H储气库注采诱发应力场及断层滑动趋势变化

H储气库运行初期出现了数次弱震或微震，研究该区注采诱发地层应力场变化及断层易滑动性对小震级地震的影响具有重要的意义。为此，通过钻孔崩落数据和震源机制解反演了工区应力方向，并且基于钻孔崩落及钻孔诱发张裂缝数据，利用应力多边形理论来约束应力量值范围；然后，根据多孔介质弹性理论来估算气田开采和储气库周期注采情况下地层应力的变化，进而讨论断层滑动趋势与小震级地震的关系；在此基础上，得出了H储气库最大允许孔隙压力值。研究结果表明：① H储气库工区最大水平主应力方向为N21°E～N23°E，与区域现代构造应力场结果较一致；②H储气库注采前储层段水平最大、最小和垂直主应力量值分别为151.3±11.6 MPa，72.7±3.0 MPa和80.2±1.2 MPa，应力结构类型为走滑型；③在H气田开采初期储层段应力结构类型为逆冲型，而随着天然气的采出逐步转换为走滑型，之后储气库的周期注采使得应力结构类型在走滑型与逆冲型之间转换；④随着天然气开采，断层滑动趋势数值降低，而储气库注气有可能会增加断层滑动的风险，H断层滑动趋势数值随着储气库周期注采而发生周期性变化；⑤诱发微震发生的临界断层滑动趋势数值为0.45±0.03，避免微震发生的最大允许地层压力为27.7 MPa。

Variation of stress field and fault slip tendency induced by injection and production in the H underground gas storage

Several weak earthquakes or micro-earthquakes happened during the early operation of H underground gas storage (H-UGS), so it is of significance to study the effects of in-situ stress field variation and fault slippability induced by injection and withdrawal on low-magnitude earthquakes in this area. In this paper, the stress direction in the working area was inverted on the basis of borehole breakout data and focal mechanism solution. And based on the data of borehole breakout and drilling-induced tensile fractures (DITFs), the magnitude range of stress was constrained using the stress polygon theory. Then, according to the elasticity theory of porous medium, the variation of in-situ stress in the stages of gasfield production and UGS cyclic injection and withdrawal was estimated. Afterwards, the relationship between fault slip tendency and low-magnitude earthquake was discussed. And based on this, the maximum allowable pore pressure of H-UGS was worked out. And the following research results were obtained. First, the direction of maximum horizontal principal stress in the working area of H-UGS is N21°E–N23°E, which is better consistent with the regional modern tectonic stress field. Second, maximum and minimum horizontal principal stresses and vertical principal stress in the reservoir before the injection and withdrawal in H-UGS are 151.3±11.6 MPa, 72.7±3.0 MPa and 80.2±1.2 MPa, respectively. And the stress regime is of strike–slip type. Third, the stress regime in the reservoir is of thrust type during the early production of H gasfield and then transforms into the strike–slip type with the production of natural gas. Afterwards, the cyclic injection and withdrawal in the UGS leads to the switch of stress regime between strike–slip type and thrust type. Fourth, as the production of natural gas continues, the fault slip tendency declines. However, gas injection in the UGS may increase the risk of fault slip. The fault slip tendency changes cyclically with the cyclic injection and withdrawal in H-UGS. Fifth, the critical fault slip tendency inducing micro-earthquake is 0.45±0.03, and the maximum allowable formation pressure to avoid the occurrence of micro-earthquake is 27.7 MPa.