Mechanisms of hydraulic fracturing induced seismicity

为了经济地开采北美巨大的致密油气储量，大规模的水力压裂技术已经变得无处不在。微震活动通常伴随着水力裂缝的发育，是压裂后储层岩体中已有裂缝和断层发生动态滑动的一种表现。这种滑移可能导致剪切裂缝的扩张，从而增加储层岩石与水力裂缝（HF）的渗透率和连通性，从而提高最终的产量。在极少数情况下，水力压裂引起的动力滑移会发展成小到中等规模的地震。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HF引入的应力和孔隙压力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)它如何影响储层性质和“受激岩石体积”；B)从地震到动力滑动的转变，以及动力破裂失控（地震）的可能性。***我们将对位于HF传播路径内或附近的先前存在的裂缝进行滑动建模。滑移是由以下机制之一引起的：a) HF前缘的应力扰动，因为它传播过先前存在的裂缝；b)与水力裂缝相交的已有裂缝的加压。由于沿HF传播的粘性流体压力下降，我们预计两种滑移诱导机制将在时间和空间上分离，前者发生在前进的HF锋面附近，而后者发生在锋面后方一定距离处，此时水力裂缝中的压力已经恢复到足以扩散增压相交剪切裂缝的水平。***现有的地震不稳定性研究——由构造荷载驱动的断层滑动从最初的缓慢速率到地震速率的转变——是基于断层泥摩擦与滑动或滑动速率的不稳定减弱的假设。由于诱发地震活动经常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性。在我们提出的工作中，我们将使用a)传统的摩擦不稳定性模型和b)一个新的模型来模拟地震成核，在这个模型中，由孔隙压力扰动和相关的摩擦加热激活的断层滑动瞬态导致孔隙流体的热加压驱动断层不稳定。我们将比较两种模型中动态滑移的成核条件和完井距离，作为HF属性、原位应力、已有裂缝的方向及其与水力裂缝面的接近程度的函数；并利用现有的实验室和地震观测资料证实了结果

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.**