Analogue modelling of leakage processes in unconsolidated sediments

International Journal of Greenhouse Gas Control 90 (2019) 102805

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Analogue modelling of leakage processes in unconsolidated sediments a,⁎

Franz May , Michael Warsitzka a b c

b,1

, Nina Kukowski

T

c

Federal Institute for Geosciences and Natural Resources, Stilleweg 2, 30655, Hannover, Germany Institute of Geophysics of the Czech Academy of Sciences, Boční II/1401, 141 31, Prague 4, Czech Republic Institute of Geosciences, Friedrich Schiller University Jena, Burgweg 11, 07749, Jena, Germany

A R T I C LE I N FO

A B S T R A C T

Keywords: Unconsolidated sediment Fluidization Fracturing Analogue modelling CO2 leakage

Cenozoic sedimentary basins typically contain young, unconsolidated sediments, potentially suitable for CO2 storage, e.g. the Utsira sand in the Norwegian North Sea Basin. During burial and compaction, formation fluids are squeezed out of the sedimentary deposits. These fluids can mobilize and transport sediments to overlying strata, forming seismic-scale clastic intrusions and shallow gas accumulations, to the Earth’s surface, or to the sea floor, where they are expelled through pockmarks or mud volcanoes, respectively. These deformation processes are described as subsurface sediment remobilization (SSR). Little attention has been paid to the formation mechanisms of SSR in the search for CO2 storage formations, though they may be relevant for storage safety, either occurring naturally, triggered by CO2 injection, or being fossil features providing pathways for fluid migration in the subsurface. Structural characteristics of SSR features are well known, but formation processes and dynamics are poorly understood. We have developed a scaled analogue experimental approach and performed systematic laboratory experiments in order to comprehend and quantify the conditions controlling and triggering SSR, the critical rheological properties of sediments, and storage structures (layer thickness, anticlines, faults). Experimental results are presented and potential implications for the selection of storage sites and storage safety are discussed in this article.

1. Motivation In shallower parts (< 1000 m) of sedimentary basins, sediments are mostly not fully lithified. These can include coarse-clastic rocks with high porosity and injectivity that could be used as storage formations, whereas fine-clastic unconsolidated rocks may provide seals that can adapt to movements and maintain continuity during geotechnical or natural deformation of a storage site. On the other hand, highly porous unconsolidated sediments can be subject to fluidization processes and migrate in multiphase mixtures of gas, oil, sediment, and formation water across formations, thus facilitating and creating pathways for fluid escape (seal bypass systems) to the surface (Duranti and Hurst, 2004; Cartwright et al., 2007; Hurst and Cartwright, 2007; Andresen, 2012). One of the sedimentary basins that has been studied and proposed for CO2 storage projects is the North Sea Basin. Storage sites are in operation or have been planned in the Norwegian (Sleipner, Snøhvit), British (Golden Eye, Endurance, Bunter), and Dutch (ROAD, K12b) sectors of the North Sea. At most of these sites, injection takes place, or is planned to do so, in consolidated rocks. However, in the case of

leakage, e.g. along wells, significant quantities of CO2 might migrate into overlying unconsolidated sediments, mobilizing these and generating intricate pathways for fluid leakage (Katzung et al., 1996; Landrø et al., 2019). Clustering of CO2 emissions from industrial areas around coastal and inland hubs of the adjacent countries has been suggested through joint transport in ships and/or trunk pipelines to offshore storage sites (e.g. Element Energy, 2010; Bentham et al., 2014; Brownsort et al., 2015; Pale Blue Dot and Axis Well Technologies, 2016; Zero Emission Technology Platform, 2016; North Sea Basin Task Force, 2017). The European Community has implemented a funding scheme for energy infrastructure projects of common interest (PCI) in order to promote the trans-national technical interconnection of its member states. International CO2 transportation networks qualify for PCIs. In November 2017, the Commission published the list of PCIs, including four projects for CO2 transportation, including the industrial hubs of Teeside (England), St. Fergus (Scotland), Eemshaven (NL) and Rotterdam (NL) (European Commission, 2018). Along the North Sea shores, plenty of power stations and industrial areas are emitting CO2. Ports and pipeline terminals provide space and infrastructure for CO2 transport. Some of the abandoned platforms located above depleted

⁎

Corresponding author. E-mail address: [email protected] (F. May). 1 Formerly at Institute of Geosciences, Friedrich Schiller University Jena, Burgweg 11, 07749, Jena, Germany. https://doi.org/10.1016/j.ijggc.2019.102805 Received 31 August 2018; Received in revised form 31 July 2019; Accepted 3 August 2019 1750-5836/ © 2019 Elsevier Ltd. All rights reserved.

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Fig. 1. Overview of the distribution of features of unconsolidated sediment deformation within the Cenozoic strata of the North Sea Basin. The map has been compiled using data published by Hovland et al. (1987), Higgs and McClay (1993), Cartwright (1994), Lonergan et al. (1998), Dewhurst et al. (1999), Duranti et al. (2002), Hovland et al. (2002), Berndt et al. (2003), Løseth et al. (2003), Schroot and Schüttenhelm (2003), Shoulders and Cartwright (2004), Evans et al. (2005), Huuse et al. (2005), Hustoft et al. (2007), Huuse et al. (2007), Shoulders et al. (2007), Andresen et al. (2008), Cartwright et al. (2008), Huuse et al. (2009), Andresen et al. (2010), Cartwright (2010), Kirk (2011), Benvenuti et al. (2012), Gafeira et al. (2012), Thöle et al. (2016), Römer et al. (2017), Müller et al. (2018), and Stück et al. (2018).

