Mechanisms of hydraulic fracturing induced seismicity

大规模的水力压裂在允许经济地开采北美巨大的致密石油和天然气储量方面已经变得无处不在。微震活动通常伴随着水力裂缝的增长，是在受刺激的储层岩石体积中预先存在的裂缝和断层上的动态滑动的表现。这种滑动可能导致剪切裂缝的扩张，以及储层岩石与水力裂缝（HF）的渗透性和连通性的相关增加，并且因此改善最终产量。在极少数情况下，由水力压裂引起的动态滑动已知会发展成小型到中型地震。这在一些国家禁止压裂的决定中发挥了作用。因此，重要的是要研究由于传播HF引入的应力和孔隙压力扰动而引起的动态和准静态断层滑动的机制，以便能够更好地理解a）它如何影响储层性质和“激发岩石体积”; B）从非地震向动态滑动的转变，以及动态破裂失控（地震）的可能性。 我们将模型滑动预先存在的裂缝位于或附近的传播HF的路径。滑动由以下机制之一引起：a）当HF传播通过预先存在的裂缝时，HF前部的应力扰动;以及B）由水力裂缝覆盖的预先存在的裂缝的加压。由于粘性流体的压力下降沿着传播的HF，我们预计两个滑移-诱导机制将在时间和空间上分离，与前者发生在附近的推进HF前，而后者在一定的距离后，在前面的液压裂缝中的压力已恢复到足以扩散加压剪切裂缝的水平。 现有的地震不稳定性研究-断层滑动从最初由构造载荷驱动的缓慢速率到地震速率的转变-是基于断层泥摩擦力随着滑动或滑动速率的不稳定减弱的假设。由于诱发地震活动往往发生在缺乏天然地震活动的地区，因此那里的地震成核机制可能不同于摩擦不稳定。在所提出的工作中，我们将模拟地震成核使用a）传统的摩擦不稳定性，和B）一个新的模型，其中断层滑动瞬态激活孔隙压力扰动和相关的摩擦加热，导致热加压的孔隙流体驱动断层不稳定。我们将比较两个模型中的动态滑动的成核和跑出距离的条件，作为HF属性的函数，在原地应力，和预先存在的裂缝的方向和它们的接近水力压裂平面;并证实使用现有的实验室和地震观测的结果。

Hydraulic fracturing on a massive scale has become ubiquitous in allowing to economically tap vast tight oil and gas reserves in North America. Microseismicity, which usually accompanies growth of hydraulic fractures, is a manifestation of dynamic slippage on pre-existing fractures and faults in the stimulated reservoir rock volume. This slip may lead to dilation of sheared fractures, and associated increase of permeability and connectivity of the reservoir rock to the hydraulic fracture (HF), and, therefore, improvement to eventual production. On rare occasions, dynamic slip induced by hydraulic fracturing is known to grow out into a small-to-moderate size earthquake. This has played a role in some countries' decision to ban fracturing. It is therefore important to study mechanisms of both dynamic and quasi-static fault slip due to stress and pore pressure perturbations introduced by a propagating HF to be able to better understand a) how it impacts reservoir properties and "stimulated rock volume"; b) the transition from aseismic to dynamic slip, and potential for dynamic rupture run-away (an earthquake). We will model slip on pre-existing fractures which lie in or near the path of a propagating HF. Slip is induced by one of the following mechanisms: a) stress perturbation at the front of the HF as it propagates past a pre-existing fracture; and b) pressurization of pre-existing fractures intersected by the hydraulic fracture. Due to a viscous fluid pressure drop along the propagating HF, we expect the two slip--inducement mechanisms to be separated in time and space, with the former taking place near the advancing HF front, while the latter at some distance behind the front where the pressure in the hydraulic fracture has recovered to the levels sufficient to diffusively pressurize intersected shear fractures. Existing studies of earthquake instability - the transition of fault slip from initially slow rate driven by tectonic loading to seismic rates - are based on the assumption of unstable weakening of the fault gouge friction with slip or slip-rate. Since induced seismicity often takes place in provinces devoid of natural seismicity, it is possible that the earthquake nucleation mechanism there differs from the frictional instability. In the proposed work, we will model earthquake nucleation using a) traditional frictional instability, and b) a new model in which a fault slip transient activated by a pore pressure perturbation and associated frictional heating, leads to thermal pressurization of pore fluid driving the fault to instability. We will compare conditions for nucleation and run--out distances of dynamic slip in the two models, as a function of the HF attributes, in situ stress, and orientation of pre-existing fractures and their proximity to the hydraulic fracture plane; and corroborate the results using existing laboratory and seismological observables.