Mechanisms of hydraulic fracturing induced seismicity

水力压裂诱发地震的机制

• 批准号: RGPIN-2016-05743
• 负责人: Garagash, Dmitriy
• 金额: $ 2.26万
• 依托单位: Dalhousie University
• 依托单位国家: 加拿大
• 项目类别: Discovery Grants Program - Individual
• 财政年份: 2020
• 资助国家: 加拿大
• 起止时间: 2020-01-01 至 2021-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=709943
• 关键词: Mechanisms; hydraulic; fracturing; induced; seismicity

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.

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。