NUMERICAL MODELLING OF WELLBORE STABILITY IN HYDRATE-BEARING SEDIMENTS

Reem Freij-Ayoub¹, Chee Tan¹, Ben Clennell¹, Jinhai Yang², and Bahman Tohidi^2
1 CSIRO Petroleum, ARRC, 26 Dick Perry Avenue, Kensington, Western Australia 6151
² Institute of Petroleum Engineering, Heriot Watt University, Riccarton, Scotland EH14 4AS

One volume of gas hydrates can contain up to 180 volumes of gas that can be released if the hydrates are destabilized. As a result gas hydrates can pose significant hazards to drilling and production operations in the deepwater environment.

Drilling causes stress redistribution around the wellbore that can, in the case of weak formations or high in-situ stresses, result in instability. The associated delays and suspension of drilling will lead to significant financial losses. Drilling in gas hydrate-bearing sediments poses an additional risk but this risk is difficult to quantify. There is uncertainty about whether hydrates are present in a locality and how much hydrates will be encountered. Added to this is a lack of knowledge of the mechanical characteristics of hydrate-bearing sediments and a poor understanding of the effect of drilling activities on their stability conditions, their dissociation and re-formation. The general strategy of drilling operators is to avoid clearly-defined hydrate occurrences where possible, but where hydrates are only suspected no mitigation is employed. It is therefore desirable to understand and quantify the risks of drilling through hydrates so that the envelope for safe operations can be defined for different types and concentrations of hydrate occurrence in submarine sediments.

In this paper, we couple the mechanical and thermodynamic responses of hydrate-bearing sediments due to the stress redistribution resulting from drilling a wellbore and the use of drilling fluid. Fluid flow and thermal transport associated with using a drilling fluid at different pressure and temperature from the formation is included in the model.

We use a continuum approach and a finite difference code, FLAC3D. The model is constructed to simulate the stability of a wellbore drilled in a formation resembling methane hydrate-bearing sand sediments. The model couples fluid flow and thermal transport to mechanical deformation. The formation mechanical behavior is represented by a Mohr Coulomb yield criterion with strain hardening or softening. The pore pressure distribution is modeled using a single-phase Darcian flow model. The effect of the varying pore pressure on the stress field is accounted for in the stability analysis.

The cohesion softening/hardening accounts for varying concentration of gas hydrates in the pore space as a result of hydrate dissociation and re-formation. Thermodynamic data of pressure and temperature corresponding to the stability of gas hydrates is used to simulate the change in hydrate stability as a result of the change in system pressure associated with drilling, fluid flow and thermal transport. The resulting amount of hydrates in the pore space can be calculated accordingly, using Boyle’s law to equilibrate pressures. The incremental change in porosity can then be calculated. The permeability is updated using an empirical power law relationship based on our own experimental results. The cohesion corresponding to the new percentage of hydrates is then fed back into the model. The cohesion and other mechanical properties are obtained from laboratory experiments for a range of hydrate concentrations. Strength parameters for porous sediments cemented by hydrates, based on correlations between cementation strength and elastic moduli deduced from Vp, Vs and small strain deformations measured in the Heriot Watt Laboratory will be used in addition to published data.

The numerical model helps to understand the response of hydrate-bearing sediments to processes associated with overbalance drilling of a wellbore. The general approach could also be applied to modeling the time-dependent response of hydrate-bearing sediments to thermal and mechanical loadings placed on the seabed.