65 / 96
A GLOBAL OUTLOOK ON METHANE GAS HYDRATES
65
• Controlling drilling rates to penetrate and case the hy-
drate-bearing strata quickly in order to stabilize the gas hy-
drate interval, while allowing sufficient time to remove the
gas hydrate or the free gas contained in mud returns; and
• Using cements with low heat of hydration for casings in
order to establish a good bond between the casing and the
surrounding formation, while minimizing thermal heat-
ing and local gas hydrate dissociation.
Over the past decade, many dedicated gas hydrate field inves-
tigations have been conducted worldwide (Text Box 3.1). These
have demonstrated that gas hydrates can occur in a variety
of reservoir settings with different overburden/underburden
sediments and physical properties of the host strata (e.g., gas
hydrate form, thickness, sediment porosity, permeability, ther-
mal properties, pressure, and temperature regimes). Such
reservoirs also vary widely in the degree of heterogeneity in
important parameters such as gas hydrate saturation, perme-
ability, and enclosing sediment characteristics. As with con-
ventional hydrocarbon fields, the specific drilling technologies
and methods employed to exploit gas hydrates depend on the
local geology and environmental setting.
A summary of drilling considerations for various gas hydrate
deposits is provided in Table 3.1. Well designs may include high-
angle, horizontal, and multi-lateral wells (Hancock
et al.
2010).
Inmarine settings, drilling will be carried out fromfloating drill-
ing structures or drill ships, employing technologies routinely
used by industry for activities in water depths of 500 to 2 000
metres (Anderson
et al.
2011; Figure 3.3). Drilling hazards and
associated environmental risks are likely to be similar to those
faced when drilling deep conventional wells, where the risks of
shallow groundwater flow, overpressure, and shallow free gas
must be assessed (Aubeny
et al.
2001; Kvalstad
et al.
2001).
Additional environmental risks relate to the challenge of
drilling and well completion in the relatively shallow depth
of many marine gas hydrate production targets, some of
which are at depths of less than 300 metres below the sea-
bed. Where soft sediments occur near the seabed, special
care will be required in the design of shallow surface casings
to carry the load of the well infrastructure. Similarly, the in-
termediate casings between the production interval and the
surface casing must be designed to ensure zonal isolation
Figure 3.3:
Marine drilling platforms. These platform designs are
currently used in various deepwater settings around the world. The
tension-leg system is founded on the bottom, whereas the other
systems are floating structures (Figure from Lamb, Robert. “How
Offshore Drilling Works” 10 September 2008. HowStuffWorks.com).
and to prevent vertical migration of produced gas through
the wellbore annulus towards the seabed. Considerable ef-
forts are in progress to improve well-bore simulation models
for gas hydrates in order to allow detailed risk assessment
and identification/ consideration of optimal drilling practice
(Birchwood
et al.
2005; Rutqvist and Moridis 2008; Rutqvist
et al.
2008; Yamamoto 2008).
In onshore areas where the gas hydrate production interval
is beneath ice-bonded permafrost, drilling technologies are
likely to be similar to those employed on the North Slope of
Alaska (Hancock
et al.
2010). A typical well design will in-
clude a shallow surface casing and an intermediate casing
that spans the permafrost interval. As with marine gas hy-
drates, Arctic gas hydrate wells will require an assessment of
the risk of overpressure and free-gas migration while drilling
through the permafrost interval.