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Abstract According to the key technical problem of deep mudstone failure under drilling fluid soaking，the weakening law of strength is analyzed by the laboratory test；and the changing mechanism of core samples is analyzed through microcosmic angle. The result shows that：(1) Under the drilling fluid soaking，the mudstone strength increases first and then decreases with increase of coring angle. The decreasing range of strength is up to 49.5%–54.7%. With the increase of confining pressure，the increasing range of strength is lower and it is only 26.4%–39.2% with coring angles of 0°and 90°. (2) The elastic modulus of mudstone decreases with the increase of soaking time，and increases first and then decreases with the increase of coring angle. But the change law of the Poisson's ratio is opposite with that of the elastic modulus. (3) Comparing with the internal friction angle，the change of mudstone cohesion presents stronger regularity. The decreasing range of cohesion is larger with coring angles of 0° and 90°，and it reaches 67.3%；but the change range of cohesion is small under other coring angles. (4) Because of the invasion of drilling fluid，the stress intensity factor of crack at the tip increases；the critical fracture toughness decreases and the cracks propagate，which is the basic reason for strength decrease of mudstone. Simultaneously，mineral composition of the fitting in bedding surface is changed by the action of drilling fluid. So，the friction factor crack face decreases and the decrease of mudstone strength is intensified.
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2012, Vol. 31 
