Abstract
The monitoring of rock volume where offshore exploitation activities take place is crucial to assess the corresponding seismic hazard. Fluid injection/extraction operations generate a pore fluid pressure perturbation into the volume hosting the reservoir which, in turn, may trigger new failures and induce changes in the elastic properties of rocks. Our purpose is to evaluate the feasibility of reconstructing pore pressure perturbation diffusion in the host medium by imaging the 4D velocity changes using active seismic. We simulated repeated active offshore surveys and imaged the target volume. We constructed the velocity model perturbed by the fluid injection using physical modeling and evaluated under which conditions the repeated surveys could image the velocity changes. We found that the induced pressure perturbation causes seismic velocity variations ranging between 2–5% and 15–20%, depending on the different injection conditions and medium properties. So, in most cases, time-lapse tomography is very efficient in tracking the perturbation. The noise level characterizing the recording station sites is a crucial parameter. Since we evaluated the feasibility of the proposed 4D imaging strategy under different realistic environmental and operational conditions, our results can be directly applied to set up and configure the acquisition layout of surveys aimed at retrieving fluid-induced medium changes in the hosting medium. Moreover, our results can be considered as a useful starting point to design the guidelines to monitor exploitation areas.
Highlights
The exploration and exploitation of geo-resources play an increasingly significant role in energy production
Our analysis relies on the following aspects: (1) we modeled the physical process of pore pressure diffusion in the investigated volume and the consequent perturbation of the velocity model; (2) in the active survey set-up, we accounted for an ad hoc attenuation law in order to build realistic arrival-time catalogues; (3) we simulated different scenarios by considering different parameters of the medium and injection typical of common cases in offshore exploitation activities; and (4) we evaluated the results in terms of the retrieved features of the velocity anomaly
Whenever the operational and environmental conditions are found to be different from those of the performed simulation, this work provides for a complete methodology, which includes the physical modeling of pore pressure perturbation propagation and the reliable evaluation of the attenuation law in offshore cases, aimed at the evaluation of the feasibility of reservoir condition monitoring with seismic time-lapse tomography and consequent active survey projecting
Summary
The exploration and exploitation of geo-resources play an increasingly significant role in energy production. During industrial operations, the extraction or re-injection of fluids may create new fractures and/or alter the frictional condition of existing faults, triggering low to moderate earthquakes [1]. Monitoring areas of geo-resource exploitation is fundamental since it allows us to follow the evolution in space, time, and magnitude of seismicity, and to reschedule or suspend industrial activities [2]. Fluid injection/extraction operations generate a pore fluid pressure perturbation into the volume hosting the reservoir. Even if the production reservoir is only a few hundred meters thick, the pore pressure perturbation diffusion can extend several kilometers [3]. Induced seismicity, with a magnitude that can be larger than M 3, has been reported to occur kilometers to tens of kilometers away from injection locations via pore pressure increase (e.g., [4,5])
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