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Assessing CO2 leak paths by analysis of borehole wavefield modes

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Abstract

Carbon capture and storage is essential to mitigating the “greenhouse” effect and ocean acidification. It involves injecting large amounts of CO2 into underground formations, followed by measurement, monitoring and verification of the surrounding site to ensure no CO2 leaks out. Evaluation of possible leakage pathways is essential (also for plug-and-abandon projects). Many leakage pathways may escape conventional well-log analysis. Boreholes are cased with steel that is cemented into place, so micro-debonding and fractures in cement are significant issues, and the cement-casing and cement-formation interfaces need to be distinguished. Multiple casings present even more challenges. The present project has developed new analyses of conventional borehole logging data. The discovery of sensitivity of the measurements to source frequency and receiver positioning may also inform new logging-tool designs for evaluating cement.
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Assessing CO2leak paths by analysis of borehole wavefield modes
Hua Wang, Michael Fehler, Aim´e Fournier
Assessing CO2leak paths by analysis of borehole wavefield modes
Hua Wang, Michael Fehler, Aim´e Fournier
Importance of the problem
Carbon capture and storage is essential to miti-
gating the greenhouseeffect and ocean acid-
ification. It involves injecting large amounts
of CO2into underground formations, followed
by measurement, monitoring and verification of
the surrounding site to ensure no CO2leaks out
[e.g., 3]. Evaluation of possible leakage pathways
is essential [e.g., 1] (also for plug-and-abandon
projects). Figure 1 shows some of the many leak-
age pathways, many of which may escape con-
ventional well-log analysis. Boreholes are cased
with steel that is cemented into place, so micro-
debonding and fractures in cement are signifi-
cant issues, and the cement-casing and cement-
formation interfaces (called I and II here) need
to be distinguished. Multiple casings present
even more challenges. The present project has
developed new analyses of conventional borehole
logging data. The discovery of sensitivity of
the measurements to source frequency fand re-
ceiver positioning may also inform new logging-
tool designs for evaluating cement.
Fig. 1:
possible leakage pathways: a) between casing and cement; b) between cement plug and casing; c) through cement pore
space cement degradation; d) through casing corrosion; e) through cement fractures; and f) between cement and
rock [2].
Background
Fig. 2:
horizontal section through borehole model showing 4 media and 4 channel-position parameters.
A cylindrical fluid channel allowing CO2leakage
may be described by 4 parameters: location (ra-
dius r, azimuth ϕ); and coverage (thickness d,
angle θ) (Fig. 2). The state-of-the-art methods
use only first-arrival time of a sonic signal, only
work if ris at Interface I, and can estimate only
one of dor θ, and then with no resolution of ϕ.
Full-waveform method
Acoustic waves are simulated by a 3D finite-
difference code [4, 5, 6, 8, 9]. The borehole
waveforms characterize the cement conditions
(Fig. 3). A slow Stonley mode (ST2) appears
when cement is replaced by water [7]. The ST2
velocity depends significantly on the fluid total
thickness at Interface I; but ST2 is not sensitive
to the spatial location or distribution of fluid.
Fig. 3:
vertical section through 60-cm wide borehole model (including formation) with a 10-KHz monopole source centered
at z= 0 m, showing acoustic waves at t= 0.8 ms for no cement (left), good cement (center) and cement containing a
channel (left).
Experiments
Full-waveform simulation enables the new bore-
hole method proposed here to estimate the Fig.-
2 parameters. To assess the sensitivities of
source-receiver design to the parameters, 98
(d, r, θ) combinations were simulated with 12
fvalues.
Results
Fig. 4:
Polar plot of received amplitude (radius) vs (1-m offset) receiver azimuth for 6 different θ(legend, deg).
The signal-amplitude variability across receiver
azimuth is significantly greater for smaller than
larger θ(Fig. 4). For a dipole source, the cross-
pole receiver phase difference is proportional to
ϕfor a range of f(Fig. 5). Many more sensi-
tivities were discovered [10].
Fig. 5:
Opposed-receiver phase difference (ordinate) vs f(abscissa) for 7 channel azimuths ϕ(legend) at d= 16 mm.
Potential for impact
Various measured waveform amplitudes, arrival
times and modes can be exhaustively combined
and interpreted to diagnose the cement condi-
tion, including radial and azimuthal location
and coverage of the CO2leak channel (Fig. 6).
Fig. 6:
Workflow to assess cement condition and leak-channel parameters.
Conclusions
Current state-of-the-art cement-bond logging
technology cannot diagnose details of CO2leak
channels. For various receiver set-ups, multi-
modal, multi-frequency, full-waveform informa-
tion can be analyzed to discover those details,
which may be necessary for CCS management.