• Can SSR features provide pathways for fluid migration or leakage in the overburden of CO stores? • Can pre-existing, naturally formed SSR features be mechanically reactivated by a pore pressure increase due to CO injection? • Can CO injection trigger the formation of new SSR structures?

hydrocarbon fields and thus could be used for storage. They are connected by abandoned pipelines to terminals close to these ports and industrial CO2 sources (Brownsort et al., 2016). The North Sea Basin is therefore an area crucial for carbon capture and storage (CCS) in Europe. Subsurface sediment remobilization (SSR) features include pipes and chimneys, clastic dykes and intrusions, mud diapirs, gravitational gliding in unstable delta systems, mass transport deposits (including debris flows resulting from slope collapse), and polygonal fault systems (Lonergan et al., 1998; Huuse et al., 2010; Andresen, 2012). The size of such structures can range from the dm-scale up to the scale of several hundreds of meters (e.g. Jolly and Lonergan, 2002; Shoulders and Cartwright, 2004; Hurst et al., 2011) and more. At the sea floor or on land, fluids escape through pockmarks, gas seeps, or mud volcanoes, respectively. Fluids may also escape along fractures, visible at the seafloor, like the Hugin Fracture in the Central North Sea (Lichtschlag et al., 2018). Such deformation features are widespread in Cenozoic sediments within the North Sea area (Fig. 1). From the perspective of storage safety, the abundance of such processes and features implies the following questions:

2

2

2

The observation of different types of large-scale SSR structures e.g. in reflection seismic or bathymetric data raises questions about the causes and mechanisms of their formation. Whereas permeable fractures usually are the result of long-term tectonic processes like movement along a fault, typically in combination with thermally efficient fluid flow that can lead to solution and therefore increasing porosity and permeability, for some types of SSR features like pipes, chimneys, mud volcanoes, or sand injections, rapid, highly dynamic formation seems reasonable (e.g. Kopf, 2000; Jolly and Lonergan, 2002; Cartwright and Santamarina, 2015). An important pre-condition for such deformation processes to occur is the creation of excessive fluid overpressure. Fluid overpressure in sediments is mostly built-up by compaction disequilibrium, which is generated during burial if expulsion of formation fluids in under-compacted sediments is restricted (Osborne and Swarbrick, 1997). However, additional processes are

2

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Table 1 List of physical properties of the analogue materials used in this study. Errors represent standard deviation of at least three measurements. Material properties were determined in the laboratories of the Institute for Geosciences (Friedrich Schiller University Jena). Intrinsic permeability and critical fluidisation velocity were measured with a self-constructed apparatus applying a defined volumetric flow rate through cylindric samples and recording the basal air pressure (referring to the procedure presented by Rodrigues et al., 2009). Frictional coefficients and cohesion were determined with a ring shear tester (type SCHULZE RST-XS; Schulze, 2014) according to the procedure described by Lohrmann et al. (2003). Parameter

Symbol

Unit

Glass powder (fine, high cohesion)

Glass powder (coarse, low cohesion)

Silicate cenospheres

Grain size (range/mean) Grain density Bulk density Porosity Intrinsic permeability Coefficient of internal peak friction Peak cohesion Critical fluidization velocity

d ρG ρB ϕ κ μ C qK

[μm] [kgm−3] [kgm−3] – [m2] – [Pa] [ms−2]

1-100/ ˜17 2600 861 ( ± 0.8) 66.9 ( ± 0.3) 3.7 × 10−11 ( ± 0.6 × 10−11) 0.73 ( ± 0.003) ˜113 ( ± 7) 0.014 ( ± 0.001)

5-300/ ˜120 2600 1316 ( ± 12) 49.4 ( ± 0.4) 3.8 × 10−11 ( ± 4.4 × 10-12) 0.85 ( ± 0.01) ˜46 ( ± 16) 0.21 ( ± 0.007)

5–300/ ˜140 750 416 ( ± 2.3) 44.6 ( ± 0.3) 9.8 × 10−11 ( ± 0.5 × 10−11) 0.47 ( ± 0.01) ˜62 ( ± 5) 0.027 ( ± 0.0004)

address the questions listed above. The results of our experiment series thus contribute to the understanding of deformation processes triggered by fluid overpressure, such as the formation of pipes, hydraulic fractures, sand injections, mud volcanoes, or pockmarks. Specific geological structures, such as anticlines and fault-bound traps are often preferred targets for CO2 storage, because of their potential for trapping buoyant fluids. Therefore, we particularly modelled these in our experimental study by including up-domed or displaced layers of different analogue materials.

