Integration of Active and Passive Seismic Data for Monitoring CO2 Storage In CoalYouli Quan and Jerry M. Harris

Geophysics Department, Stanford University

Introduction

Proper ties of Coal for CO2 Seismic Monitoring

Existing Applications of Coal Seismology

Proposed Seismic Methods for Coal Bed CO2 Storage Monitoring

Seismic Monitoring with Active & Passive Data

Summar y

· Relatively shallow· Fracturing induced by stress changes· Multi-component adsorption of CH4 and CO2

· Little seismic velocity change due to replacement of CH4 by CO2

· Low density· Low seismic velocity· High seismic attenuation· High seismic anisotropy

Coal is one of the possible geological storages for CO2 sequestration. The injection process of CO2 can cause changes in many properties of the coal bed, for example, the state of stress and seismic velocities. The purpose of this subproject is to develop monitoring strategies for CO2 storage in coal using those property changes due to the injection.

Active and passive seismic data are combined for monitoring CO2 storage in coal. Active seismic data include 3-D surface surveys and vertical seismic profiling (VSP). Passive seismic data are microseismic events induced by the change in stress and pore pressure associated with fluid injection. Different data are sensitive to different property changes caused by CO2 injection. Surface seismic data show the reflectivity changes. VSP tomography gives detailed velocity models that show the pressure and saturation changes associated with CO2 injection. Microseismic locations indicate the fluid front. Joint passive and active seismic tomography and imaging provide more reliable monitoring with low cost. This is a way to maximize the use of field instru-ment and data.

In the case of enhanced coal bed methane (ECBM), CO2 replaces CH4 in coal. In this multi-component case, the CO2 injected may not cause detectable velocity changes; the passive seismic monitoring should be an effective method because it detects the fluid front instead of the seismic velocity changes.

Compared with deep aquifer and gas/oil reservoir storages, coal beds have some special features related to in-situ monitoring.

· High resolution 3D surface seismic surveys · Vertical seismic profiling and Cross well profiling· Seam waves· Passive seismic monitoring for safety decisions

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Fehler, M., A. Jupe, H. Asanuma, 2001, More than cloud: New techniques for character-izing reservoir structure using induced seismicity, The Leading Edge

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References

· A circular array can be used to collect 3-D seismic data. This geometry has the maximum coverage for the same number of detectors.

· Passive seismic monitoring may be useful for CO2 storage in coal, especially for multi-component case when the velocity changes might be undetectable.

· Active monitoring and passive monitoring detect different features caused by the injection. The integration of them should provide more reliable results.

· Dynamic inversion is one of the approaches to integrating different types of data collected at different time.

Passive Seismic Monitoring

· Continuously monitor injection pressure front by listening to the sound (or microearthquakes) caused by fluid injection

· Locate the sources of those micro seismic events and map the fault and fracture structures

· Passive seismic reflectivity imaging

Reflection image of passive seismics in a thermal field.

Albright et al.,1994 Fehler et al.,2001

3-D Numerical Simulation· Finite difference method is used to simulate the wave field for dif-ferent observation configurations and different time-lapse models.

· Stanford CEES computer clusters are used for this heavy comput-ing task. It takes 50 days CPU time for 5 time-lapse models, 300 sources, 360x360x270 grid size, and 1700 time steps.

· The simulated seismic data are then imaged using Kirchhoff mi-gration.

Circular array simulation Cross array simulation

(a) Model

(b) A common source gather

(a) Model (b) Slice view of the model

(c) Amplitude difference of two time-lapse source gathers

(d) Time slice view. The diffraction pattern indicates the leakage along the fault.

Seismic Arrays for Active Monitoring· Use surface array(s) and a linear vertical array to collect 3-D surface reflection data and VSP data with control sources. Three different surface arrays are proposed.

· Under the condition that each configuration has a fixed number of detectors

Three surface source/receiver arrays for 3D seismic data acquisition. The shaded area shows the reflection coverage. The circular array has maximum coverage for the same number of detectors.

Grid array Cross array Circular array

Depth slices of 3D migration using synthetic data computed for grid array, linear array, and circular array

Injection well & vertical array

(b) A depth slice of 3D image cube. It is the difference of depth migration cubes corre-sponding to two time-lapse surveys.

(a) Model. Two 3D time lapse seismic surveys are simulated using finite difference method.

A simple time-lapse simulation with circular array

CO2 injected

CO2 front change from time 1 to time 2Grid array Cross array Circular array

Improve passive seismic locations using detailed velocity model

Feature-enhanced inversion. Location map of microseismics as

initial model

Dynamic travel time inversion

with sparse data

Containment assessment

Active seismic data reflectivity imaging

Passive seismic data reflectivity imaging

On-going tasks

Future tasks

3-D numerical simulation of

active & passive seismic data

Dynamic aperture considerations

Finished tasks