Environmental Engineering Reference
In-Depth Information
Figure 16 Appalachian Basin cross-section showing the relationship between the
deep Marcellus Shale Formation reservoir and shallow aquifers.
reservoir indicates sealing from shallower layers by regional seals within the
overlying section. This thickness of overburden is more than sucient to
prevent vertical connection between a completed well and near-surface
aquifers, as illustrated for the major shale plays in the USA, 54 the largest frac
on record attaining a height of 588 m but with a chance of the frac-height
exceeding 350 m of around 1%. 56 However, circumstances can be envisaged
where this overburden seal may compromised and Figures 17(a) and 17(b)
illustrate potential gas leakage scenarios during drilling and completion. In
shallow sections by penetration of shallow gas pockets, influx of gas due to
lowering pressure by borehole depletion or leakage from deeper sources
which could reach shallow aquifers along the borehole or via faults pene-
trated by either the vertical or horizontal sections of the well-bore; these
risks can be mitigated by proper drilling procedures with the use of drilling
mud of adequate weight and composition and with the subsequent setting of
additional surface casing deeper to below any potential aquifer or fault
zones, sealed by full cementing. Failure of well casing and abandoned well
capping integrity is cited as the most likely cause of well-related gas leak-
age. 62 Completion of well-bore sections containing faults and natural frac-
ture is a further potential risk and should be avoided; at best, frac'ing such
intervals is a waste of frac energy, fluids and proppant, while the risks are
much greater as faults can be re-activated as conduits to overlying water-
bearing sections which can flow into the well, resulting in loss of the well
due to water rather than hydrocarbon production, with the potential risk of
contamination of the water-bearing section with drilling and completion
fluids and, potentially, gas.
As discussed above, minor faults and fractures zones should be avoided in
production well-bores, either vertical or horizontal, as completing across
such zones greatly increases the risk of well failure, contamination by
leakage of frac fluids or hydrocarbons and the risk of low-level seismic ac-
tivity. Disposal of fluids into the sub-surface has been implicated in several
 
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