|
Titel |
Methane adsorption and micropore characterisation of Posidonia shales and isolated kerogen |
VerfasserIn |
Thomas Rexer, Eliza Mathia, Andrew Aplin, Mark Thomas |
Konferenz |
EGU General Assembly 2013
|
Medientyp |
Artikel
|
Sprache |
Englisch
|
Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 15 (2013) |
Datensatznummer |
250077317
|
|
|
|
Zusammenfassung |
Gas in shales is stored in three different states: adsorbed, compressed (free) and dissolved;
quantifying each underpins calculations of gas storage capacity and also the mechanisms by
which gas must be transported from pore (surface) to fracture to well. While compressed gas
dominates in meso- and macropores, it is often assumed that (a) sorbed gas occurs mainly in
micropores (< 2nm) and (b) micropores are mainly associated with organic matter. Here, we
test those ideas by characterising the porous structure of six shales and isolated kerogens
from the Posidonia Formation by low-pressure sorption techniques and in combination with
high pressure methane sorption isotherms at 45, 65 and 85Ë C. Together, these data help us to
understand the extent to which (a) small pores control CH4 sorption and (b) whether
small “sorption” pores are associated with the organic and inorganic phases within
shales.
Samples were selected with vitrinite reflectance of 0.6, 0.9 and 1.45%. Pore volumes –
named sorption pore volumes here - were determined on dry shales and isolated
kerogens by CO2 isotherms measured at -78Ë C and up to 0.1 MPa. These volumes
include micropores (pore width < 2nm) and narrow mesopores; according to the
Gurvitch Rule this is the volume available for sorption of most gases. Sorption
pore volumes of both immature and gas window shales range from 0.010 to 0.021
cm3 g-1, accounting for 33 - 43% of total porosity. For shales in the oil window,
sorption pore volumes are 55 – 87% of total porosity. Whilst sorption pore volumes of
isolated kerogen are much higher, between 0.095 – 0.147 cm3 g-1, normalization by
TOC shows that only half the shales’ sorption pore volume is located within the
kerogen.
Methane sorption isotherms at 45Ë C for shale show excess maxima between
0.07 – 0.12 mmol g-1. Uptake on isolated kerogen is much higher, varying from
0.47 mmol g-1 in immature kerogen to 1.02 mmol g-1in gas window kerogen.
Combining kerogen and shale isotherms shows that around half the CH4 sorption
in shales occurs within the organic matter, entirely consistent with the division
of sorption pores between organic and inorganic phases. There is a very strong
correlation between the maximum methane excess uptake and the sorption pore
volume indicating that methane sorption capacities are indeed governed by this
volume. Our experiments were performed on dry shales; in the subsurface, it is
possible that inorganic pores are water-filled and thus unavailable as sorption sites. |
|
|
|
|
|