required to create excessive overpressure, which can be for instance: fluid influx from deeper, overpressured sources through e.g. highly permeable faults, shear-induced liquefaction during seismic shaking, or lateral pressure transfer due to e.g. sediment loading (e.g. Jolly and Lonergan, 2002; Cartwright et al., 2008; Jonk, 2010). Many of the SSR structures identified in the geological record of the North Sea are associated with natural gas accumulations in the shallow subsurface. Wells drilled for hydrocarbon production from deep reservoirs frequently penetrate such shallow gas accumulations, providing pathways for fluid leakage (Vielstädte et al., 2017). Processes generating SSRs however, are difficult to study in the subsurface. Hydraulic properties of various types of pipes and chimneys are difficult to assess from seismic images alone (Karstens, 2015). While the surface expression of natural SSR features and fluid expulsion can be observed directly, seismic images reveal the conduits of material transport, but they are of limited resolution. Erosion may lead to the exposition of fossil systems in outcrops in geological times (e.g. Hurst et al., 2016). However, physical conditions and dynamics of deep fluid transport and deformation processes are not accessible to direct observation. In well constrained laboratory experiments, however, the observed formation of SSR structures enables to understand the dynamics of such systems. Both, physical analogue experiments and numerical simulations provide useful strategies to better understand the conditions and dynamics of SSR. Numerical simulations use well constrained boundary conditions and in contrast to analogue experiments, they are not limited by physical scales but by the adequate coupling of the many physical processes that interact simultaneously in highly dynamic non-equilibrium conditions. Few attempts have been made to model SSR and associated fluid flow numerically (e.g. Räss et al., 2018). Fluid flow modelling through existing chimneys usually considers chimneys as static porous media (Karstens et al., 2017), including uncertain assumptions about internal structures and permeability. Analogue experiments provide a unique opportunity to study large deformations as well as processes potentially evolving with a fluctuating speed in high spatial and temporal resolution, although scaling of the experiments limits quantitative interpretation and thus, potential upscaling to nature. Analogue experiments have been undertaken in studies of dynamic processes of sediment deformation driven by fluid overpressure, e.g. Cobbold and Castro (1999), Rodrigues et al. (2009), Mourgues et al. (2011, 2012), Bureau et al. (2014). Some aspects, such as the propagation of fluid pathways and fractures or changes of hydraulic conductivity caused by SSR, have not been studied in these experiments though. Altogether, adequately scaled laboratory experiments employing suitable analogue materials seem to be appropriate to simulate the complex nature of SSR formation. Therefore, we developed a sandbox type analogue experimental approach including the built-up of fluid overpressure to study large-scale (> ˜100 m) SSR phenomena to

2. Experimental setup and procedure Details of the experimental setup are described in Warsitzka et al. (2017). Here, we outline the main features of the experiments, necessary to understand the results presented. To adequately represent deformation processes and the dynamics of natural processes, geometrical, kinematic and dynamic variables should be suitably scaled between model and nature (Hubbert, 1937). The scaling procedure in our study follows approaches by Cobbold and Castro (1999) and Mourgues et al. (2012). We chose a geometrical scaling ratio (model/ nature) of about 10−4 so that 10 cm in the model represent about 1000 m in nature. To achieve dynamical scaling, forces driving deformation, which are related to the fluid pressure and fluid flow, need to be scaled with the same ratio as forces resisting deformation, which mainly depend on material strength and lithostatic pressure. Based on the applied scaling procedure, the dynamic scaling ratio relating stresses between model and nature is ˜5 × 10-5. This means for instance that an experimental lithostatic pressure of 1000 Pa (of a roughly 7 cm thick layer; Tab. 3) equals ˜20 MPa in nature, which is a suitable order of magnitude for shallow sediments. Experimental fluid pressures should then be in the same order of several hundreds of Pa (Table 1). The strength of the materials is basically described by the frictional shear strength, which depends on the cohesion and the coefficient of internal friction according to the Mohr-Coulomb criterion (e.g. Lohrmann et al., 2003). Cohesion needs to be scaled by the same dynamic scaling factor as stresses. However, cohesion of natural rocks might vary over more than one order of magnitude (Contardo et al., 2011; Abdelmalak et al., 2012), e.g. cohesion of marine sediments is in the order of a few MPa (e.g. Hoshino et al., 1972). Assuming this, cohesion of the analogue materials should be in the range of several tens of Pa (e.g. Abdelmalak et al., 2012), which is in good agreement with the measured values of cohesion of the materials used in this study (Table 1). It should be emphasized that dynamical scaling can only be achieved, if inertia forces are negligible (Hubbert, 1937), i.e. if deformation processes in the analogue experiment are relatively slow. Studies by Rodrigues et al. (2009) showed that fluid flow during fluidization of analogue materials is turbulent and deformation processes are fast so that inertia forces are not negligible. Thus, dynamical scaling 3

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differently coloured particles in the analogue materials, are cross-correlated between two successive images (e.g. Adam et al., 2005). An offset of the patterns in the image plane is then converted into 2D motion vectors. Streaming air has been used to produce a fluid pressure in the dry analogue materials (Cobbold and Castro, 1999). Using air as analogue for natural fluids has the advantage that capillary effects and clumping of wet material can be avoided, which would fundamentally change material properties measured under dry conditions. Controlling the flow of air into a basal pressure chamber created a laterally constant fluid overpressure at the base of the layered analogue materials (Fig. 2). Air flow, injected locally through a needle valve, simulated CO2 injection. Air flow through the basal chamber and through the needle valve was controlled independently. Increasing the flow of air through the bottom or the needle valve stepwise, fluid pressure in the layers was gradually increased. When a stable state of constant pressure was achieved, the flow of air was increased, until deformation of the analogue materials commenced. Further increasing flow resulted in progressive deformation and ultimately in breakthrough of reservoir material to the surface. Reducing the flow of air stopped the transport of fluidized material. After finishing the experiment, the air flux was increased again in some experiments (Table 2) to quantify and compare the critical pressure necessary to reactivate deformation structures. Measuring the air pressure in the basal chamber at the beginning of and after finishing each experiment, facilitated calculations of the intrinsic permeability and its change due to deformation according to Darcy’s law.

