FROZEN HEAT | Volume 2

• 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.

A GLOBAL OUTLOOK ON METHANE GAS HYDRATES 65