A Study of Factors Influencing the Mechanical Properties of Deep Permafrost
Authors: Ruedrich R.A. , Perkins T.K. 
Published: December 2007

In a detailed experimental investigation, seven factors affecting the mechanical behavior of permafrost have been evaluated. That behavior is dictated by a combination of the resistance of ice to deformation and the elastic and flow behavior of the soil constituents.

Introduction

The permafrost in the arctic regions has become a major new design factor for the practicing petroleum engineer. The drilling and completion practices of the petroleum industry must be modified in view of the petroleum industry must be modified in view of the mechanical and thermal behavior of the permafrost. At Prudhoe Bay, for example, completion procedures will be greatly influenced by the presence of nearly 2,000 ft of permafrost consisting of layers of gravel, sand, silt, and clay that are permanently frozen. Studies of the thermal behavior of wells completed in permafrost have indicated that some thawing around wellbores is generally to be expected during drilling and production operations. other studies have shown that pressures will rise as saturated, thawed regions around a wellbore are refrozen. The level to which external pressures rise will be significantly influenced by the elastic and flow behavior of the surrounding frozen soil. There are a number of publications dealing with the mechanical behavior of frozen soils. Much of the civil engineering literature deals with the behavior of shallow permafrost at relatively low stress levels. Chamberlain et al., however, have reported behavior at very high stress levels that might be of interest for nuclear excavation or detection. In this paper we report a study of the factors influencing the mechanical behavior of deep permafrost. We shall discuss the influence of these permafrost. We shall discuss the influence of these seven factors:confining stress,pore pressure,mineralogy,electrolyte concentration,temperature,strain, andstrain rate.

These data can be incorporated into numerical or analytical models to predict stresses produced by refreezing thawed zones in the permafrost.

The Interaction of Ice and Soil

Since permafrost is a composite material, we should expect it to exhibit mechanical characteristics similar to those of its constituents, ice and soil. A formal approach to ice/soil interaction is enlightening. Fig. 1 illustrates a cylindrical element of permafrost subjected to a total axial stress, at, and a total lateral stress, lt. Some fraction of the axial stress, as, is carried through the soil matrix; and the remainder, ai, is carried through the ice phase.(1)at = as + ai,

Similarly, the total lateral stress can be divided into that portion carried by the soil matrix, ls, and a portion carried through the ice phase, li. portion carried through the ice phase,(2)li.lt = ls + li

The uniaxial strength (i.e., when the sample is loaded along the axis but no lateral force is applied) of pure multicrystalline ice samples is known to be a function of temperature, strain rate, and strain. It has been reported in the literature, and confirmed in our laboratories up to lateral stresses of 1,000 psi, that for ice under triaxial stress, the difference between axial and lateral stress is a function only of temperature, strain rate, and strain.

JPT

P. 1167