is probably only fulfilled before the onset of fracture breakthrough or fluidisation, respectively. Furthermore, experiments using flowing air to produce pore fluid pressure are not scaled for time, i.e. lack kinematic scaling (Mourgues and Cobbold, 2006). However, since our models intend to simulate mechanical effects due to changes in stress conditions during CO2 injection, a suitable dynamic scaling of the pore pressure in relation to lithostatic pressure and material strength is more important. Basically, CO2 storage formations consist of a basal high-permeability reservoir layer overlain by a low-permeability, mechanically strong caprock. For convenience, the terms “reservoir layer” (RL) and “cover layer” (CL) are used here for these respective layers to describe the layering in the models. In order to find suitable analogue materials to achieve this layering, numerous dry granulates and powders, all of which exhibit a Mohr-Coulomb elasto-plastic rheology (e.g. Schellart, 2000; Lohrmann et al., 2003; Ritter et al., 2016), have been tested for their mechanical and hydraulic properties (Warsitzka and Kukowski, 2017). Based on these tests, we chose hollow silicate spheres, which are characterized by a high permeability, a low density and a small frictional strength (Table 1), as representative reservoir material. Furthermore, two types of glass powder were selected as analogue for a low-permeability caprock. These materials are characterised by a low permeability, a large density, and a large frictional strength (Table 1). The finer glass powder has a significantly higher cohesion compared to the coarser one so that the impact of cohesion on sediment remobilization processes could be studied through our experiments (Table 2). In all experiments (besides experiments R2, A3D1, A3D2), there was an additional top layer consisting of reservoir material, which was added to better identify fluid leakage through the underlying cover layer. Natural potential storage sites are typically characterized by various structural geometries, such as domes, anticlines or faults, resulting from, for instance, syn- or post-depositional uplift of salt domes or tectonic deformation. Thus, we included such structures in the suite of our experiments. Additional information on experiments carried out in the framework of this project can be found in a digital supplement (Warsitzka and Kukowski, 2019). Experiments were performed in a box of 80 cm length and 5–40 cm width (Fig. 2) representing a region of 8 × 0.5 or 8 × 4 km, respectively, in nature. The variable width allowed to perform pseudo-2D and 3D experiments. As 3D experiments require considerable more effort than 2D experiments, while evolving internal structures are not easy to observe, we conducted most experiments in 2D, but included some 3D experiments to observe surface deformation not biased by boundary effects caused by the bordering glass panes (Table 2). Acrylic glass walls enabled monitoring of deformation processes in frontal and in top view with digital cameras. For kinematic analyses, displacement vectors were calculated by digital image correlation (DIC) using the software OpenPIV (http://www.openpiv.net/; Taylor et al., 2010). In the DICanalysis, differences in pixel patterns of grey values, e.g. due to

3. Results We performed several systematic series of experiments. Their main results are presented using some of our experiments as examples to illustrate the importance of cohesion and the effects of “tectonic” structures like anticlines and faults on the formation of SSR structures. 3.1. Reference experiments – effect of cohesion In order to test the role of the cohesion of the cover layer, two reference experiments were conducted (R1 and R2; Fig. 3) in both of which layers were flat. Air was injected into the basal pressure chamber at a predefined constant rate such that the air pressure in the basal chamber equaled approximately 40% of the lithostatic pressure of the material column. During the experiment, air was injected through the needle valve in the center of the reservoir layer with stepwise increasing flow rate. This resulted in an increasing local air pressure in the surrounding of the tip of the needle valve. The reference experiments reveal that generally in experiments with a high-cohesion cover layer, its uplift precedes fracturing, followed by the injection of fluidized material into the cover layer and its extrusion at the surface. In

Table 2 Main characteristics of the experiments described in this article. The Lab codes refer to the experiments as documented in Warsitzka and Kukowski (2017). Reactivation means that air was again injected after the end of the main experiment. CL – cover layer, PC – pressure chamber, NV – needle valve. Name

Dimension

Type

Air flux

Cohesion of CL

Height of CL above NV [cm]

Tot. normal stress above NV [Pa]

Reactivation

Lab code

R1 R2 A2D1 A2D2 A2D3 A2D4 A3D1 A3D2 F2D1 F2D2 F2D3 F2D4

2D 2D 2D 2D 2D 2D 3D 3D 2D 2D 2D 2D

Reference exp. Reference exp. Anticline Anticline Anticline Anticline Anticline Anticline Fault Fault Fault Fault

PC + NV PC + NV only PC only PC PC + NV PC + NV PC + NV PC + NV only PC PC + NV PC + NV PC + NV

High High High High High Low high Low high high high Low

6.7 7.9 6.2 6.4 6.2 6.8 6.4 6.4 6.8 6.7 6.2 7.1

796 1267 825 841 829 1263 723 1082 867 865 910 1240

x x – – x x – x – – x –

HV33 HV26 SV05 SV04 SV06 SV10 SV15 SV16 SV11 SV12 SV18 SV13

4

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Fig. 2. Schematic sketch of the experimental box and the air supply system in (a) frontal view and (b) top view. The pseudo 2D experiments were performed in the narrow, 5 cm wide configuration, whereas the wider, 40 cm wide configuration was used to conduct 3D experiments. Arrows indicate air flow directions. Flow rate controllers regulate the air flux into the pressure chamber and through the needle valve. PRL – pressure in the lower reservoir layer, PPC – pressure in the basal chamber, PNV – pressure in the needle valve (NV), QNV – volumetric air flow through the needle valve, QPC – volumetric air flow into the basal chamber.

decreased during the ongoing extrusion of material of the lower reservoir layer. In the reference experiment R2 consisting of a low-cohesion cover material (glass powder, coarse; Table 1), a different, eruptive type of deformation was observed. At critical air flux (QNV = 10.3 l min−1), the cover layer directly above the tip of the needle valve expanded (Fig. 3b), i.e. the granular packing was disintegrated due to fluid pressure and flowing air in the pore space. This process took only a few seconds (< 5 s) and was accompanied by surface uplift. Shortly after, without a further increase of the air flux in the needle valve, abrupt fluidization of the cover material occurred within a roughly 5 cm wide, V-shaped zone. This fluidization resulted in continuous mixing and fountain eruption of the cover and the reservoir material and a widening of this zone. A sudden decrease of air pressure was detected during the beginning of the fluidization (Fig. 3d).

