SPE Journal
Volume 17, Number 1, March 2012, pp. 219-229

SPE-131772-PA

Shale Gas-in-Place Calculations Part I: New Pore-Scale Considerations

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DOI  More information 10.2118/131772-PA http://dx.doi.org/10.2118/131772-PA

Citation

  • Ambrose, R.J., Hartman, R.C., Diaz-Campos, M., Akkutlu,I.Y., and Sondergeld, C.H. 2012. Shale Gas-in-Place Calculations Part I: New Pore-Scale Considerations. SPE J.  17 (1): 219-229. SPE-131772-PA. http://dx.doi.org/10.2118/131772-PA.

Discipline Categories

  • 6.9.2 Shale Gas
  • 6.7.1 Estimates of Resource in Place
  • 6.2.1 Phase Behavior and PVT Measurements

Keywords

  • Shale, Gas-in-place, Correction

Summary

Using focused-ion-beam (FIB)/scanning-electron-microscope (SEM) imaging technology, a series of 2D and 3D submicroscale investigations revealed a finely dispersed porous organic (kerogen) material embedded within an inorganic matrix. The organic material has pores and capillaries having characteristic lengths typically less than 100 nm. A significant portion of total gas in place appears to be associated with interconnected large nanopores within the organic material.

Thermodynamics (phase behavior) of fluids in these pores is quite different; gas residing in a small pore or capillary is rarefied under the influence of organic pore walls and shows a different density profile. This raises serious questions related to gas-in-place calculations: Under reservoir conditions, what fraction of the pore volume of the organic material can be considered available as free gas, and what fraction is taken up by the adsorbed phase? How accurately is the shale-gas storage capacity estimated using the conventional volumetric methods? And finally, do average densities exist for the free and the adsorbed phases?

We combine the Langmuir adsorption isotherm with the volumetrics for free gas and formulate a new gas-in-place equation accounting for the pore space taken up by the sorbed phase. The method yields a total-gas-in-place prediction. Molecular dynamics simulations involving methane in small carbon slit-pores of varying size and temperature predict density profiles across the pores and show that (a) the adsorbed methane forms a 0.38 nm monolayer phase and (b) the adsorbed-phase density is 1.8 - 2.5 times larger than that of bulk methane. These findings could be a more important consideration with larger hydrocarbons and suggest that a significant adjustment is necessary in volume calculations, especially for gas shales high in total organic content. Finally, using typical values for the parameters, calculations show a 10 - 25% decrease in total gas-storage capacity compared with that using the conventional approach. The role of sorbed gas is more important than previously thought. The new methodology is recommended for estimating shale gas in place.

© 2011. Society of Petroleum Engineers

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History

  • Original manuscript received: 19 January 2010
  • Meeting paper published: 24 February 2010
  • Revised manuscript received: 2 December 2010
  • Manuscript approved: 11 December 2010
  • Published online: 7 November 2011
  • Version of record: 13 March 2012