The Mechanical Behavior of Synthetic Permafrost

Abstract

Abstract Discoveries of oil in Arctic regions have led to several engineering problems that are relatively new to the petroleum industry. An understanding of some of the new problems associated with construction of surface facilities as well as with the drilling and completion of wells requires an understanding of the mechanical properties of permafrost. permafrost. Synthetic permafrost samples have been prepared from quartz sand as well as from natural soils taken from Prudhoe Bay permafrost cores recovered from depths as great as 1,753 It. All samples have been recompacted and frozen under a condition of zero confining stress. Samples prepared in this way should exhibit behavior similar to that of shallow permafrost. Samples have been tested in uniaxial permafrost. Samples have been tested in uniaxial compression at constant strain rates as well as with constant axial stress. At constant temperature and low strain rates, the log of the maximum shear strength will plot as a straight line vs the log of the strain rate. For sand-ice samples at high strain rate, another mode of failure was evident that led to a maximum shear strength independent of strain rate. Under triaxial conditions, the maximum shear strength of sand-ice samples was generally increased with increasing stress level. In uniaxial tension, the tensile strength of sand-ice samples was found to be a function of temperature and strain rate. Elastic response of these samples was obscured by the more dominant flow behavior at low strain rates. Only at clearly high strain rates was an elastic response clearly discernible. Young's modulus measured after 10 to 15 percent plastic strain increases with increasing stress level. Introduction Within the last few years significant oil discoveries have been made in Arctic regions. There is much speculation that additional oil will be found in regions that are characterized by quite low ambient and soil temperatures. The drilling of wells and production of oil under these environmental conditions poses new problems not traditionally faced by the petroleum industry, but which presumably will be of increasing concern within the presumably will be of increasing concern within the next few years. One new engineering challenge is that of dealing with permafrost, soil which has been continuously frozen for a number of years. Already at Prudhoe Bay a number of wells have been drilled through about 2,000 ft of permafrost. As an example of permafrost influence, measurements have shown that, when thawed permafrost around a well refreezes, significant pressures can be generated. In order to understand this phenomenon, it will be necessary to understand the mechanical behavior of permafrost. In addition, surface facilities have been permafrost. In addition, surface facilities have been constructed where there is a thin, active region (which thaws during summer months) underlain by permafrost. An understanding of permafrost permafrost. An understanding of permafrost mechanical behavior will aid in the design of foundations for surface facilities. There are a number of variables that can influence the mechanical behavior of frozen soils such as minerology, percent of ice saturation, presence of excess ice, salt content, etc. In this paper we will describe a laboratory study of relatively fine-grained granular materials with pore spaces saturated with ice. The results presented here may not be applicable to frozen clays or gravels, where pore spaces are undersaturated or where a large amount of excess ice is present. Since permafrost is composed of ice and soil, its behavior will naturally reflect that of its constituents. The rate of yield or flow of ice is known to be a function of temperature, shear stress and strain, but is independent of hydrostatic pressure level. Soil, on the other hand, exhibits pressure level. Soil, on the other hand, exhibits yield behavior that is independent of temperature over the small range of permafrost temperatures of interest. For sandy soil, yield behavior is relatively independent of strain rate, but is significantly influenced by strain and stress level. Under stress, a dominant characteristic of shallow permafrost is that of yield or flow. Its rate of flow will be a function of all the variables mentioned above. Over-all deformation results from a combination of elastic and flow behavior. SPEJ P. 211