experiments with a low-cohesion cover layer, expansion of the reservoir layer precedes fluidization of material that results in a V-shaped structure of mobilized and mixed material. In R1, the cover layer consisted of high-cohesion glass powder (fine) (Table 1). Reaching a critical volumetric flow rate (QNV = 7.21 l min−1), the cover layer was uplifted in an approximately 26 cm wide anticline (Fig. 3a). Due to bending, initial fractures formed at the upper crest and at the lower flanks of the anticline. The crestal fracture than propagated from the top of the cover downwards. Fractures at the anticline’s flanks propagated upwards vertically before branching laterally in the middle of the cover layer. The uplift of the CL remained stable until QNV was again increased. During the next step (QNV = 7.49 l min−1), branched, tilted fractures formed originating from the tips of the initial, vertical fractures (Fig. 3a). The CL above these fractures was further uplifted. As soon as the fracture broke through to the surface, material of the lower reservoir layer intruded into the fractures and then extruded at the surface accompanied by the formation of Vshaped fluidization zones in the upper reservoir layer. While reservoir material was continuously mobilized and extruded, the uplifted cover layer subsided into the depleting lower reservoir layer. The pneumatic data sets (Fig. 3c) reveal that air pressure in the basal chamber, the needle valve and the reservoir layer proportionally increased with increasing air flux, even if initial deformation structures occurred. A sudden pressure release, however, was recorded at the onset of the breakthrough of the fractures at the surface and the subsidence of the uplifted anticlines. Afterwards, fluid pressure gradually

3.2. Cover layer geometry – anticline Using high-cohesion cover material, two different shapes of anticlines were modelled (Fig. 4), one in which the cover layer possessed a constant thickness (A2D1) and two others, in which the curved cover layer had a laterally variable thickness (A2D2, A2D3; Table 2). In both types of experiments, overpressure had been generated only by air injection into the basal pressure chamber. Deformation structures in these experiments were similar to those observed in the reference experiment R1 (Fig. 3). This indicates that the 5

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Fig. 3. Deformation structures and pneumatic data of two reference experiments with even cover and reservoir layers. In both experiments, air pressure in the basal chamber was kept at 40% of the lithostatic pressure and air flux through the needle valve was increased stepwise. (a) R1 consisted of high-cohesive glass powder with a small average grain size (Table 1). The distance between the fractures at the base of the cover layer is about equal to the width of the anticlinal uplift. (b) R2 consisted of low-cohesive glass powder with larger average grain size (Table 1). CL – cover layer, RL – reservoir layer. (c) and (d) Pneumatic data of the flow rate controller and the pressure sensors. The orange line marks the onset of initial deformation (uplift of the cover layer or expansion, respectively). The red lines mark the time of maximum deformation (breakthrough of fractures or fluidization of the cover layer, respectively). PRL – pressure in reservoir layer, PNV – pressure in the needle valve, PPC – pressure in the basal pressure chamber, QPC – volumetric flow rate into the basal pressure chamber, QNV – volumetric flow rate through the needle valve. The suffix “_crit” defines values for the initial (init.) and the maximum (max.) deformation. ΔP represent the amounts of pressure decrease during deformation.

pressure within the reservoir was relatively low (436 Pa, Fig. 4f), compared to a critical reservoir pressure of 671 Pa in experiment A2D2. In the 3D experiment A3D1, an anticline of high-cohesion cover material was modelled. Experimental procedure and the shape of the anticline in cross section were similar to those in A2D3, i.e. the long axis of the anticline was elongated parallel to the long axis of the box. To monitor surface deformation at the top of the cover layer, no upper reservoir layer was added in A3D1. Since the reservoir material had a relatively small bulk density (Table 1), the total lithostatic pressure was still similar in A2D3 and A3D1 (Table 2). During initial deformation (QNV = 7.5 l min−1), the trace of a tiny fracture could be observed, which struck parallel to the elongation of the anticline (Fig. 5a). Additional fractures perpendicularly branched off this central fracture. The calculated displacement field reveals several distinct regions of horizontal movement away from these fractures (Fig. 5b). This indicates tilting of specific regions, which was caused by the uplift and bending of the cover layer.

geometry of the cover layer has only a minor influence on the fracture pattern and its propagation. However, there are some characteristic differences: In case of a cover layer with a curved surface (A2D1), uplift of the cover layer commenced in the center of the anticline, while almost the entire cover was uplifted in the case of flattop anticline (Fig. 4c). The larger thickness of the cover layer at the flanks of the anticline in A2D2 lead to a smaller width of uplift compared to A2D1 (Fig. 4a). The initial deformation commenced at pressures of 725 Pa in A2D2 and 671 Pa in A2D1, respectively, within the reservoir layer (Fig. 4b, d). These values correspond to 81 and 86% of the lithostatic pressure in the two experiments. In A2D3, that is equivalent to A2D2, air was also injected through the needle valve in addition to generating a constant fluid pressure in the basal chamber (40% of the lithostatic pressure). The width of the uplifted region of the cover layer fold in A2D3 (Fig. 4e) was significantly smaller than in A2D2. Due to the high local fluid pressure at the needle valve, the deformation of the cover commenced, when the 6

