Environmental Engineering Reference
In-Depth Information
2
Shale Resource Systems - Concepts
Shale plays exhibit high levels of heterogeneity and present complex
geology/mineralogy, with ''nanopores'' and very low matrix permea-
bility, generally with natural fractures, and gas storage related to both
porosity and adsorption on both organic matter and clay minerals.
Comprises a hydrocarbon source rock which is likely to have previously
charged ''conventional'' plays within the basin with oil and gas.
Non-expelled hydrocarbons retained in source rock - shale gas, al-
though often ''liquids-rich'', dependent on thermal maturity.
Local migration into associated more-permeable lithologies - shale oil
''hybrid'' reservoir.
No ''natural'' permeable reservoir: equivalent to trap cap-rock or seal in
''conventional'' plays - porous but with very small pore throats.
Reservoir man-made - hence the need for long horizontal wells and
hydraulic fracturing.
Shale ''plays'' can be described as fine-grained reservoirs defined by a micro-
structure with a
o
5 mm grain size, of variable lithology and mineralogical
composition, characterised by nanometer-scale pores (''nanopores'') with ex-
tremely small pore throats and negligible permeability, which are producible
only by multiple-frac'ed horizontal wells. Many, but not all, such reservoirs are
organic-rich and are generally hydrocarbon source rocks with a variable ther-
mal maturation state, examples being the Barnett, Marcellus, Haynesville and
Eagle Ford Shales. The Niobrara Formation is a typical carbonate-rich, non-
source shale reservoir. Mineralogy is variable and is critical to a successful
shale play, generally requiring a large proportion of ''brittle'' minerals such as
silica or carbonate forming detrital matrix and cement components compared
to ''ductile'' clay and organic matter components. Silica-rich shale plays in-
clude the Barnett, Marcellus, Mowry and Duvernay Shales, while the Eagle
Ford, Niobrara and Vaca Muerta formations are carbonate-rich shale plays.
The Haynesville Shale is a mixed silica-carbonate shale play but, importantly,
none of these shales is totally clay-rich, although the Marcellus can reach up to
50% clay. Natural fractures are commonplace, generally related to local
faulting and to development of regional tectonic stress patterns: these can be
either open or cemented by minerals such as calcite and can influence the
effectiveness of well stimulation. The aim of hydraulic fracturing a well is to
achieve a large artificial ''flow-unit'' or stimulated reservoir volume by creating
a complex fracture network to connect the hydrocarbon-bearing ''nanopores''
with the well-bore. Small-scale local fracture systems can be exploited and
extended from the well-bore into the reservoir by the stimulation, but ce-
mented fractures may be deleterious as the cements generally have a lower
fracture threshold than the surrounding shale and much of the stimulation
energy is diverted into opening these fractures rather than the stronger shale
matrix. Gas storage within organic-rich shales occurs as both free gas within
the pores of the rock matrix and any fracture system and, importantly,
