Coupled Poroelastic Modeling of Hydraulic Fracturing‐Induced Seismicity: Implications for Understanding the Post Shut‐In M_L 2.9 Earthquake at the Preston New Road, UK

Abstract

Post‐injection seismicity associated with hydraulic stimulation has posed great challenges to hydraulic fracturing operations. This work aims to identify the causal mechanism of the post shut‐in ML 2.9 earthquake in August 2019 at the Preston New Road, UK, amongst three plausible mechanisms, that is, the post shut‐in pore pressure diffusion, poroelastic stressing on a non‐overpressurized fault, and poroelastic stressing on an overpressurized fault. A 3D fully coupled poroelastic model that considers the poroelastic solid deformation, fluid flow in both porous rocks and fracture structures, and hydrofracturing‐induced pressure perturbations was developed to simulate the hydromechanical response of the shale reservoir formation to hydraulic fracturing operations at the site. Based on the model results, Coulomb stress changes and seismicity rate were further evaluated on the PNR‐2 fault responsible for the earthquake. Model results have shown that increased pore pressure plays a dominant role in triggering the fault slippage, although the poroelastic stress may have acted to promote the slippage. Amongst the three plausible mechanisms, the post shut‐in pore pressure diffusion is the most favored in terms of Coulomb stress change, seismicity rate, timing of fault slippage and rupture area. The coupled modeling results suggested that the occurrence of the post shut‐in ML 2.9 earthquake was a three‐staged process, involving first propagation of fracture tips that stimulated surrounding reservoir formations, then hydraulic connection with and subsequent pore pressure diffusion to the conductive PNR‐2 fault, and eventually fault activation primarily under the direct impact of increased pore pressure.

How Injection History Can Affect Hydraulic Fracturing–Induced Seismicity: Insights from Downhole Monitoring at Preston New Road, United Kingdom

“…Plotting the distance of the events from the injection point versus the occurrence time (Figure 4) shows that a pore pressure diffusion front can explain the observed migration of seismicity during injection, as also indicated by . This interpretation is further supported by the numerical modeling studies of and Cao et al (2022). After injection, migration occurred along pathways expanding in multiple directions, making it difficult to analyze the diffusion process from distance-time plots.…”

“…The migration of seismicity studied by and the 3D coupled poroelastic modeling of Cao et al (2022) suggest that the fault triggering was probably caused by a decrease in normal stress due to pore pressure diffusion. In contrast, at the nearby PNR-1z well, tensile fracture opening due to the injection of high-pressure fluids is also thought to play an important role in triggering seismicity (Kettlety et al, 2020).…”

“…The sleeves were evenly spaced at 14.5 m. In the first five stages, fluids were injected at a constant rate of 0.075 𝑚 3 ∕𝑠 for approximately 1.5 hours. The injection plan was not to exceed the maximum injected volume of 432 𝑚 3 per stage (Cao et al, 2022). In stage 6, 𝑀 𝐿 > 0.5 events occurred, resulting in a pause of the injections, as imposed by the adopted Traffic Light Scheme.…”