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Fig. 4. Comparison of the deformation structures and pneumatic data of three experiments with anticlinal reservoir structures. (a/b) In A2D1, the cover layer was shaped in an anticlinal structure with uniform thickness and air flux was only injected into the basal chamber. (c/d) In A2D2, the cover layer anticline possessed an even upper surface and air flux was applied only in the pressure chamber. (e/f) Similarly, in A2D3, the cover anticline had an even upper boundary. The air pressure in the basal chamber was kept at 40% of the lithostatic pressure, additionally air had been injected through the needle valve. Orange and red lines in the diagrams mark the onset of initial (uplift of the cover layer) and maximum deformation (breakthrough of fractures). CL – cover layer, RL – reservoir layer. PRL – pressure in the lower reservoir layer, PNV – pressure in the needle valve, PPC – pressure in the basal pressure chamber, QPC – volumetric flow rate into the basal pressure chamber, QNV – volumetric flow rate through the needle valve. The suffix “_crit” defines values for initial (init.) and maximum (max.) deformation. ΔP represent the amounts of pressure decrease during deformation.

gradually increased. Approaching the critical air flux (QNV = 10.2 l min−1), dome-shaped surface uplift took place, which was likely caused by expansion of the cover material in an internal conical structure, similar to that observed in the 2D experiment (Fig. 6a). During surface uplift, radially striking fractures could be observed in the uplifted region (Fig. 7b). Immediately before the eruption of fluidized material occurred, concentric fractures appeared, which is visible in Fig. 7b and in a concentric ring of low radial displacement in Fig. 7a, at t + 3 s. The processes of expansion lasted roughly 5 s. Afterwards, fluidization and material extrusion lead to the formation of a crater, surrounded by a ˜15 cm wide wall of ejected material.

After several further steps of increasing the air flux through the needle valve (QNV = 10.5 l min−1), extrusion of reservoir material began. Other than in the similar 2D experiment A2D3, breakthrough of reservoir material in A3D1 occurred through the central fracture directly above the position of the needle valve (Fig. 5a, time t + 40 s). This indicates that the fractures formed at the flanks of the anticline did not penetrate to the surface. Instead, the central fracture at the anticline’s crest propagated downward into the reservoir layer. The 2D experiment A2D4 consisted of a low-cohesion cover layer, which formed an anticlinal structure with a flat upper boundary (Fig. 6). Air pressure was set to 580 Pa in the basal chamber (equivalent to 40% of the lithostatic pressure of the material column). Additionally, air was injected through a needle valve in the center of the reservoir layer. Approaching the critical air flux (QNV = 10.6 l min−1), material expansion and surface uplift commenced in a V-shaped, conical zone, which can be recognized by upward directed displacement vectors (Fig. 6a, t = 4750 s). Two seconds later, a sudden breakthrough of the conical structure of fluidized cover material occurred accompanied by a rapid pressure drop within the reservoir layer (Fig. 6b). The cover layer was bent upwards at the margin of the conical fluidization structure. The displacement vectors, shown in Fig. 6a at 4753 s, reveal downward movement of material at the boundary of the fluidized zone, while in the center upwards directed movement prevailed. V-shaped structures within the fluidization zone result from material of the upper reservoir layer sinking into the steep-sided fluidization structure. In the 3D experiment A3D2, an anticlinal structure made of lowcohesion cover material similar to the one in A2D4 was modelled. Surface deformation in A3D2 was monitored in top view, while air flux in the basal chamber was constant and air flux in the needle valve was

3.3. Cover layer geometry – fault In this experiment series, a pre-existing normal fault in the cover layer was modelled to examine effects of lateral variations in thickness and height of the cover layer on the structural evolution. The fault geometry however, was not produced by mechanical displacement of the layers, but shaped by filling analogue materials of different height into the box. Thus, the fault planes do not represent zones of mechanical weakness or hydraulic pathways. In the experiments with high-cohesion cover material (F2D1, F2D2, F2D3; Table 2), deformation styles similar to those of the reference experiment could be observed (Fig. 8). Nevertheless, the width of the uplifted region and the positions of fractures were affected by the location of the “normal fault”. In the experiment with a relatively small offset along the “normal fault” and when air was only injected into the basal pressure chamber (F2D1), uplift of the cover layer focused on the shallower part (“foot wall block”) (Fig. 8a). The crestal fracture first 7

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Fig. 5. (a) Detailed images of the surface deformation of the 3D experiment A3D1. (b) Vector fields of the (2D) horizontal displacement derived from the DIC analysis. Movement of the surface indicates tilting of certain regions of the cover layer. The air pressure in the basal chamber was kept at 40% of the lithostatic pressure. Additionally, air has been injected through the needle valve.

(QNV = 6.57 l min−1) a relatively small part of the cover layer (˜12.5 cm wide) was uplifted. A fracture developed at the left edge of the “hanging wall block”. Focused air flux through this fracture lead to fluidization of the upper reservoir layer directly above the tip of the fracture. After 2 s, without further increase of the air flux, the material of the lower reservoir layer became fluidized directly above the needle valve, whereas the former fracture was closed (Fig. 8e). Mixing and surficial extrusion occurred within this sub-vertical fluidization structure. During ongoing material extrusion, the right part of the “footwall block” sunk into the depleting reservoir. Compared to experiment F2D2 (PPCcrit =555 Pa), deformation of the cover layer commenced at significant lower pressures (PPCcrit =402 Pa) (Fig. 8f), since the thickness of the cover layer above the needle valve was significantly smaller. In F2D4 with a low-cohesion cover material, air was only injected into the basal pressure chamber (Fig. 9) so that air pressure was uniformly increased across the entire base of the material. Nevertheless, expansion and fluidization focused in the region of reduced thickness of the cover layer. This indicates that the air flow concentrated in this region, where the overall permeability was small, because of the reduced thickness of the cover layer. During ongoing mobilization and material mixing, the fluidization zone increased laterally and affected the entire region of the normal fault. Similar as in A2D4, the vector field

