1
Innovative CO
2
injections in carbonates and advanced modelling for
numerical investigation
J.Carlos de Dios
a*
, Yann Le Gallo
b
, Juan A. Marín
a
a
Foundation Ciudad de la Energía (CIUDEN). Avenida del Presidente Rodríguez Zapatero 24492
Cubillos del Sil (Spain)
b
Geogreen, 2 Rue des Martinets 92500 Rueil-Malmaison (France)
Corresponding author
#
: jc.dedios@ciuden.es
e-mail address authors: ylg@geogreen.fr ; ja.marin@ciuden.es
1. ABSTRACT
CO
2
geological storage in deep saline aquifers was recently developed at industrial scale mainly
in sandstone formations. Experiences on CO
2
injections in carbonates aquifers for permanent
trapping are quite limited, mostly from US projects such as AEP Mountaineer, Michigan and
Williston Basin.
The behavior of fractures in carbonates plays a key role in those reservoirs in which porous
matrix permeability is very poor, which drives the CO
2
plume migration through the fracture
network where hydromechanics and geochemical effects take place due to injection.
Hontomín (Spain) is the actual on-shore injection pilot in Europe (EP Resolution of 14 January
2014), whose reservoir is comprised of fractured carbonates. Existing experiences from field
scale tests conducted on site have supported to better understand the behavior of this type of
reservoirs for CO
2
geological storage.
Innovative CO
2
injection strategies are being carried out in ENOS Project (EU H2020 Programme,
http://www.enos-project.eu). First results based on field tests conducted at Hontomín, and the
advanced modelling developed so far will be analyzed and discussed in this article, as well as,
the description of future works. The evolution of operating parameters such as flow rate,
pressure and recovery term during the tests confirm the CO
2
migration through the fractures.
Keywords: CO
2
Storage, carbonate fractures, ENOS, operating parameters, advanced modelling
Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 27 July 2018 doi:10.20944/preprints201807.0537.v1
© 2018 by the author(s). Distributed under a Creative Commons CC BY license.
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2. INTRODUCTION
Most of experiences on CO
2
geological storage worldwide have been conducted in rock
formations with high permeability in the pore matrix, mainly in sandstones [Michael et al.,
2011] [Krevor et al., 2012] [Torp and Gale 2004] and in some cases in carbonates such as
AEP Mountaineer Project [Gupta 2008] [Mishra et al., 2013], Michigan and Williston Basin
[Finley et al., 2013] [Worth et al., 2014] in USA. There are also CO
2
-EOR projects which injects
CO
2
in carbonate formations such as the IEAGHG Weyburn-Midale CO
2
project [Wilson and
Monea, 2004] in Canada and the Uthmaniyah CO
2
-EOR demonstration project in Saudi
Arabia [Liu et al., 2012]
The design of safe CO
2
injection strategies and the understanding of trapping mechanisms
in carbonates with poor matrix porosity and fluid transmissivity through the fractures are
challenging matters so far. To give a proper solution, the study of hydrodynamic and
mechanical effects induced by the CO
2
plume migration in the fractures, and those ones
produced by the geochemical reactivity due to the acidification of reservoir water, is needed
to increase the knowledge on the behavior of these reservoirs for CO
2
geological storage
and later industrial deployment [de Dios et al., 2017].
Hontomín Pilot Plant [Neele et al., 2014], operated by Fundación Ciudad de la Energía
(CIUDEN), is the only current onshore injection site in Europe for CO
2
geological storage,
recognized by the European Parliament [EP resolution 2014] as key test facility for CCS
technology development. The pilot is located close to Burgos in the north of Spain, and its
reservoir is comprised of fractured carbonates with poor matrix porosity [Campos et al.,
2014].
To demonstrate innovative injection strategies and history matching approaches for
increased confidence of operators in safely managing sites is a priority within ENOS Project
(EU H2020 Programme, http://www.enos-project.eu). It is expected to increase the
understanding on CO
2
injection in fractured carbonates with low primary permeability, and
to develop safe and efficient operational procedures using real-life experience from running
the Hontomin pilot [Gastine et al., 2017].
CO
2
injection in rock formations with main fluid transmissivity through fractures usually
requires high values of pressure, which means a risk for the pair seal-reservoir integrity
[Vilarasa et al., 2014]. On the other hand, as mentioned above, the geochemical reactivity
due to reservoir water acidification impacts on the carbonate permeability [Gaus et al.,
2015]. These matters must be considered to design safe and efficient injection strategies in
ENOS project to improve the hydrodynamic stability and control of storage integrity. First
injections conducted at Hontomín using synthetic brine and CO
2
will be described in the
article, analyzing the evolution of operational parameters and discussing the results.
To model the CO
2
migration through the fractures and predict the injection effects in the
carbonates is a challenge as well. An advanced modeling workflow with FracaFlow™ used to
elaborate a Digital Fracture Network (DFN) [Bourbiaux et al., 2005] around the injection well
and characterize the main properties of the fracture network will be described in the article.
The dynamic characterization of fracture properties was then performed using an advanced
automated history matching with CMOST™ to model the pressure behavior around the
injection well based upon a previous modelling work [Le Gallo et al., 2017].
Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 27 July 2018 doi:10.20944/preprints201807.0537.v1
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Taking into account the first results from injections and the modelling developed so far, the
authors will describe the planned works to be conducted in ENOS project in order to find
solutions based on real life experiences.
The results from first injections performed at Hontomín site confirmed the singularity of this
reservoir where CO
2
migration is through the carbonate fractures. The evolution of main
operational parameters such as well head pressure (WHP), flow rate, bottom-hole pressure
(BHP) and distributed temperature along the well tubing confirm the injection of CO
2
in
liquid phase. Taking into account the information provided by the first results, it is necessary
to determine the long term evolution of BHP and flow regarding the injection strategy used
and particularly the pressure recovery period during the fall-off phase according to the
cumulative amount of CO
2
injected on site.
3. DESCRIPTION OF PILOT PLANT
Hontomín site represents a structural dome where the pair seal-reservoir is located within
Jurassic Formations (Marly Lias and Sopeña respectively). Overburden is formed of Dogger,
Purbeck and Weald and the underlying seal is located at Triassic Keuper [Rubio et al., 2014].
Figure 1 shows the lithological column of Hontomín site and the geological cartography of
the area.
Fig 1.- Lithological column and geological map of Hontomín area
Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 27 July 2018 doi:10.20944/preprints201807.0537.v1
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Pair seal-reservoir is located at the depth of 900 in the top of the dome and 1832 m in flanks.
Marly Lias and Pozazal form the main seal (160 m thick) comprised of marls, shales, limestones
and calcareous mud stones. Carbonates reach the average of 50% in the seal composition.
Reservoir is Sopeña Formation (120 m thick) comprised of limestone at its upper part and
dolomites at the bottom [Kovacs 2014], with a high level of fracturing in different geological
blocks which does not affect the seal integrity.
Two wells were specifically drilled and monitored during the site construction reaching the
depth of 1600 m, one for injection (HI) and other for observation (HA) [de Dios et al., 2016]. HI
well is equipped with super duplex tubing anchored to the liner by a hydraulic packer (1433 m
MD), two P/T sensors below, Distributed Temperature Sensing System (DTS) and Distributed
Acoustic Sensing System (DAS) along the tubing, six ERT electrodes and a deep water sampling
(U tube) installed in the bottom hole.
On the other hand, HA well is equipped with a fiber glass tubing anchored to the liner with 3
inflatable packers (1275 m, 1379 m and 1497 m MD) which distribute the open hole in intervals
with different permeability, 4 pressure/temperature (P/T) sensors and 28 ERT electrodes
installed in the seal and reservoir. Both well schemes are shown in figure 2.
Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 27 July 2018 doi:10.20944/preprints201807.0537.v1
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Fig 2.-Schemes of injection well HI (left side) observation well HA (right side) and panoramic
view of the pilot
Facilities for CO
2
injection and water conditioning, the seismicity monitoring network comprised
of 30 passive seismic stations covering an area of 18 Km
2
and hydrogeological monitoring wells
to control shallow aquifers also form part of the pilot.
The main challenge faced during Hontomín hydraulic characterization was the low injectivity
existing on site. The injection of brine and CO
2
to characterize the pair seal-reservoir produced
geomechanical changes and geochemical reactivity effects that improved the permeability in
the fracture network while the matrix does not appear to significantly contribute to the storage
capacity for the time being [de Dios et al., 2017].
Hontomín is at the early injection phase, thus, all long term effects that condition the safe and
efficient CO
2
geological storage must be determined and analyzed.
4. INNOVATIVE CO
2
INJECTION STRATEGIES
It is planned to inject on site up to 10 000 metric tonnes of CO
2
during the period 2016-2020 in
ENOS project, with the purpose of better knowing the behavior of this tight fractured reservoir,
mainly what concerns the improvement of hydrodynamic stability and control of storage
integrity, for finding safe and efficient operation conditions.
Therefore, the design of CO
2
injection strategies to be conducted in the project must be based
on criteria of efficiency and safety, for later up-scaling to industrial deployment. The operating
procedures must ensure efficient energy consumption, maximizing reservoir capacity and
preserving seal integrity [Gale et al., 2001].
As mentioned before, CO
2
injection in carbonate reservoirs with low matrix permeability shows
specific features that are different from injection in porous media. Considering the CO
2
migration
is dominated by the fracture network, the following gaps need to be studied in order to define
proper strategies for the injection:
• Bottom-hole pressure (BHP) evolution and its influence in the cap-rock integrity and
reservoir behavior
• Bottom-hole temperature (BHT) evolution and the analysis of thermal effects due to
injection
• Monitoring CO
2
evolution along the well tubing and the fluid density reached at the
bottom hole.
• Energy consumption and operation performance for each planned injection
Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 27 July 2018 doi:10.20944/preprints201807.0537.v1