Environmental Engineering Reference
In-Depth Information
borne in mind that these shales form the cap-rock seals to conventional
reservoired hydrocarbon traps. Natural fracture systems, both open and
cemented, are also an important feature of overall shale permeability and
complicate the measurement and determination of effective connectivity
and flow in shale reservoirs. Heterogeneities within the shales also impart
major controls; for example, permeability measurements parallel to bedding
planes can be up to an order of magnitude greater than those measured
perpendicular to bedding.
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Accurate permeability measurements are re-
quired to gain an understanding of the fundamental controls on hydro-
carbon flow within shales and, importantly, to derive parameters which can
be used in the construction of reservoir models used in the determination of
the commerciality of a shale play. These measurements also need to be re-
producible and comparable between different laboratories. Permeability
measurements have traditionally been measured on core plugs but these can
readily be seen to be dependent on many factors, not only on orientation of
the plug as discussed above, which often leads to these problems. In an
attempt to overcome the diculties and to produce a standardised, fast
method to determine shale permeability, the US Gas Research Institute de-
veloped a technique using crushed and sieved core,
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thus removing the
effects of both natural fracturing and rock heterogeneity. However, it should
be noted that these may be critical controls on fluid flow within a shale
reservoir. Measurements of both porosity and permeability in shales are
therefore very dependent on both the analytical methods and the experi-
mental conditions used.
Various flow types can occur in very fine-grained rocks, depending on the
nature of the porosity and fracturing:
Natural fractures and macro-pores tend to have Darcy flow (flow rate
proportional to pressure gradient);
Meso-Pores range from Darcy flow through transitional flow to Knudsen
diffusion (molecular slippage along the flow channel walls);
Micro-pores are restricted to Knudsen diffusion; and
Hydraulic fractures typified by turbulent non-Darcy flow or Darcy flow.
These observations suggest that while fracture flow, either natural or in-
duced, may dominate early reservoir performance, matrix properties control
performance over longer periods.
Non-steady-state permeability measurements show that the state of the
core, whether ''as received'', dried or reservoir/room temperature and pres-
sure, has a major effect on the analytical results, with increased moisture
decreasing permeability with increasing effective stress compared to dry
samples.
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The composition of the analytical gas is also critical,
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with
smaller size gas molecules having a greater flow potential. Confining stress
and pore pressure have also been shown to be important controls,
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with
permeability decreasing as confining stress increases, which can result
from reservoir depletion, while decreasing pore pressure increases the
