Mechanisms of hydraulic fracturing induced seismicity

大规模的水力压裂已经变得无处不在，使人们能够经济地开采北美巨大的致密油气储量。微震活动通常伴随着水力裂缝的发育，是模拟储集层岩石体积中原有裂缝和断层发生动态滑动的一种表现。这种滑动可能会导致剪切裂缝的扩张，并相应地增加渗透率和储集岩与水力裂缝(HF)的连通性，从而改善最终的产量。在极少数情况下，水力压裂引起的动态滑动会发展成中小型地震。这在一些国家禁止压裂的决定中发挥了作用。因此，研究由传播的HF引起的应力和孔压扰动引起的动态和准静态断层滑动的机制是很重要的，以便能够更好地理解a)它如何影响储集层性质和“受刺激的岩石体积”；b)从地震到动态滑动的转变，以及动态破裂失控(地震)的可能性。*我们将对位于HF传播路径内或附近的预先存在的裂缝上的滑动进行模拟。滑动是由下列机制之一引起的：a)当HF穿过预先存在的裂缝时，在其前部的应力扰动；以及b)与水力裂缝相交的先前存在的裂缝的加压。由于沿传播的HF的粘性流体压降，我们预计两种滑动诱导机制在时间和空间上是分开的，前者发生在前进的HF锋面附近，而后者发生在前锋后面的某个距离处，那里水力压裂中的压力已经恢复到足以对相交的剪切裂缝进行扩散加压的水平。*现有的地震不稳定性研究--断层滑动从构造载荷驱动的最初的低速向地震率的转变--是基于断层泥摩擦随着滑动或滑移率的不稳定减弱的假设。由于诱发地震活动往往发生在没有自然地震活动的省份，因此那里的地震成核机制可能不同于摩擦不稳定。在拟议的工作中，我们将使用a)传统的摩擦不稳定性和b)一个新的模型来模拟地震成核，在该模型中，由孔压扰动和相关的摩擦加热激活的断层滑动瞬变导致孔隙流体的热增压导致断层失稳。我们将比较两个模型中动态滑移成核和跳动距离的条件，作为HF属性、地应力和预先存在的裂缝的方向及其与水力裂缝平面的接近程度的函数；并利用现有的实验室和地震学观测资料来证实结果。

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.**