Environmental Engineering Reference
In-Depth Information
but cannot differentiate between ''good'' and ''not so good'' quality reservoir;
this is best identified from a cross-plot of bulk density versus V
p
: V
s
ratio.
Other combinations of V
p
, V
s
and bulk density, such as Young's Modulus,
Lambda-Rho and Mu-Rho, can be used for litho-typing, proxies for geo-
mechanical properties ('frac'ability') and potentially also for addressing fluid
effects. Seismic velocity within shales is strongly affected by lithological
variations and heterogeneities such as laminated organic rich and organic
poor units and vertical litho-type variations, leading to high vertical het-
erogeneity at all scales. In particular, strong vertical anisotropy can affect the
interpretation of micro-seismic data.
3-D seismic data are generally used for such shale-quality ''sweet-spot''
evaluation, but existing good quality 2-D regional seismic data can also be
utilised. In general, at shale thicknesses exceeding the seismic tuning effects
of the formation boundaries, an organic-rich shale containing gas in par-
ticular will have a much lower density than organic-poor or non-gas-bearing
shale, which creates a seismic attribute anomaly which can be mapped.
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Calibration of seismic attributes such P and S-wave anisotropy to well-log
and core data allows potentially resource-rich areas of the shale to be de-
termined and mapped, allowing leasing to be optimised and reducing the
need for the pilot hole drilling.
Seismic data are also critical in the location of wells, particularly with
lateral production wells typically extending 5000-10 000 feet from the ver-
tical pilot well location, as drilling across even very minor faults can lead to
lost circulation problems or displacement from the optimal well target
horizon, while completion across such faults can result in loss of hydro-
carbon production by water influx from a breached cap-rock or seismic ac-
tivity by fault-plane lubrication; seismic data along a proposed well-bore are
therefore critical for geo-hazard determination. Location of a well-pad from
which six to eight laterals may be drilled is also critical, with location pref-
erentially in areas of minimal faulting. Vertical pilot wells with detailed wire-
line logging and core data collection are generally drilled at spacing intervals
of around 5 km or when shale parameters have potentially changed, as in-
dicated by logs from non-cored wells or seismic-attribute variation.
Lateral production wells are generally drilled from the pad in opposite
directions perpendicular to normal to the direction of maximum horizontal
stress, the parallel well-bores generally being 600-800 feet apart. Geological
stress is the most important control of fracture growth,
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with fractures
propagating perpendicular to the direction of least principal stress, fol-
lowing the direction of maximum stress. At depths greater than 3000-4000
feet, maximum stress is generally due to overburden, so vertical fracture
propagation predominates, the horizontal direction being controlled by
tectonic stress direction. Fracture orientation can be obtained from image
logging of the reservoir interval of a vertical pilot hole, providing valuable
information for determining principal stress directions for drilling the lat-
eral production well.
