Abstract

In Arctic regions, permafrost and gas hydrate bearing sediments below the seabed present a technical problem for drilling operations. The first and most important challenge is to be able to control the well if pressured gas is encountered in or below such formations, and the second problem is the stability of the borehole if these sediments begin thawing during drilling. Both these issues are strongly dependent on the choice of mud - and mud-weight. In the present paper we outline a methodology that can aid mud and mud-weight choices during Arctic drilling. It involves using a numerical borehole stability code, which calculates safe mud-weight windows by applying time-dependent physico-chemo-thermo rock mechanical models. It takes as input available information on the subsurface that is to be drilled, and if field data is not available it can be given input from scientific literature or from correlation analyses in a large database of laboratory test results on North Sea core samples.

As a part of the process of adapting the numerical borehole stability code to Arctic environments, several simulations were done using Arctic well parameters from literature as input. These are presented in this paper, and they reveal that the mechanical and petrophysical properties of gas hydrate bearing sediments make them difficult to drill - even without taking thawing or thermal/gas induced changes in mud rheology into account. Our analyses indicate that similar drilling problems as those encountered when drilling in shales are likely to be met also when drilling through clay/silt based permafrost and gas hydrate sediments. Even if our simulations do not include the tricky sediment thawing during drilling - the impact of this process on borehole stability is discussed. It is concluded that with more detailed input on the Arctic subsurface, the presented methodology has the potential to predict safe mud-weight windows for drilling through permafrost and gas hydrate bearing sediments - and it can thereby contribute to safer and more cost-efficient exploitation of Arctic petroleum resources.

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