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Summary Induced seismicity associated with hydraulic fracturing has become a significant regulatory issue in Western Canada, after the occurrence of felt events in three specific reservoirs. Seismicity results from elevated pressure of the stimulated fracture network reducing the effective clamping force and triggering slip of tectonically stressed faults. Several published examples indicate activation over progressively larger fault regions, as multiple stages interact with critically stressed faults in various relative orientations to the treatment well. Geomechanical modeling is used to examine progressive fault slip from multi-stage fracturing and the associated seismicity. The modeling explores different operational scenarios to examine progressive fault activation as hydraulic fracture stages sequentially pressurizes more of the fault. Testing these mitigation strategies on faults in different orientations provides potential operational guidelines to support seismic traffic light systems, typically used to mitigate injection induced seismicity. Simulations show that the amount of injected fluid interacting with the fault plane controls the intensity of observed seismicity for a specific fault. Stages farther away from the fault can have an impact on fault slippage but with a delayed effect. Sequence of propagation of the hydraulic fracture stages compared to fault orientation is important. If the first stage is closest to the fault, more of the injected fluid will interact with the fault, triggering a large slipping patch on the fault plane. Successive stages will have a lesser effect due to stress shadowing. However if the first stage is the most distant from the fault, slippage on the fault plane will be gradual, thus reducing the amount of seismic moment release. The sequence in which wells on a multi-well pad are stimulated could also impact the associated seismicity. Introduction There has been increasing occurrences of recent seismicity associated with fault activation during hydraulic fracturing, resulting from elevating pore pressure on optimally-oriented, pre-existing faults leading to triggered release of stored tectonic energy (Maxwell, 2013). Anomalous seismicity is similar to microseismicity, although larger magnitude fault seismicity corresponds to inelastic slip over a larger area. However, triggered fault slip has in certain conditions lead to felt ground shaking. For example, three Western Canadian reservoirs located close to the tectonically-active trust belt have experienced anomalous activity: specifically at localized regions of the Horn River Basin, Montney and Duvernay Shales (Atkinson et al., 2016). Operators and regulators in Canada have proactively engaged the issue, establishing operational practices including seismic monitoring for a traffic light system to guide seismic hazard mitigations strategies. Clearly the topic continues to be of concern and establishing mitigation best practices is increasingly important.
Ritz, V. A. (Swiss Seismological Service) | Rinaldi, A. P. (Swiss Seismological Service) | Zbinden, D. (Swiss Seismological Service) | Nespoli, M. (University of Bologna) | Karvounis, D. (Swiss Seismological Service) | Wiemer, S. (Swiss Seismological Service)
ABSTRACT The risk of inducing seismic events is nowadays compromising the full development of new forms of exploitation of the georesources. Understanding the physical mechanisms is pivotal to the development of numerical tools to forecast induced seismicity and to elaborate mitigation strategies. Modelling tools constitute the base of the so-called Adaptive Traffic Light System (ATLS), which could provide in the future a real-time evaluation of the geoenergy system performance. While several coupled simulators (continuum or DFN) are able to describe most of the physical processes, their application in real-time could be quite computationally expensive. For this reason, a hybrid approach combining physical/mathematical models with a stochastic description of some of the processes might be required for future applications. In this work, we present the current development of modelling tools for an ATLS and review case studies applications of the TOUGH2-Seed simulator: a coupled hydro-mechanicalstochastic approach providing a good representation of various physical mechanisms. The modelling approach has been applied to simulate injection operations at Enhanced Geothermal Systems (Basel) and hydrothermal (St. Gallen) projects in Switzerland that have seen a significant number of induced sequences of earthquakes, as well as to the hydrothermal area of Hengill in Iceland, where reinjection has been seen to trigger induced seismicity crises. 1. INTRODUCTION Induced seismicity has become the topic of discussion in multiple areas, especially pertaining to the exploitation of geo-resources. Large magnitude induced seismic events are a risk for populations and structures, as well as an obstacle for the development of viable industrial activities. Mining, hydrocarbon and geothermal exploitation as well as underground storage can alter the stress field of the shallow Earth's crust by pore pressure changes, or volume and/or mass changes inducing or triggering seismicity (Ellsworth, 2013). In particular, the injection or withdrawal of fluids from the subsurface that occurs in geothermal plants carry significant risk of inducing seismic events (Evans et al., 2012).
