Managing Wellbore Instability Risk in Gas-Hydrate-Bearing Sediments

Abstract

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.

RETRACTED ARTICLE: Modeling and molecular simulation of natural gas hydrate stabilizers

“…From a macroscopic aspect, the hydrate crystal dissociation rate can be greatly reduced by lecithin in the liquid phase. The results of the laboratory evaluation (Chen & Kamath, 2006;Jianguo et al, 2010;Yan et al, 2012) and field application (Schofield et al, 1997;Uchida et al, 1999) also confirmed that lecithin is a very effective type of hydrate stabilizer (or promoter). Thus, the simulation results in this paper conformed with the objective laws.…”

“…However, hydrate is a safety hazard in drilling operations. When drilling in NGH sediments, the changes in temperature and pressure conditions as a due to in-situ formation disturbance can result in hydrate decomposition, thereby resulting in a series of drilling problems, such as borehole wall instability (Tan et al, 2005), gas-cutting trouble, and drilling fluid performance deterioration, which can result in submarine landslides and serious environmental pollution (Glasby, 2003;MacDonald, 1990;Paull et al, 1996).…”

Polyethylene Glycol Drilling Fluid for Drilling in Marine Gas Hydrates-Bearing Sediments: An Experimental Study

“…The dissociation of gas hydrate can lead to a series of problems related to drilling safety. In the case of an uncased borehole, the gas-hydrate dissociation will cause the borehole to become unstable, and the gas released by gas-hydrate dissociation will erode the drilling pipe and leak at the seafloor [9][10][11][12][13]. The gas circulated with drilling fluids may also re-form as a hydrate, blocking the drilling pipe terminating the fluid circulation [14][15][16].…”

Design Considerations for Isolating Gas-Hydrate-Bearing Zones in Deepwater Environments

“…Investigation of natural hydrates in various drilling fluids to determine the equilibrium curves and salt requirements in deepwater drilling muds was carried out by Lai and Dzialowski (1989). Tan et al (2005a and2005b) illustrates time-dependent wellborestability analysis in gas-hydrate-bearing sediments by means of laboratory measured petrophysical and mechanical properties. The approach combines engineering simulations with expert knowledge under consideration of uncertainty to assist engineers during scenario planning.…”