developed in the center of the “footwall block” at the top of the cover layer and proceeded downwards. On the “hanging wall block” faults propagated upwards from the bottom of the cover layer and branched laterally in the middle of the cover layer. At higher flow rates (Fig. 8b), fluidization and extrusion of the reservoir material along the fractures occurred contemporaneously with the subsidence of the uplifted cover layer. In the experiment F2D2 (Fig. 8c) in which air was injected through the needle valve in addition to a constant air flux into the basal chamber (PPC = 40% of the lithostatic pressure), the width of the uplifted cover layer fold was smaller than in F2D1. The uplift was bounded by two large fractures that penetrated from the bottom of the cover layer, while a small fracture developed at the top of the cover layer in the center of the uplifted area. The two bounding fractures were inclined, dipping sub-parallel to the fault plane. Extrusion of fluidized material was restricted to the fracture connected to the shallower part of the reservoir layer in the footwall block. In experiment F2D3, the throw of the fault equaled the thickness of the cover layer so that a highly permeable connection existed between the upper and the lower reservoir layer (Fig. 8e). Thus, the overall permeability in this experiment was roughly twice as high as that in F2D2. When reaching the critical air flux through the needle valve 8

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Fig. 6. Structural evolution of a fluidization structure on top of an anticline of low cohesive materials and pneumatic data of experiment A2D4. Air pressure in the basal chamber was kept at 40% of the lithostatic pressure. Air flux through the needle valve was increased stepwise. (a) Sequential images including the vector field of the vertical displacement derived from the DIC analysis (upward in red, downward in blue). (b) Detailed image showing the V-shaped fluidization structure. Fluidized material in the center erupts through the center of the structure, while material in the boundary zone sinks down. The lamination within the cover material is bent upwards along the flanks of the V-shaped structure. (c) Pneumatic data of the main experiment shown in (a) and (b). (d) Pneumatic data of the reactivation experiment show that the critical pressures and volumetric flow rates required to reactivate the fluidization are lower than to initiate deformation in the main experiment. CL – cover layer, RL – reservoir layer. PRL – pressure in the lower reservoir layer, PNV – pressure in the needle valve, PPC – pressure in the basal pressure chamber, QPC – volumetric flow rate into the basal pressure chamber, QNV – volumetric flow rate through the needle valve. The suffix “_crit” defines values for the initial (init.) and the maximum (max.) deformation. ΔP represent the amounts of pressure decrease during deformation.

created pathways along the glass panes. Fortunately, the boundary effects had little impact on the pressure/flow relation and overall deformation pattern in most experiments. This was confirmed through performing several experiments twice. In 3D experiments, boundary effects were not present, however internal fluidization could not be observed directly. In the 3D setting, the view on an experiment’s surface is analogous to observations at the surface in nature, e.g. by acoustic images of the seafloor. In the 3D experiment with a high-cohesion cover layer, the lateral migration of uplift and faulting of the surface could be observed prior to the breakthrough of fluidized material and subsidence of the uplifted fold (Fig. 5). For the full understanding of SSR processes and resulting features, both types of experiments were necessary and provided complimentary information. The experiments were performed with a single fluid phase only. In nature, two or three fluid phases might be present and capillary forces might complicate fluid flow as long as the solid phases maintain their position and contact of grains. Air compressed in the order of a few hundreds of Pa behaves like an ideal gas, whereas CO2 is strongly nonideal. In nature, CO2 streams would pass by the critical point during their passage from the reservoir to the surface and locally drastic

reveals the downward movement of material at the boundaries of the fluidization zone at 2191s (Fig. 9b), whereas a turbulent upward movement prevailed in the center. 4. Discussion In this section, we first discuss some limitations of our experimental approach that have to be kept in mind when quantitative results obtained in the laboratory experiments are compared to natural situations. Then we deal with fluid pressure, which plays a dominant role in initiating SSR, and compare our laboratory SSR systems with those occurring in nature. Finally, we strive topics of the actual debate about storage safety and discuss possible implications for geotechnical CO2 storage. 4.1. Limitations of laboratory experiments Most of the experiments were conducted in the 2D setting, because internal deformation could be observed through the side glass pane. Boundary effects were visible in some experiments, where the air flow 9

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Fig. 7. (a) Displacement of the analogue material surface of the 3D experiment A3D2. Sequential images of the vector field of the (2D) horizontal displacement derived from the DIC analysis. Horizontal movement illustrates accelerated uplift and radial expansion of the material. (b) Ring faults strike radial at the uplifted surface. (c) Pneumatic data show a pressure drop during the breakthrough and fluidization of the material. The air pressure in the basal chamber was kept at 40% of the lithostatic pressure, additionally air has been injected through the needle valve. PRL – pressure in the reservoir layer, PNV – pressure in the needle valve, PPC – pressure in the basal pressure chamber, QPC – volumetric flow rate into the basal pressure chamber, QNV – volumetric flow rate through the needle valve. The suffix “_crit” defines values for the initial (init.) and the maximum (max.) deformation. ΔP represent the amounts of pressure decrease during deformation.