Hui, Gang (University of Calgary, Alberta, Canada) | Chen, Shengnan (University of Calgary, Alberta, Canada) | Gu, Fei (PetroChina Research Institute of Petroleum Exploration and Development, Beijing, China)
Abstract The recent seismicity rate increase in Fox Creek is believed to be linked to the hydraulic fracturing operations near the region. However, the spatiotemporal evolution of hydraulic fracturing-induced seismicity is not well understood. Here, a coupled approach of geology, geomechanics, and hydrology is proposed to characterize the spatiotemporal evolution of hydraulic fracturing-induced seismicity. The seismogenic faults in the vicinity of stimulated wells are derived from the focal mechanisms of mainshock event and lineament features of induced events. In addition, the propagation of hydraulic fractures is simulated by using the PKN model, in combination with inferred fault, to characterize the possible well-fault hydrological communication. The original stress state of inferred fault is determined based on the geomechanics analysis. Based on the poroelasticity theory, the coupled flow-geomechanics simulation is finally conducted to quantitatively understand the fluid diffusion and poroelastic stress perturbation in response to hydraulic fracturing. A case study of a moment-magnitude-3.4 earthquake near Fox Creek is utilized to demonstrate the applicability of the coupled approach. It is shown that hydraulic fractures propagated along NE45° and connected with one North-south trending fault, causing the activation of fault and triggered the large magnitude event during fracturing operations. The barrier property of inferred fault under the strike-slip faulting regime constrains the nucleation position of induced seismicity within the injection layer. The combined changes of pore pressure and poroelastic stress caused the inferred fault to move towards the failure state and triggered the earthquake swarms. The associated spatiotemporal changes of Coulomb Failure Stress along the fault plane is well in line with the spatiotemporal pattern of induced seismicity in the studied case. Risks of seismic hazards could be reduced by decreasing fracturing job size during fracturing stimulations.
As regulators introduce operational protocols based on seismic magnitudes occurring during hydraulic fracturing, operation strategies are required to mitigate the seismic hazard. In certain cases, pressure increases associated with the hydraulic fracture injection, can induce slip of tectonically stressed faults leading to triggered seismicity. A coupled hydrogeomechanical model is used here to examine fault activation during multi-stage hydraulic fracturing and to examine operational scenarios. The model shows that the relative stage sequencing relative to the direction of fault slip can significantly impact the fault slip. Changing the viscosity of the fracturing fluid is also explored as an operational mitigation scenario, which is found to have a strong impact on fault slip and associated seismic magnitudes.
Presentation Date: Tuesday, September 26, 2017
Start Time: 11:25 AM
Presentation Type: ORAL
Abstract Induced seismicity resulting from fluid injection is a growing concern with a number of operations, including hydraulic fracturing. The vast majority of hydraulic stimulations results in no felt seismicity. However, three examples of larger, anomalous seismicity have been attributed to hydraulic fracturing, which seem to be associated with operations in unique geologic and geomechanical settings. In response, a number of operational protocols have been developed and include specific requirements for seismic monitoring. Seismological aspects are obviously central to these protocols, including characterizing the seismic source strength and associated seismic hazard. The typical microseismicity recorded during hydraulic fracturing represents a small portion of the hydraulic energy associated with the injection. However, the energy balance of the relative amount of seismic energy increases in the cases of anomalous seismicity, which may provide a monitoring tool to potentially help mitigate induced seismicity. Although the number of cases with anomalous seismicity is relatively small, other examples have been observed from geothermal stimulations. In these cases, the ratio of seismic energy is relatively larger but of potentially interest remains significantly less than the hydraulic energy. Furthermore, the ratio of seismic moment to injected volume also increases but typically remains less than a limit suggested by McGarr (1976). Potentially the energy and volume balances could be useful monitoring tools to assist in ongoing operation decision processes.