References
[1] L. Deremble, M. Loizzo, B. Huet, B. Lecampion, D. Quesada (2011) Stability of a leakage
pathway in a cemented annulus.Energy Procedia,4, 5283–5290.
[2] S.E. Gasda, S. Bachu, M.A. Celia (2004) Spatial characterization of the location of potentially
leaky wells penetrating a deep saline aquifer in a mature sedimentary basin.Environmental
Geology,46, 707–720.
[3] Litynski, J., T. Rodosta, B. Brown. Best practices for monitoring, verification, and ac-
counting of CO2stored in deep geologic formations – 2012 Update. US DoE, 2012.
[4] H. Wang, G. Tao, M.C. Fehler (2015) Investigation of the high-frequency wavefield of an
off-center monopole acoustic logging-while-drilling tool,Geophysics,80, D329–D341.
[5] H. Wang, M. Fehler and D. MillerThe wavefield of acoustic logging in multiple casing models.
SPWLA 57th Annual Technical Conference Rejkiavik, Iceland, 25-29 June, 2016.
[6] H. Wang, G. Tao, X. Shang (2016)Understanding acoustic methods for cement bond logging
Journal of Acoustical Society of America 139, 2407–2416.
[7] H. Wang, M. Fehler (2017) A cement-bond evaluation method based on the full waveform
from a monopole tool.SEG Technical Program Expanded Abstracts 875–879.
[8] H. Wang, M. Fehler (2018) The wavefield of acoustic logging in a cased-hole with a single
casing—Part I: A monopole tool.Geophysical Journal International 212, 612–626.
[9] H. Wang, M. Fehler (2018) The wavefield of acoustic logging in a cased hole with a single
casing—Part II: A dipole tool.Geophysical Journal International 212, 1412–1428.
[10] H. Wang, M. Fehler, A. Fournier (2018) Assessing CO2leak paths by analysis of borehole
wavefield modes.MITEI Seed Fund Review and Selection Meeting.
ResearchGate has not been able to resolve any citations for this publication.
Conference Paper
Full-text available
Successful operations for plug and abandonment (P&A) must seal the well bores to ensure that there is never leakage between geological horizons penetrated by the wellbore or to the surface. Acoustic logging methods for cement bonding, being designed for material evaluation of a single casing string, are currently unable to characterize cement and bonding of cement to casing when multiple concentric casing strings are present. When this is the situation, the inner pipes of the wells must be removed to leave only one layer of steel and cement that can be evaluated which increases the cost of the P&A. With the goal of improving the reliability of acoustic logging methods for material bonding evaluation in multiple casing bonding conditions, we use a 3D Finite Difference (3DFD) method to simulate wave propagation in cased borehole models including single casing and dual casing boreholes with different bonding conditions. Pressure snapshots for different models are shown which allow us to better understand the wave propagation. Data processing methods such as velocity-time semblance and dispersion analysis facilitate the identification of propagation modes in the different models. A modal decomposition method is also used in the data with an eccentered source. For single casing models, we assume that the basic formation modes can be easily discerned when the casing is well bonded. The P wave is submerged in the casing mode and the S wave has poor coherency when the cement is replaced with fluid. Acoustic logging tool with a monopole sonic source can be used for determine the bonding condition of different interfaces for the single casing model. For the dual casing models, it is easy to determine if there is good cement in both annuli and whether the outer casing is fully bonded by using monopole logging data. However, if the first annulus is not bonded well, the monopole data are not useful but it may be possible to use dipole or higher order modes to distinguish the bonding condition. The modal decomposition method for the data with an eccentered source helps us understand the higher order modes. New data processing methods and tool designs can be developed when we have a full understanding of the wavefield excited by an eccentered source.
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This work was motivated by considerations of potential leakage pathways for CO2 injected into deep geological formations for the purpose of carbon sequestration. Because existing wells represent a potentially important leakage pathway, a spatial analysis of wells that penetrate a deep aquifer in the Alberta Basin was performed and various statistical measures to quantify the spatial distribution of these wells were presented. The data indicate spatial clustering of wells, due to oil and gas production activities. The data also indicate that the number of wells that could be impacted by CO2 injection, as defined by the spread of an injected CO2 plume, varies from several hundred in high well-density areas to about 20 in low-density areas. These results may be applied to other mature continental sedimentary basins in North America and elsewhere, where detailed information on well location and status may not be available.
Best practices for monitoring, verification, and accounting of CO 2 stored in deep geologic formations -2012 Update
  • J Litynski
  • T Rodosta
  • B Brown
Litynski, J., T. Rodosta, B. Brown. Best practices for monitoring, verification, and accounting of CO 2 stored in deep geologic formations -2012 Update. US DoE, 2012.