reasons is that cohesion of a material is a crucial factor that is little known for sediments under in-situ conditions and also dependent on grain shape, grain size distribution, and the type of fluids bonding grains. In clay rich sediments, large internal surface areas and electrical charges facilitate strong bonding forces. In our experiments, fluid overpressure was maintained by a laterally constant air flux. Lasting, “unlimited” fluid supply during expulsion of fluidized sediments may be rarely met in nature. The Lusi mud volcano in Indonesia (Mazzini et al., 2007) is such an extreme example. Generally, fluid fluxes are constrained by the permeability of the conduit and by the size of catchment areas around conduits and thus, should decrease after the fluids in its near environment have been drained. We continued air flux to extreme stages in order to find out threshold values of over-pressuring and to gain comprehensive understanding of inherent physical process. Therefore, the final stages of our experiments should not be considered as likely leakage scenarios for CO2 storages. However, the non-linear expansion of compressed CO2 could lead to enhanced leakage processes, especially from accumulations at shallow depth (Pruess, 2008). The dynamics of such vigorous eruptions cannot be studied with the chosen experimental approach though. The experimental approach followed here has proven a suitable and versatile tool for investigating deformation processes of unconsolidated sediment driven by fluid overpressure. It is useful to understand processes that have formed natural features or that feed active surface manifestations of SSR, difficult to observe in real time. On the other hand, observations of natural examples may help evaluating experimental results and further improving the experimental setup as well as scaling of the experiments. Well constrained laboratory experiments

changes of pressure, volume and temperature could occur (e.g. Pruess, 2008). This might induce additional forces on the grain package of the unconsolidated sediments promoting their fluidization and fracturing. Thus, in the advanced stages of deformation, the expulsion of fluidized sediments by expanding CO2 may be more dynamic in nature compared to the laboratory experiments using air instead. If ideal Coulomb materials are used as analogue materials, cohesion can be neglected, and time does not need to be scaled (Krantz, 1991; Schellart, 2000; Lohrmann et al., 2003). However, powder-like materials as used in our study possess a significant cohesion, which has a major impact on the scaling as well as on the deformation style of the cover layer. Scaling issues, in the sense that time and velocities are not scaled as soon as material fluidization begins, cannot be ruled out in models using streaming air as pore fluid (e.g. Rodrigues et al., 2009). However, in our experimental study the pressure conditions and fracture initiation processes before onset of major fluidization were addressed. As indicated by the systematic relation between experimental results (e.g. critical pressure, width of uplifted anticline) and varied parameters (e.g. thickness of the cover layer), the experiments apparently did not suffer from major scaling inconsistencies. Furthermore, structures and deformation processes observed in our models (e.g. branched fractures in the high-cohesion cover layer, conical fluidization zones in low-cohesion cover layers), are similar to those observed in other experimental studies (e.g. Rodrigues et al., 2009; Nermoen et al., 2010; Mourgues et al., 2012) reflecting their overall reproducibility. Critical pressures at which the analogue materials began to deform are difficult to upscale to natural pressure thresholds required for optimized use of storage capacity or safe storage operation. One of the 10

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Fig. 8. Stages of the structural developments and pneumatic data of experiments F2D1, F2D2 and F2D3 in which the cover layer was shaped in a normal fault geometry. (a/b) In F2D1, only the basal pressure chamber was used. (c/d) In F2D2, the air pressure in the basal chamber was kept at 40% of the lithostatic pressure, additionally air has been injected through the needle valve. (e/f) In F2D3, the fault offset was equal to the thickness of the cover layer and air flux was injected into the basal chamber and through the needle valve. Orange and red lines in the pneumatic data mark the onset of initial (uplift) and maximum deformation (breakthrough of fractures) of the cover layer. CL – cover layer, RL – reservoir layer. PRL – pressure in the lower reservoir layer, PNV – pressure in the needle valve, PPC – pressure in the basal pressure chamber, QPC – volumetric flow rate into the basal pressure chamber, QNV – volumetric flow rate through the needle valve. The suffix “_crit” define the values for the initial (init.) and the maximum (max.) deformation. ΔP represent the amounts of pressure decrease during deformation.

caprock failure might be better modelled in laboratory experiments.

could serve as test cases for numerical simulators and calibration of numerical approaches. Initial fluid dynamics triggering instabilities in the packing of grains and fluidization might well be simulated with numerical methods. The highly dynamic processes during leakage and

Fig. 9. Formation of a fluidized zone in low cohesive material in experiment F2D4. The air pressure in the basal chamber was kept at 40% of the lithostatic pressure and air flux through the needle valve was increased stepwise. (a) Sequential images showing the vector field of the vertical displacement derived from the DIC analysis (upward in red, downward in blue). (b) Detailed images of the V-shaped fluidization zone. CL – cover layer, RL – reservoir layer. QPC – Volumetric air flow into the basal chamber. 11

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in experiments with low-cohesion cover material (Fig. 11). In the first case, this increase resulted from open fractures filled by material of the reservoir layer and penetrating the low-permeability cover layer. The maximal outlier of ˜0.74 (Fig. 11) was estimated for experiment A2D1, in which a widely opened fracture in the centre of the box lead to outburst and flushing of the cover layer material. In other experiments with high-cohesion cover layer material, the fracture width was smaller and the fractures were partly closed after reducing air flux. In low-cohesion cover layer material, grain packing becomes disintegrated due to fluidization causing an increase of porosity, which usually goes along with increased permeability.

4.2. Fluid pressure evolution The critical pressure required to initiate SSR is one of the technical parameters of particular interest, as injection permits usually prescribe maximum values for safe storage operation. Apart from pressures required for hydraulic fracturing and capillary entry pressures, the critical pressure for SSR should be taken into account in the calculation of maximum tolerable reservoir pressures in CO2 storage sites. In our experiments, pressures have been measured within the basal chamber, in the air supply tube to the needle valve and within the lower reservoir layer (Fig. 2). The different positions of the sensors have to be taken into account when interpreting the pressure history of experiments. