The Effect of Freezing of Slightly Consolidated Cores

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Summary This paper presents laboratory experimental techniques, procedures, and routine core analysis data of unconsolidated to slightly procedures, and routine core analysis data of unconsolidated to slightly consolidated core plugs from North Sea reservoirs. The effect of freezing on permeability has been investigated, and results from about 30 core plugs with permeabilities ranging from 0.2 to 12 darcies indicate a considerable change in permeability after freezing. Both increased and decreased permeability were obtained after freezing, For one rock type, the freezing permeability were obtained after freezing, For one rock type, the freezing seemed to increase the permeability in cores with initial permeability less than 2 darcies, while decreasing the permeability of core plugs with initial permeability greater than 2 darcies. The observed changes in permeability depend on the amount of kaolinite in the core plugs and the permeability depend on the amount of kaolinite in the core plugs and the changes in kaolinite morphology during freezing and thawing. Introduction Freezing of rock is a widely used technique for both core handling and preservation. In the North Sea, freezing is most frequently applied in the storage of unconsolidated rock and in drilling core plugs from unconsolidated material. Little consideration has been given, however, to how freezing affects the core material. Previous works consider the change in permeability and porosity after freezing of consolidated plug samples containing water and report an average increase in permeability and porosity after freezing, yet decreasing values are also porosity after freezing, yet decreasing values are also observed. The average change in permeability ranges from 5.2 to 18.6%, while the corresponding values for porosity are 0.7 to 9.6%. No conclusive trends are observed between the effects of freezing and water saturation, initial porosity, or initial permeability. porosity, or initial permeability. Three studies that investigate the effect of freezing on permeability are presented here. The core materials are from reservoir rock in the Troll and Snorre fields in the North Sea, which are Upper Jurassic sandstones. Background When water freezes and expands, this expansion is independent of the rate of freezing. The crystal size is dependent on the rate of freezing, however: a few large crystals result from a low rate of freezing and many small crystals from a high rate. When water expands inside the core, it is probable that the internal grain structure will be altered. After the thawing process, there is no guarantee that the grains will be oriented in exactly the same manner as before freezing, especially in unconsolidated and friable rocks, where the original cement bonds between grains are weak. Clay materials can be loosened easily from the pore walls during the freeze/thaw process, and these times may start moving in a fluid-flow process, and these times may start moving in a fluid-flow situation. The effect of water expansion on the rock material is probably a function of water saturation. Most cores, probably a function of water saturation. Most cores, however, contain gas and oil in addition to water. Oil and especially gas will tend to act as a buffer against the water expansion. Some authors claim that water expansion will have no effect as long as the gas saturation is greater than 10%. In a water-wet rock, however, the water is concentrated within the smallest pores. pore throats, and grain contacts, thereby reducing the effect of gas and oil as a buffer. The effect of ice on the mineral grains depends on the structure and strength of the minerals. For minerals with a smooth surface and no surface vugs, such as quartz, it appears natural that the ice will have no effect. For minerals with a surface that water can penetrate, water expansion may alter the mineral structure. As an example, a kaolinite mineral is shown in Fig. 1. In this figure it appears that freezing water between the layers may burst the mineral, producing small. migrating particles. It is well known that some of the water bound to the clay will never freeze; however. this interstitial water layer is probably thinner than the distance between the constituents probably thinner than the distance between the constituents of the clay mineral.

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