seismic monitoring

Available online at www.sciencedirect.com
ScienceDirect
Energy Procedia 114 (2017) 4028 – 4039
13th International Conference on Greenhouse Gas Control Technologies, GHGT-13, 14-18
November 2016, Lausanne, Switzerland
Optimization Study of Seismic Monitoring Network at the CO2
Injection Site – Lessons Learnt from Monitoring Experiment at the
Cranfield Site, Mississippi, U. S. A.
Makiko Takagishia*, Tsutomu Hashimotoa, Tetsuma Toshiokaa, Shigeo Horikawab,
Kinichiro Kusunosec, Ziqiu Xuea, Susan D. Hovorkad
a
Research Institute of Innovative Technology for the Earth (RITE), 9-2 Kizugawadai, Kizugawa-shi, Kyoto, 619-0292, Japan
b
Suncoh Consultants Co., Ltd., 1-8-9 Kameido, Koto-ku, Tokyo, 136-8522, Japan
c
National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1, Higashi, Tsukuba-shi, Ibaraki, 305-8567 Japan
d
Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas, U.S.A.
Abstract
In this paper, we discuss the design (seismometers’ types and layout) of seismic monitoring network that monitors microseismic
events induced by CO2 injection and natural earthquakes that occurs around the CO2 injection site. We also elucidate CO2
injection and occurrences of microseismic events. Firstly, we tested a monitoring network at the Large-scale CO2 injection site in
the U.S.A to obtain lessons learnt. Then, we review case studies of microseismic monitoring at the two CO2 injection sites.
Finally, we discuss and propose the monitoring network that meets our requirements based on findings from the monitoring
experiment and case studies.
© 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
© 2017 The Authors. Published by Elsevier Ltd.
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Peer-review
under responsibility of the organizing committee of GHGT-13.
Peer-review under responsibility of the organizing committee of GHGT-13.
Keywords: CO2 geological storage; microseismic events; seismic monitoring; seismic monitoring network; Optimization; Cranfield; Large-scale
CO2 injection site
* Corresponding author. Tel.: +81-774-75-2312; fax: +81-774-75-2313.
E-mail address: [email protected]
1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Peer-review under responsibility of the organizing committee of GHGT-13.
doi:10.1016/j.egypro.2017.03.1543
Makiko Takagishi et al. / Energy Procedia 114 (2017) 4028 – 4039
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1. Introduction
Geological storage of CO2 that has been captured at the large, point source emitters represents a key potential
method for reduction of anthropogenic greenhouse gas emissions. Supercritical CO 2 is injected into reservoir, deep
sedimentary formation reservoir, which mainly consists of porous and permeable sandstone [1].
Monitoring at CO2 injection sites is essential to demonstrate that the injected CO2 is stored safely within the
reservoir. Various geophysical and chemical monitoring techniques are conducted at CO2 injection sites [e.g. 2].
Passive microseismic monitoring is the monitoring technique that monitors the geomechanical state of the ground
by using source locations of microseismic events induced by fluid injection including CO2.The microseismic events
are induced by pore pressure elevation within the reservoir, and these events are usually too small to be felt by
human being [3]. Microseismicity is monitored at CO2 injection sites, and reported events are all unfelt [4]. These
microseismic events are not likely to cause any damage, so we intend to evaluate microseismicity and use the result
for CO2 injection rate control (same rate, reduced rate and stop injection).
When CO2 geological storage is conducted in Japan, because of the high seismicity area, natural seismicity is also
observed during microseismic monitoring. Since we want to use microseismic events for CO2 injection rate control,
we need to extract the microseismic events from the recorded data. We also need to monitor natural earthquakes to
demonstrate safe CO2 injection and obtain public acceptance for CO2 geological storage operation. If felt
earthquakes occur around the CO2 injection site, some people might relate the earthquakes and CO2 injection. So we
need to show that the natural earthquakes are not related to CO2 injection operation. To achieve that, we monitor
natural seismicity from prior to CO2 injection (baseline monitoring period) and show that no significant increase of
natural earthquakes.
Therefore we monitor both microseismic events and natural earthquakes and distinguish them at the processing to
demonstrate the safe CO2 injection. To monitor both events, designing the seismic monitoring network which
includes choosing layout and type of seismometers is important. It is also important to obtain the relationship
between CO2 injection and occurrences of microseismic events such as magnitudes, location distribution, and
seismicity patterns.
To design the seismic monitoring network which monitors microseismic events and natural earthquakes, first we
designed seismic monitoring network and monitored seismicity at the CO2 injection site to obtain lessons learnt.
Next, we reviewed case studies at the CO2 injection sites based on our monitoring experiment. Finally, we discussed
and proposed the seismic monitoring network which meets the requirements.
2. Monitoring experiment
2.1. Design of seismic monitoring network deployed at the CO2 injection site
We designed seismic monitoring network by choosing layout and type of seismometers to test at the real CO2
injection site. First, we describe two monitoring targets; microseismic events induced by CO2 injection and natural
earthquakes. Then we explain the layout and type of seismometers.
Microseismic events induced by CO2 injection (hereinafter, microseismic events) are monitored to examine
geomechanical state of the reservoir, and we intend to utilize the locations of microseismic events for CO 2 injection
rate control. Microseismic events are monitored at several CO2 injection sites, and these events are reported unfelt
with magnitudes are ranging from Mw -3 to Mw +1.7 [e.g. 5-7]. The hypocentral distributions are around the
reservoir for both horizontal and vertical extents.
Natural earthquakes are monitored to show that CO2 injection gives no significant influence on the natural
seismicity. The hypocentral depths of earthquakes are in the seismic basement deeper than those of microseismic
events. Monitoring area is also broader than those of microseismic events, e.g. up to 20 ~ 30 km. Target magnitude
range is around unfelt to felt natural earthquakes, the required monitoring frequency range is from a few seconds to
a few tens of Hz, accordingly.
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To detect the both events, required frequency range and monitoring area are from a few seconds to at least tens of
Hz and up to around 20 ~ 30km from CO2 injection well, respectively.
Based on the monitoring targets, we decided a layout of seismometers. There are two types of the layouts of
seismometers as described in Fig. 1; one is a vertical downhole monitoring array (Fig. 1a), and the other is a planar
monitoring network (Fig. 1b). The vertical downhole monitoring array includes multiple seismometers deployed
vertically at different depths in a single borehole well. The downhole monitoring can detect microseismic events
with negative magnitudes, but the detectable distance for small magnitude events is limited. This array is used, for
example, at the Weyburn, Otway, and In Salah CO2 injection sites [5-7].
The planar monitoring network deploys several seismometers at the surface or at shallow borehole wells. It can
detect more distant microseismic events and natural earthquakes, but the minimum detectable magnitudes are larger
than those detected by vertical downhole monitoring array.
Since our monitoring target is both microseismic events and natural earthquakes, we need to detect seismic events
with variable magnitudes up to 20 ~ 30 km from the injection well. Thus we decided to test the planar monitoring
network at the CO2 injection site.
Next, we chose a type of seismometers. There are some classification methods; frequency range, measurement
principles, and measuring objects (acceleration, velocity, displacement). In this study, we classified frequency
ranges of seismometers. There are four types of seismometers used at the field [8]; long period (used for worldwide
monitoring, 0.01 Hz to 1Hz), short period (used for regional monitoring, 0.1 Hz to 20 Hz), broadband (0.01 Hz to 50
Hz) and microseismic systems (1Hz to 10,000 Hz). The monitoring range is wide ranging from seconds to at least
tens of Hz to detect both microseismic events and natural earthquakes. Long period and short period seismometers
cannot cover the higher frequency ranges, and microseismic systems measurable up to 10,000 Hz are more than
sufficient frequency bands. Therefore, we decide to use the broadband seismometers which cover the required
frequency range of both events.
As a result of designing seismic monitoring network, we decided to use a planar monitoring network with
broadband seismometers.
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Fig. 1 Layout of seismometers (a) Vertical downhole monitoring array; (b) Planer monitoring network
2.2. Monitoring experiment at the Cranfield site, Mississippi, U. S. A.
We conducted a monitoring experiment at the large-scale CO2 injection site in the United States. Monitoring site is
the Cranfield Oilfield located in a rural area of southwestern Mississippi, in the United States (Fig. 2, left bottom).
This oilfield was selected for three reasons; large site area with suitable geometry to deploy the planar monitoring
network, low regional seismicity, and large-scale CO2 injection with annual injection rate of more than 1million
metric tonnes. In addition to those reasons, the site operator was willing to host.
Due to the low regional seismicity around the Cranfield site reported by [9], we could focus on detecting
microseismic events. Because only one monitoring station exists more than 100km northeast to the site, there was a
possibility that small natural earthquakes were unreported by the existing network. So we intended to detect the
Makiko Takagishi et al. / Energy Procedia 114 (2017) 4028 – 4039
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unreported natural earthquakes, too. We also expected that the large amount of CO 2 injection had a high possibility
to induced microseismic events.
Seismic
Monitoring
Station
䐠 24-3
䐡 EGL#7
䐟 44-6
䐢 31DAS
Highway
3km in radius
Cranfield, MS
䐣 45F2&F3
䐤 68-3
Fig. 2 Map of the planar monitoring network deployed at the Cranfield oilfield. Six near-surface borehole wells were deployed in a 3km radius
area to cover the entire area, and broadband seismometers were deployed at the depth of around 90m. The left bottom figure indicates a location
of the site.
At the Cranfield site, CO2-EOR floods started in 2008. A total of 11.6million metric tonnes of CO2 have been
injected, and 5.37 million metric tonnes have been stored as of February 2015. A total of 29 injection wells exist at
the site as of 2012, and the production wells are about the same number. Injection zone is the Lower Tuscaloosa
formation consisting of fluvial deposit in a broad four-way structural closure at a depth greater than 3,000m with
high porosity and permeability [10]. Injection and production are conducted from the north part of the site around
the (1)44-6 station in a clockwise. The oil and gas are produced by self-flow, elevating pressure at the reservoir by
means of CO2 injection. It takes about one year from CO 2 injection to beginning of the production. When we started
seismic monitoring in 2011, CO2 injection front was around (5) 45 F2&F3 station, and there was no activity around
the (6) 68-3 station area. CO2 injection began in summer of 2013 around (6) 68-3 station area.
We constructed a planar monitoring network which consists of six monitoring stations deployed about in 3km
radius (Fig. 1). Three component broadband velocity seismometers were installed at the depth of about 90 m (300ft).
Signals were measured continuously at the sampling frequency of 200 Hz. In our experiment, measurable frequency
range is from 0.018 to 80 Hz. The monitoring started on December 15, 2011 and finished on February 19, 2015. The
recorded data was processed by semi-automated processing and by visual judgement. The details of the signal
processing and results for the first two years are reported at [11] and no microseismic events are detected at the site
even though the monitoring network worked normally.
As a result of monitoring for more than three years, no microseismic events were detected within the Cranfield site.
Natural earthquakes around the CO2 injection site were not also detected, either. The detected waveforms were
identified as background noises, artificial noises, noises due to weather changes such as lightning strikes, and natural
distant earthquakes (Epicentral distances more than 300km with Mw>2.5). The recorder of the (2) 24-3 station at the
north part of the site (Fig.1 encircled in red) was not temporarily working properly, but no obvious influence was
found for the overall monitoring result.
To interpret this monitoring result, we estimated minimum detectable magnitudes of the deployed monitoring
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network by comparing frequency spectra calculated from Abercrombie’s spectral model at given magnitudes and
distances [12], and those calculated from background noise. This spectral model is suitable for small and near-field
seismic events, and calculation details are described in [13]. Signals of a perforation shot conducted at the reservoir
depth were detected at the (4) DAS31 station during the monitoring period, and we also estimated a magnitude of
the event based on the spectra model.
Fig. 3 indicates the magnitude – distance relation of the Cranfield site calculated using Abercrombie’s spectral
model. Circle symbols indicate the minimum detectable magnitudes at the distance ranging from 3 to 30km. If the
microseismic events occur at the reservoir depth just beneath the monitoring station, the minimum detectable
magnitude is estimated Mw -0.5 (distance of 3km with a blue circle), and the monitoring network can detect as low
as Mw0.0 if the events occur within the site (distance of 6km with a green circle). Seismic events with Mw +1.0 at
the distance of 30 km are able to be detected by this monitoring network. The perforation shot with a gray diamond
symbol with an error bar was estimated between Mw -0.5 and 0.
Perforation shot
Within the site (6km)
Above the injection point (3km)
Fig. 3 Magnitude-distance relation of the Cranfield site calculated with Abercrombie’s spectral model. Circles indicate the minimum detectable
magnitudes at the distance ranging from 3 to 30km. A gray diamond symbol (with an error bar) is obtained by a perforation shot.
At the beginning, we assumed that a large amount of CO2 injection was likely to cause microseismic events, but we
did not detect the events. Then we focused on investigating pressure changes at the reservoir. Since CO 2-EOR was
operated at the Cranfield site, the pressure change at the reservoir was limited when both CO2 injection and
production were operated.
CO2 injection started in summer of 2013 around the (6) 68-3 station. Reservoir pressure elevation is expected at
that area and time. So we reprocessed the data obtained at the (6) 68-3 station by visual judgment for a half year
including summer of 2013. If signals were identified, we examined signals by plotting particle motions for two
horizontals (EW-NS) and horizontal – vertical (EW-UD and NS-UD). Despite the condition, no events were
detected from the data. Therefore we concluded that induced microseismic events more than the minimum
detectable magnitude Mw -0.5 did not occur at the site.
As a result of the seismic monitoring at the Cranfield site, we obtained three points for conclusions.
1. No microseismic events were detected for more than three years of monitoring at the Cranfield site. Detected
signals were all identified as background noises, artificial noises, noises due to weather changes and natural
distant earthquakes at the distance of more than 300km with Mw>2.5.
2. The monitoring network worked normally overall. Even though a recorder at the (2)24-3 station did not work
properly temporarily, there were no apparent influences on the monitoring results.
3. Microseismic events with the magnitude of Mw -0.5 are detectable at the distance of 3km as a result of
Abercrombie’s spectral model analysis. Seismic events with M w 0.0 are detectable within the site. The
recorded perforation shot was estimated with magnitude between -0.5 and 0.
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2.3. Lessons learnt from the seismic monitoring experiment at the Cranfield site
Through the monitoring experiment at the Cranfield site, we obtained five lessons learnt for designing seismic
monitoring network to detect both microseismic events and natural earthquakes.
1. Spectral model analysis using Abercrombie’s model was effective to evaluate monitoring network performance.
This analysis can deal with small earthquakes in a short distance and should be conducted during the baseline
monitoring period.
2. The broadband seismometers were effective to measure natural earthquakes that occurred outside of the site.
The seismometers could detect events of Mw 2.5 at the distance of 300km from the Cranfield site. It would be
suitable to monitor natural earthquakes around the CO2 injection site.
3. Minimum detectable magnitude of Mw -0.5 was not sufficient to detect microseismic events at the Cranfield
site. This result suggests that seismic monitoring requires detecting events with smaller magnitudes.
4. We could not detect microseismic events by deploying a near-surface planar monitoring network with
broadband seismometers. There are two possible reasons why microseismic events were not detected. At first,
hypocentral distance was around 3km, and it was too far to detect events. The seismometers should be
deployed close to the injection point. The vertical downhole monitoring array is required to detect
microseismic events. Next, the sensors require higher detectable frequency range. Sensors which can detect up
to several hundred Hz might be required to monitor microseismicity. It is required at least two kinds of
seismometers to detect both microseismic events and natural earthquakes.
5. Occurrences of microseismic events are not simply related to injection amount or pressure changes at the
reservoir. It seems that there are multiple reasons to induce microseismic events.
Since we did not detect any microseismic events at the Cranfield site, we need to obtain information on
relationship between CO2 injection and occurrences of microseismic events by reviewing case studies of other CO2
injection sites. We also need to obtain information on monitoring network from the case studies.
3. Case study review
We reviewed case studies at the Decatur, and Lacq sites based on the lessons learnt from the monitoring
experiment at the Cranfield site. We focused on the monitoring network and relationship between CO 2 injection and
occurrences of microseismic events such as magnitudes, range of hypocenters, and seismicity patterns. Table 1
summarizes the case study review.
Table 1 Summary of review at the two CO2 injection sites
Monitoring network/
M range/
Hypocenter range
Purpose
microseismicity rate
Site/
Country
Reservoir
Decatur,
USA
(MGSC)
Saline
Aquifer
Two borehole arrays
(sf: 2,000Hz)
(Injection period)
2,573 located over 22months
Ö 117 events/month
Mw -1.91to +1.0 , Ave. Mw -0.85
Decatur,
USA
(USGS)
Saline
Aquifer
Surface and nearsurface network
(aperture: 8km)
sf: surface 200Hz
sf: near-surface: 500Hz
(Injection period)
179 located over 19 months
Ö ~ 10 events/month
Mw -1.13 to +1.26
1) 1 borehole array
fr: 1-800Hz
Lacq,
France
Depleted
Gas Field
2) Shallow-borehole
array (radius: 2km)
fr: 10-1000Hz
3) one surface
seismometer
fr: 0.5-50 Hz
*sf : sampling frequency [Hz]; fr: frequency range
Horizontal:
305 to 1,372m
Vertical: At and
below reservoir
3) 2 events (Mw - 0.5~0.3)
References
16 clusters
Post injection
seismicity
b =1
[14], [15]
Up to 2,600m
Depth :1.9km to
2.7km
3 clusters
Post injection
seismicity
[16], [17]
within 500m
Cluster
Post injection
seismicity
b=2.1
[18]
1) ~ 600 located/ 25months
Ö ~ 24 events/month
Mw -2 to -0.3; Mostly Mw -2 to -1
2) 14 located/ 39 months
Ö Less than 1 event /month
Mw > -1
Pattern
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3.1. Decatur site
3.1.1. Downhole monitoring by MGSC
The Decatur site is located in Illinois, United States [14]. The Illinois Basin-Decatur Project (IBDP) is conducted
by the Illinois state Geological Survey is a project of Midwest Geological Sequestration Consortium (MGSC). A
total of about one million metric tonnes of CO2 (1,000 tonnes per day) is injected to a saline aquifer at the depth of
about 2,140m for three years. The reservoir is the Cambrian Mt. Simon sandstone which consists of fluvial
deposited sandstone with high porosity and permeability. The pre-Mt. Simon with low porosity and permeability,
and the Precambrian basement are underlying the reservoir.
Microseismic monitoring network consists of a combination of two borehole geophone arrays in the injection well
(CCS1) and in a geophysical monitoring well (GM1) located approximately 61m west to CCS1. CCS1 is outfitted
with two levels of four component tools located within the Mt. Simon formation at depths of 1,751m, and 1,871m,
and one level located at the depth of about 1,500m. GM1 is outfitted with 31 levels of three component tools at
15.2m intervals between 624 and 1,052m depth with two of them located near ground level. The lowest seven levels
of geophones are not working. Microseismicity is monitored continuously with sampling frequency of 2,000 Hz.
The baseline survey for 18 months reveals the level of natural seismic activity is minimal.
During the first 22 months of the injection period, 10,123 microseismic events have been detected; of these, 2573
events are located and calculated moment magnitudes. Magnitudes have ranged between Mw -1.91 and +1.0 with a
mean of Mw -0.85. A total of 16 microseismic clusters have formed with a majority of the located events occurring
at north to northwest of CCS1, and half of the clusters have clear NE striking trends. 95% of the located events are
within a 1,372m radius of CCS1 excluding one cluster at 2,287m northwest of CCS1. It is reported that the area of
seismicity is nearly concordant with the predicted pressure elevate zone. Richer-Gutenberg analysis illustrates a
distinct tectonic characteristic evidenced by b-value of around 1. Events clusters span a vertical extent including the
injection interval, the lower Mt. Simon formation, and the upper portion of the Precambrian basement rock. Based
on several analyses of geology, microseismic activity tends to be spatially associated with Precambrian
paleotopographic features [14]. After CO2 injection stopped, microseismic events have still measured but the
seismicity rate and magnitudes decrease compared to during CO2 injection period [15].
3.1.2. Surface and near-surface monitoring by USGS
The U. S. Geological Survey (USGS) also has monitored seismicity at the Decatur site since 2013 [16-17]. The
purposes of monitoring are to monitor induced seismicity as a possible seismic hazard and to elucidate the general
patterns of the seismicity.
The near-surface and surface monitoring network consists of thirteen seismic stations. Nine stations are equipped
with both three component broadband seismometers and three component force-balance accelerometers installed at
the surface. The sensors of nine stations are recorded at the sampling frequency of 200 Hz [16]. Four stations are
equipped with three component force balance accelerometers at the surface and three component high-sensitivity
geophones installed in ~150 m deep boreholes recorded at the sampling frequency of 500 Hz[16]. The aperture of
this network is about 8km, centered on CCS1; with the four stations located within the radius of about 2km, and
surface stations located outside of the shallow boreholes. This network provides azimuthal coverage for
microseismicity with moment magnitudes (Mw) above about -0.5 [17].
For nineteen months of monitoring since July 2013, 221 events are detected and 179 of them are located with
magnitudes ranging from Mw -1.13 to +1.26. Hypocenters fall into three clusters distributed from north to northwest
at the maximum distance of 2.6km from CCS1with hypocentral depths between 1.9 and 2.7 km corresponding to the
reservoir and the Precambrian basement. Fault mechanism analysis indicates right-lateral to oblique strike-strip
mechanisms consistent with the regional maximum principal stress. Seismicity pattern indicates heterogeneity of the
redistribution of pore pressure within the reservoir. The redistribution probably caused by heterogeneous and
anisotropic permeability of the reservoir.
Makiko Takagishi et al. / Energy Procedia 114 (2017) 4028 – 4039
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3.2. Lacq-Rousse site
An integrated CCS project is conducted by TOTAL in the Lacq-Rousse area, Southwest of France [18]. From
January 2010 to March 2013, a total of 51 thousands metric tonnes of CO 2 (average about 70 tonnes /day) are
injected into a fractured dolomite depleted gas reservoir (Mano formation) at the depth of around 4,500m in the
Rousse field. Seismic monitoring is conducted by deploying a hybrid multi-scale passive seismic monitoring
network to address three monitoring objectives: watch triggered fault type seismicity around the field, distinguish
natural seismicity at north Pyrenees from induced seismicity, and assess CO2 injection-induced seismicity.
To watch triggered fault type seismicity within 5km, Shallow Borehole Array (SBA) consisting of seven 200m
borehole wells are deployed; one at the next to the injection well (RSE-1) and others in a 2km radius. Four-level
three component velocity geophones (bandwidth: 10-1,000Hz) ranging from about 120 to 200m are deployed at
boreholes. To distinguish natural seismicity from induced seismicity, a broadband surface seismometer (bandwidth:
0.05-50Hz) is deployed at the site. To assess injection induced seismicity, a deep downhole array (R&D network) of
three triaxial accelerometer-type sensors (bandwidth: 1-800 Hz), has been installed along an optical fiber cable in
the injection well between 4,200 and 4,400m with 100m spacing.
Monitoring period includes baseline, during and post injection periods. About 2,200 events have been detected by
R&D network, and a quarter of events are located (monitoring since 2011 due to the technical trouble). Most of the
located events are with magnitudes ranging from M w -2 to -1, and the largest event is Mw - 0.3. The majority of
events occurred within 500m from RSE-1, and the depth are within the reservoir depth with trends following
mappable fault directions located a few kilometers northeast. Microseismic events are also detected at the postinjection phase with magnitudes not exceeding Mw -1. The b-value based on Gutenberg-Richter’s equation is 2.1. It
appears that seismicity rate changes with average CO 2 injection rate. Fourteen CO2 injection induced events with
>Mw -1.0 and two events > Mw -0.5 are also detected by SBA and by a surface seismometer, respectively. Thirtyfive events related to activity of mappable faults at the depth of about 6km near the injection area are located by
Master network. A surface seismometer recorded natural seismicity that occurred at the north Pyrenean tectonic
activity about 30km away from the injection well.
3.3. Summary of the case study review
Through the case study review of two CO2 injection sites, we obtained three points for seismic monitoring network
and five points the relationship between CO2 injection and occurrences of microseismic events, respectively.
Seismic monitoring network
1. The vertical downhole monitoring arrays are used to monitor microseismicity at the both sites. The minimum
vertical distances from injection depth to the sensor of the vertical downhole monitoring arrays are 269 m at
the Decatur site, and 100 m at the Lacq site. Maximum detectable frequencies of the sensors are at least 100 Hz
for both borehole array and near-surface and surface network monitoring at the two sites.
2. Near-surface or surface monitoring network can detect microseismic events at the two sites. Even though those
events are detectable, numbers of detected events per month are less than 10 % of those detected by using
vertical downhole arrays (see event rate at Table 1). At the Lacq site, the SBA has a main role to monitor
seismicity related to the known fault located at a few kilometers from the injection well.
Relationship between CO2 injection and occurrences of microseismic events
1. Reported microseismic events are all unfelt. Many of them are with magnitudes of around Mw -1 (Lacq: Mw -2
to -1, Decatur: Mw -0.85 on average). The maximum magnitude is Mw +1.26 at the Decatur site.
2. Microseismic events are detected at the lateral distance of up to 2.6 km from the injection well. The horizontal
extents of event locations are different at two sites. Microseismic events are mainly located at the reservoir
depth, and some of them are located above or below the reservoir.
3. Hypocenters of microseismic events form clusters, and many of them are extending to the similar directions as
the horizontal maximum principle stress or existing faults around the CO2 injection sites.
4. CO2 injection induced seismicities continue after CO2 shut-in at the both sites.
5. Gutenberg-Richter’s relation of b-values is different at the two sites. At the Decatur site, b- value is around 1.0
similar to that of the natural seismicity; b-value exceeds 2 at the Lacq site. This is due to difference of
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geological heterogeneity and stress condition at both sites.
4. Seismic monitoring network
4.1. microseismic monitoring network to detect microseismic events
Microseismic monitoring network to detect and locate CO2 injection induced seismicity is necessary to meet the
following requirements.
First, many of the microseismic events are with magnitude of around M w -1; seismic events with Mw -1 should be
located by the monitoring network with high accuracy. Even though near-surface or surface monitoring network can
monitor events with Mw -1, the number of detected events are less than 10 % compared to vertical downhole
monitoring array. Therefore, at least one vertical downhole monitoring is necessary to ensure detecting and locating
the microseismic events.
Next, the maximum detectable frequency of the seismometers should be at a few hundred Hz. Microseismic
systems shown at 2.1 such as high sensitivity seismometers or geophones are suitable to monitor the microseismic
events.
Then, the minimum distance from a sensor to the injection point should be close up to a few hundred meters. Case
study review showed that the minimum distances of downhole arrays are up to 300m.
Monitoring period should include before/during/post CO2 injection periods. Baseline monitoring is essential to
obtain background natural seismicity. The microseismicity still continue after CO 2 shut-in, so the post-injection
monitoring is necessary. Continuous monitoring is conducted at the Cranfield, Decatur, and Lacq sites. Summary of
the microseismic monitoring to detect and locate microseismic events is shown at the Table 3.
4.2. Seismic monitoring network to detect natural earthquakes around the CO2 injection site
The planar monitoring network is suitable to monitor natural earthquakes that occur around the CO2 injection sites.
The monitoring stations encircle the CO2 injection well, the predicted CO2 plume and the pressure elevated zone.
The radius of the monitoring coverage area is a few kilometers. Our monitoring experiment showed that the planar
monitoring network was effective to detect natural earthquakes that occur outside of the network. The Decatur case
study also indicates that the horizontal extent of the microseismic events correspond to the area of pressure elevated
zone [14].
Next, seismometers are deployed at the surface or preferably shallow borehole wells to suppress the cultural noise.
At least four monitoring stations are required to determine four parameters; hypocenters (x, y, z coordinates) and
occurrence of times. All the monitoring examples fulfilled these two requirements.
The broadband seismometers can monitor the target natural earthquakes ranging from unfelt (Mw> 0) to felt,
medium or moderate events. Corresponding frequency range is from seconds to a several tens of Hz. Broadband
seismometers of Cranfield and Decatur sites have similar frequency ranges.
It is also cost effective to utilize existing earthquake observation network. For example, in Japan, Hi-net (High
Sensitivity Seismograph Network Japan) operated by NIED (National Research Institute for Earth Science and
Disaster Resilience) is a dense seismic monitoring network with station to station distance of around 20 km on
average. This network can detect and locate seismic events down to around Mw 0 [19], equal to our required
detectability.
4.3. Seismic monitoring network at the CO2 injection site in Japan
We propose the seismic monitoring network to monitor microseismic events and natural earthquakes at the CO2
injection site based on the monitoring experiment and a case study review. A summary table and a schematic view
of the monitoring network are shown in Table2 and in Fig. 4, respectively.
A hybrid monitoring network, a combination use of a vertical downhole array and a planar monitoring network is
necessary to monitor microseismic events and natural earthquakes. A clear monitoring role should be established for
each monitoring network like the Lacq site; a vertical downhole array monitors microseismic events, and the planar
monitoring network monitors natural earthquakes.
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Makiko Takagishi et al. / Energy Procedia 114 (2017) 4028 – 4039
We have two reasons to deploy hybrid monitoring network. The first is the wide frequency monitoring range.
Monitoring frequency range of two events is from seconds to a few hundred Hz. There are no seismometers that can
cover the entire frequency range at the current technology. The Cranfield site experiment showed that only
deploying broadband seismometers did not meet the monitoring requirements. Therefore at least two different types
of seismometers need to be deployed.
The second reason is the required monitoring accuracy of seismic events. Since we intend that hypocenters of
microseismic events are utilized for CO2 injection rate control, the high location accuracy within a few kilometers is
required. Thus, seismometers are required to be deployed close to the CO2 injection point. Natural earthquakes are
monitored e.g. up to 20 ~ 30km to make sure that there is no significant change for natural seismicity. So,
monitoring microseismic events requires much more precise detection and location.
Natural earthquakes site are monitored to gain public acceptance for CO2 injection operation, thus the planar
monitoring network needs to continue monitoring even if a part of the monitoring network does not work properly.
Besides the four monitoring stations, a few more stations are required to deploy to keep the monitoring detectability
and location accuracy. Preferably, the planar monitoring network can detect a portion of microseismic events,
because a borehole measurement sometimes does not work properly due to technical problems (e.g. [18]).
Furthermore, a frequency spectrum analysis is effective to evaluate the detectability of monitoring network.
Estimating the minimum detectable magnitude helped interrupting the monitoring result at the Cranfield site. This
analysis should be conducted for both vertical downhole array and planar monitoring network before CO2 injection
starts. The analysis is also preferred to perform during and post injection period since the frequency and noise
characteristics may change.
㻟㻯㻌㼟㼑㼕㼟㼙㼛㼙㼑㼠㼑㼞
㻿㼑㼢㼑㼞㼍㼘㻌㼗㼙
䡚㻝㻜㻜㼙
㻿㼡㼞㼒㼍㼏㼑㻌㼛㼞㻌
㼟㼔㼍㼘㼘㼛㼣㻌㼎㼛㼞㼑㼔㼛㼘㼑
㻮㼛㼞㼑㼔㼛㼘㼑
㼍㼞㼞㼍㼥
㻵㼚㼖㼑㼏㼠㼕㼛㼚 㼣㼑㼘㼘
㻺㼍㼠㼡㼞㼍㼘
㼑㼍㼞㼠㼔㼝㼡㼍㼗㼑
㻹㼕㼏㼞㼛㼟㼑㼕㼟㼙㼕㼏 㼑㼢㼑㼚㼠㼟
Fig. 4 Proposed seismic monitoring network to monitor microseismic events and natural earthquakes
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Makiko Takagishi et al. / Energy Procedia 114 (2017) 4028 – 4039
Target events
(4.1)
Microseismic
events
Table 2 Monitoring network requirements
Pattern/ Knowledge
Requirements for monitoring network
Magnitude: Unfelt events (Mw -2 to +1.26)
Vertical downhole monitoring array
Detectability: Mw -1 located with high accuracy
Many of events around Mw -1
Distance: Up to 3km from an injection well
Stations: At least 1 borehole with multiple-level sensors
Depth: Reservoir depth, and above and
Sensors: Measurable up to several hundred Hz
Sensors and injection point should be close.
below the reservoir
Monitoring period: Pre/during/after CO2 injection
Seismicity pattern: clusters, b-values are
different at each sites.
Monitoring method: Continuous monitoring
*Seismicity continues after CO2 shut-in.
(4.2)
Natural
earthquakes
Magnitude: Unfelt to felt earthquakes
Monitoring distance: e.g. ~ 20 to 30km
from a site
Depth: Within the seismic basement
Seismicity occur from baseline monitoring.
Planar monitoring network that covers predicted pressure elevated
zone with the radius of several km
Detectability: Events with Mw0 should be located.
Number of stations: At least four stations
Sensor depth: Surface or shallow borehole wells
Sensors: broadband velocity seismometers ( seconds ~ up to 50/100Hz)
Monitoring period: Pre/during/after CO2 injection
Monitoring method: Continuous monitoring
(4.3)
Seismic
monitoring
network
Detect microseismic events and natural
earthquakes to confirm safe CO2 injection.
Hybrid monitoring network: Combination use of a vertical
downhole monitoring array and planar monitoring network
Monitoring role: A vertical downhole array for microseismic events and
planar monitoring network for natural earthquakes
Monitoring period: Pre/during/after CO2 injection
Monitoring method: Continuous monitoring
*Detectability evaluation using frequency spectra is effective to check
the monitoring performance.
5. Conclusions
In this paper, we discussed designing a seismic monitoring network which monitors both microseismic events and
natural earthquakes.
At first, we tested the planar seismic monitoring network with broadband seismometers at the Cranfield site. No
microseismic events with more than Mw -0.5 were detected even though the monitoring network worked normally.
We obtained lessons learnt for designing seismic monitoring network.
Then, we reviewed two case studies at the Lacq and the Decatur sites focusing on the design of monitoring network
and relationship between CO2 injection and occurrences of microseismic events. Microseismic events are all unfelt,
and many of the events are around Mw -1. The vertical downhole monitoring array is suitable to detect and locate
microseismic events. Maximum detectable frequencies of the sensors require at least a few hundred Hz.
Finally, we discussed and proposed the monitoring network that can monitor both microseismic events and natural
earthquakes based on the monitoring experiment and a case study review. A hybrid monitoring network, a
combination use of a vertical downhole monitoring array and a planar monitoring network is required to monitor
both events. Clear monitoring roles should be established for each monitoring network. A vertical downhole
monitoring array monitors microseismic events; the planar monitoring network monitors natural earthquakes that
occur around the CO2 injection sites. Planar monitoring network needs to continue monitoring even though a part of
the monitoring network does not work properly. Continuous monitoring for pre/during/post injection periods is
necessary. Detectability evaluation using frequency spectra is also effective to check the monitoring performance.
Future work
In Japan, more than 90% of CO2 storage capacity is stored at the offshore area [20], the monitoringnetwork is
deployed at the offshore area. Seismic monitoring is expected more difficult due to technical difficulty of drilling
deep borehole wells, and the borehole monitoring might be limited. Therefore we have to rely on the surface
monitoring deploying Ocean Bottom Seismometers (OBSs) or Ocean Bottom Cables (OBCs) on the sea floor.
Signal to noise ratio of the recorded data gets lower compared to the onshore monitoring because of hydrographical
phenomena. Improved signal processing methods to distinguish events from noises and the event location method
are required.
Makiko Takagishi et al. / Energy Procedia 114 (2017) 4028 – 4039
4039
In Japan, CO2 injection at the Tomakomai CCS site started in April, 2016 [21]. Seismic monitoring network in
Tomakomai is a combination use of two inclined borehole arrays and planar seismic network including OBCs,
OBSs encircling the injection point at the radius of 2km, and an onshore seismometer. Besides the own network,
recorded data at the 10 Hi-net stations around the Tomakomai site are used to monitor natural seismicity around the
site including the active fault and volcano which are located 20km to 30 km from the site [21-22].
This monitoring network is not the same but similar to our proposed monitoring network. Therefore, we can test
the effectiveness of the proposed monitoring network by investigating the monitoring data obtained at the
Tomakomai site.
Acknowledgements
This work is a part of an R&D project “The Development of Safety Management Technology for Large-Scale CO2
Geological Storage”, Commissioned to the Geological Carbon Dioxide Storage Technology Research Association
by the Ministry of Economy, Trade and Industry (METI), Japan.
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