Seawater is injected into the high temperature North Sea chalk reservoirs to improve oil recovery with great success.Increased compaction of the rock,which also is a significant contribution to the oil displacement,is observed in the water-flooded area.Very recent laboratory studies at high temperature documented that enhanced compaction took place when chalk cores under stress were flooded by seawater.The special composition of seawater regarding reactive ions towards the chalk surface appeared to be the reason.In this paper,two different water-weakening mechanisms are discussed: 1) Substitution of Ca~(2+) from the chalk surface by Mg~(2+) present in seawater(dolomitization) at inter granular contacts and 2) Chemical dissolution.The effect of temperature is opposite for the two mechanisms,and in the present paper,the impact of temperature on chalk compaction is studied both in a static and dynamic way using triaxial cells under hydrostatic conditions. Distilled water was used as a reference fluid in the static tests at 80-130 °C.The mechanical strength of chalk containing seawater was opposite to chalk containing distilled water.With seawater,the chalk became weaker as the temperature increased, while using distilled water the chalk became stronger as the temperature increased.In the dynamic creep tests,at 70-90 °C,the cores flooded with seawater were significantly weaker than cores flooded with seawater without sulfate,and the difference in mechanical strength decreased as the temperature decreased.The mechanism for enhanced chemical induced weakening of chalk is discussed.
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