EXPERIMENTAL STUDY ON STRENGTH PROPERTIES OF DEEP MUDSTONE UNDER DRILLING FLUID IMMERSION

In view of the key technical problem of deep mudstone failure under drilling fuid soaking the weakening law of strength is analysed by a laboratory test. The result shows that under the drilling fluid soaking the mudstone strength at first increases and then it decreases with increase of coring angle. The decreasing range of strength is up to 49.5-54.7%. With the increase in confining pressure the increasing range in strength is smaller and it is only 26.4-39.2% with coring angles of 0 degrees and 90 degrees. The elastic modulus of mudstone decreases with the increase of soaking time and it increases at first and then it decreases with the increase in coring angle.