New Pore-scale Considerations for Shale Gas in Place Calculations

Abstract

Using FIB/SEM imaging technology, a series of 2-D and 3-D submicro-scale investigations are performed on the types of porous constituents inherent to gas shale. A finely-dispersed porous organic (kerogen) material is observed imbedded within an inorganic matrix. The latter may contain larger-size pores of varying geometries although it is the organic material that makes up the majority of gas pore volume, with pores and capillaries having characteristic lengths typically less than 100 nanometers. A significant portion of total gas in-place appears to be associated with inter-connected large nano-pores within the organic material. This observation has several implications on reservoir engineering of gas shales. Primarily, thermodynamics (phase behavior) of fluids in these pores are known to be quite different. Most importantly, gas residing in a small pore or capillary is rarefied under the influence of organic pore walls and shows a density profile across the pore with damped-oscillations. This raises the following 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 for the free gas phase and what fraction is taken up by the adsorbed phase? If a significant fraction of the organic pore volume 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 and how large would a typical density contrast be in an organic pore for an accurate gas reserve calculation? In order to answer these questions we combine the Langmuir equilibrium adsorption isotherm with the volumetrics for free gas and formulate a new gas-in-place equation accounting for the organic pore space taken up by the sorbed phase. The method yields a total gas-in-place prediction based on a corrected free gas pore volume that is obtained using an average adsorbed gas density. Next, we address the fundamental-level questions related to phase transition in organic matter using equilibrium molecular dynamics simulations involving methane in small carbon slit-pores of varying size and temperature. We predict methane density profiles across the pores and show that (i) an average total thickness for an adsorbed methane layer is typically 0.7 nm, which is roughly equivalent to 4% of a 100 nm diameter pore volume, and (ii) the adsorbed phase density is 1.8-2.0 times larger than that of the bulk methane, i.e., in the absence of pore wall effects. These findings suggest that a significant level of adjustment is necessary in volume calculations, especially for gas shales high in total organic content. Finally, using typical values for the parameters, we perform a series of calculations using the new volumetric method and show a 10-25% decrease in total gas storage capacity compared to that using the conventional approach. This additionally could have a larger impact in shales where the sorbed gas phase is a more significant portion of the total gas-inplace. The new methodology is recommended for estimating shale gas-in-place and the approach could be extended to other unconventional gas-in-place calculations where both sorbed and free gas phases are present.