韩城区块老井经过长期开采，面临产量下滑的问题，需要进行重复压裂改造，从而形成新裂缝沟通煤层气富集区，以实现煤层气稳产；由于煤层初始应力差小，地应力在压裂和长期排采后，大小和方向均发生显著变化，直接影响重复压裂时机的选择；为此，结合测井资料得到研究区块Y3井的建井的初始地应力，利用有限元模型模拟压裂过程中煤层地应力变化，对排采过程中的孔隙压力变化进行研究，根据重复压裂转向机理分析判断水平主应力和诱导应力大小，得到重复压裂的最佳时机，并对Y3井进行现场试验。研究结果表明：在煤层中首次压裂产生的应力影响较小，未造成应力反转；Y3井在排采约1 500 d后，孔隙压力非均匀性差值约0.6 MPa，等于煤层段初始地应力差，即出现应力方向反转，在该时间进行二次压裂，有利于在近井眼处产生能延伸70 m的新裂缝，沟通井底煤层气富集区；按此方法优选重复压裂时机，同层重复压裂改造后Y3井日产气量由114 m³增至1 626 m³，效果显著。

The old wells in Hancheng Block face the problem of production decline after long-term exploitation, and need to be re-fractured to form new fractures to communicate with CBM rich areas, so as to achieve stable CBM production. Due to the small difference of initial stress in coal seam, the magnitude and direction of in-situ stress change significantly after fracturing and long-term drainage, which directly affects the choice of re-fracturing time. Therefore, we firstly obtain the initial ground stress of well Y3 in the research block combined with logging data, then simulate the change of coal seam ground stress during fracturing by using finite element model, then study the change of pore pressure during drainage and production, and determine the horizontal principal stress and induced stress according to the steering mechanism of refracturing, so as to obtain the best time for refracturing. Field tests were carried out on well Y3. It is found that the stress produced by the first fracturing in coal seam has little influence and does not cause stress reversal. After about 1 500 days of drainage, the difference of non-uniformity of pore pressure is about 0.6 MPa, which is equal to the difference of initial in-situ stress in the coal seam section, that is, the stress direction reverses. Secondary fracturing at this time is conducive to generate new fractures that can extend 70 m near the hole and communicate the CBM rich area at the bottom of the hole; according to the method proposed in this paper, the daily gas production of well Y3 increased from 114 m³ to 1 626 m³ after the same zone re-fracturing, and the effect is remarkable.