reservoir could act as a potential hydrate-cemented seal during production.

32 Conclusions Channel systems point to the presence of potential sand dominated gas hydrate reservoirs Seismic velocity analysis clearly identifies: a high velocity zone above the BSR that is likely to correspond to the presence of gas hydrates a low velocity zone below the BSR, which is interpreted to be caused by the presence of free gas AVO analysis reveals the presence of free gas and possibly indicates the TGHZ Results of the AVO inversion indicate a ~200 m thick gas hydrate zone LMR analysis indicate that the gas hydrates zone lies in a sand dominated environment. Rock-physics model suggests a GH saturation of 25%. The potential hydrate reservoir at the BGHS is favourable for exploration.

33 Acknowledgements The authors would like to thank: Foundation for Research, Science, and Technology (FRST, C05X0908 ) for funding this work. Ministry for Economic Development (MED) for providing the seismic data. National Institute of Water & Atmospheric Research (NIWA) for providing the bathymetry data.

34 References Barker, D. H. N., R. Sutherland, S. Henrys, and S. Bannister, 2009, Geometry of the Hikurangi subduction thrust and upper plate, North Island, New Zealand: Geochemistry Geophysics Geosystems, v. 10, p. 23. Castagna, J. P. and Swan, H. W., 1997, Principles of AVO crossplotting: The Leading Edge, 16, no. 04, Field, B. D., 2005, Cyclicity in turbidites of the Miocene Whakataki Formation, Castlepoint, North Island, and implications for hydrocarbon reservoir modelling: New Zealand Journal of Geology and Geophysics, 48, Goodway, B., Chen, T. and Downton, J., 1997, Improved AVO fluid detection and lithology discrimination using Lamé petrophysical parameters, 67th Ann. Internat. Mtg: SEG, Pecher, I. A., and S. A. Henrys, 2003, Potential gas reserves in gas hydrate sweet spots on the Hikurangi Margin, New Zealand, Institute of Geological & Nuclear Sciences science report 2003/23, p. 36. Pecher, I. A., M. Fohrmann, A. R. Gorman, and P. Barnes, 2010, New developments in gas hydrate exploration implications for the Hikurangi Margin: New Zealand Petroleum Conference, p. 10. Schwalenberg, K., Haeckel, M., Poort, J., and Jegen, M., 2010, Evaluation of gas hydrate deposits in an active seep area using marine controlled source electromagnetics: Results from Opouawe Bank, Hikurangi Margin, New Zealand: Marine Geology, v. 272, no. 1-4, p