With the petroleum industry endeavouring to develop oil and gas fields in deepwater and to increase its activities in onshore arctic environments, greater emphasis should be placed on quantifying the hazards to drilling operations posed by gas hydrates. In spite of gas hydrate-bearing sediments having been drilled successfully in the past, it is important, as future drilling operations progress into deeper and ultradeep waters, to develop a sound understanding of gas hydrate-related hazards and thereby identify ahead of time when problems are likely to occur. It is also highly desirable to define the envelope of risk-free operations so that unnecessary costs and delays associated with mitigation of the problems are not incurred.
This paper describes the specific requirements to develop a comprehensive risk management capability for drilling in gas hydrate-bearing sediments. The methodology adopted concentrates on reducing risk and uncertainty associated with gas hydrates and associated shallow gas geohazards in deep water. These will be achieved by means of determining the petrophysical, mechanical and thermodynamic properties of gas hydrate-bearing sediments, and developing a fully coupled model for wellbore stability in the sediments, and methodology for drilling fluid design optimization and a risk assessment and optimization framework. Examples of laboratory measured petrophysical and mechanical properties of gas hydrate-bearing sediments, and results of time-dependent wellbore stability in such sediments are presented and discussed.
Introduction
With the petroleum industry endeavouring to develop oil and gas fields in deepwater and to increase its activities in onshore arctic environments, greater emphasis should be placed on quantifying the hazards to drilling operations posed by gas hydrates. Hydrates, or clathrates, are crystalline compounds formed as a result of combination of suitably sized non-polar or slightly polar molecules with water under low temperature and high pressure conditions. The necessary conditions for their formation could exist within the sediments of continental margins where thermogenic or biogenic sources of gas are present[1]. Biogenic source gas is generated from breakdown of organic matter by bacteria under the seafloor whilst gas of thermogenic source is from hydrocarbons generated much deeper that have flowed upward through fractures and faults. Many studies suggest that the source of naturally occurring hydrates is predominantly biogenic[2], composed maily of methane (generally >95%), carbon dioxide, ethane and traces of higher hydrocarbons.
Drilling and testing efforts have been on-going in three hydrate accumulations viz. Mallik field in Northwest Territories, Canada, Prudhoe Bay/Kuparuk River area in Alaska, USA and Nankai Trough in offshore Japan[2,3,4]. Problems associated with stabilisation of in-situ hydrates so as to allow wells to be drilled safely with minimal gas levels have been reported in the development of the Cascade Field in the North Slope, Alaska[5]. Various other operating companies have described a range of problems associated with gas hydrates and free gas pockets, including uncontrolled gas release (including blowouts), fire, stuck pipe, catastrophic well-site subsidence, disruption of casing cementing operation, gas leakage to the surface outside of the casing and casing collapse[3].
The cost of a deep water well can be up to $70 million with the cost of drilling amounting to tens of millions of dollars. Although gas hydrates potentially pose significant hazards to drilling operations, the documentation of actual incidents is somewhat limited. The operational database is lacking because little monitoring has taken place, there are few well logs available from the shallowest section and this section of the well has typically been drilled without the use of a riser return system. A cost effective approach to minimizing exposure to hydrate-related drilling risks must take into account the general operational environment of deep and ultradeep waters. Hence, a greater emphasis should be placed on quantifying the hazards and risk to drilling operations, including those in gas hydrates-bearing sediments, and developing risk envelope for safe deep water drilling.
