Abstract
In many subsurface industrial applications, fluids are injected into or withdrawn from a geologic formation. It is of practical interest to quantify precisely where, when, and by how much the injected fluid alters the state of the subsurface. Routine geophysical monitoring of such processes attempts to image the way that geophysical properties, such as seismic velocities or electrical conductivity, change through time and space and to then make qualitative inferences as to where the injected fluid has migrated. The more rigorous formulation of the time-lapse geophysical inverse problem forecasts how the subsurface evolves during the course of a fluid-injection application. Using time-lapse geophysical signals as the data to be matched, the model unknowns to be estimated are the multiphysics forward-modeling parameters controlling the fluid-injection process. Properly reproducing the geophysical signature of the flow process, subsequent simulations can predict the fluid migration and alteration in the subsurface. The dynamic nature of fluid-injection processes renders imaging problems more complex than conventional geophysical imaging for static targets. This work intents to clarify the related hydrogeophysical parameter estimation concepts.
Highlights
There are many scenarios in which the earth’s subsurface is being altered by anthropogenic activity
As will be outlined in detail, these concepts and overall goals are distinct from those underlying time-lapse geophysical monitoring approaches such as crosswell seismic (e.g., Landrø and Stammeijer, 2004; Daley et al, 2008; Marchesini et al, 2017), seismic coda monitoring (e.g., Kanu et al, 2014; Obermann et al, 2016), crosswell electromagnetics (EM) (e.g., Binley et al, 2001; Day-Lewis et al, 2003; Marsala et al, 2008), and crosswell electrical resistivity tomography (ERT) (e.g., Daily et al, 1992; Bergmann et al, 2012)
The main distinction of the processimaging approach presented here is in the definition of the forward model that simulates how time-lapse geophysical signals vary during the course of fluid injection
Summary
There are many scenarios in which the earth’s subsurface is being altered by anthropogenic activity. As will be outlined in detail, these concepts and overall goals are distinct from those underlying time-lapse geophysical monitoring approaches such as crosswell seismic (e.g., Landrø and Stammeijer, 2004; Daley et al, 2008; Marchesini et al, 2017), seismic coda monitoring (e.g., Kanu et al, 2014; Obermann et al, 2016), crosswell electromagnetics (EM) (e.g., Binley et al, 2001; Day-Lewis et al, 2003; Marsala et al, 2008), and crosswell electrical resistivity tomography (ERT) (e.g., Daily et al, 1992; Bergmann et al, 2012) In this tutorial, we elaborate on the concepts around estimation of parameters that control the migration of injected fluids. The process-imaging formulation entails modeling of geophysical signals as one component of a larger coupled multiphysics forward model
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