IN-SITU STRESS, IN-SITU STRENGTH OF ROCKS AND THE CHINA MAINLAND STRESS FIELD

Based on the data of hydrofracturing from 150 segments in 8 oil fields in China, in this paper, we studied the variation in horizontal stress with depth in some areas, and found that the ratios of average horizontal stress to vertical stress are about 0.8-1.2 at the depth of 4000 m, the high values corresponding to reverse faults, and low ones to normal faults. The results of in situ measurements coinside with the features of fault activity in every area. We find that the more distant from the boundary between the Eurasian and Indian plates the less the maximum horizontal stress becomes; the values are normalized to the depth of 4000 m with respect to the sea level after making the elevation correction of well sites. This result and the hypothesis of Force Source Center hinted by the radiant distribution of the direction of the principal stress in the China mainland from focal mechanisms solutions support each other. We also got the in-situ tensile strength of rocks by use of the hydrofracturing curve tests. The actual result shows a certain degree of regularity: the strength increases with the depth of rocks, also varies linearly with the logarithm of the age of rocks. These field strength data coinside roughly with experimental results.