在地质断层构造中地应力状态演变研究

Study of in situ stress state evolution in geologic fault structure

在国内86个断块油气田近千口钻井中进行了水压致裂试验。试验结果表明,无论是在发育正断层还是发育逆断层的地层中,最小主应力均为水平应力,且其方向往往平行于断层走向。这与传统观点所认为的最小主应力方向垂直于断层走向(正断层)或最小主应力为垂直应力(逆断层)相矛盾。指出传统观点所解释的是断层开始发生时的应力状态和主应力与不同断层构造的关系,而压裂试验所测得的应力是断裂活动停止之后的应力状态。在正断层发生过程中,垂直于断层走向的水平应力(最小主应力)不断增大,直到足以阻止正断层的继续发展。因此断裂活动终止时此水平应力往往会超过平行于断层走向的水平应力(断裂发生前的中间主应力),成为中间主应力,平行于断层走向的水平应力则成为最小主应力。在逆断层发生过程中,垂直于断层走向的水平应力(最大主应力)不断减小,直到不能推动断层继续移动;平行于断层走向的水平应力在此过程中亦随之减小,且始终小于垂直于断层走向的水平应力,到断裂活动终止时必然成为最小主应力。图5参2(梁大新摘)

The hydrofracturing tests have been conducted in near thousand oil wells drilling at 86 oil gas fields of 13 big oil areas in China. Test results show that minimum principle stress, δ 3 is always horizontal and parallel to the strike of normal and reversed faults. This is discrepant with traditional point of view, which states that minimum principle stress is horizontal and perpendicular to the strike of normal fault or is vertical for reversed fault. This paper points out that traditional point of view explains the stress state on fault occurrence and the relationship of three principle stresses with different fault structure. The measured stresses based on hydrofracturing tests in oil fields are stresses after completion of fault movement. Obviously, the stress state before fault occurrence continuously changes during fault formation. In the system of normal fault the horizontal stress, perpendicular to the strike of fault increases continuously up to the level which is enough to stop normal fault movement. Thus after formation of normal fault the horizontal stress becomes larger than the horizontal stress, parallel to the strike of normal fault. The last one becomes minimum principle stress. During reversed fault formation the horizontal stress, perpendicular to the strike of the fault decreases continuously up to the level which becomes incapable of moving the block forward. The horizontal stress, parallel to the strike of the fault decreases continuously too and always is less than the horizontal stress, perpendicular to the strike of the fault, being minimum principle stress after fault completion.