Environmental Engineering Reference
In-Depth Information
adsorbed onto organic matter and clay mineral surfaces, particularly the walls
of organic-matter hosted pores. While free gas predominates,
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the relative
importance of adsorbed gas is highly variable, both within and between shales,
varying as a function of the amount and type of organic matter present, pore
size distribution, mineralogy, level of diagenesis or maturation state and res-
ervoir conditions (temperature and pressure).
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Shale resources represent a complete petroleum system,
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comprising the
hydrocarbon source, reservoir and seal within a single rock unit.
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They are
generally described as ''statistical plays'', as each well not only accesses a
separate portion of the continuous shale reservoir with its very low inherent
permeability and variations in geological parameters, but also due to the re-
quirement for the completion process to create an artificially permeable res-
ervoir. Hydraulic fracturing of horizontal reservoir sections, optimally located
within the shale, is the key to successful access to such resources. While shale
resources can be determined by well log data calibrated to cores, as discussed
below, due to the need to artificially create a permeable reservoir post-drilling,
conversion to reserves is necessarily made by the use of production data to
determine the Estimated Ultimate Recovery (EUR).
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The EUR for a shale well is
largely dependent on the following factors discussed below:
Shale quality (thickness, richness, reservoir potential);
Horizontal well-bore length;
Number of hydraulic fractures created in the reservoir section; and
Effectiveness of these fractures in accessing the hydrocarbon-bearing
pores.
The EUR is calculated from the well initial production rate (IP) in com-
bination with an estimated type decline curve (see Figure 4) and is generally
in the range of 15-35% of the calculated gas resource (gas in place).
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Figure 4 Typical production decline curves for US gas shales.
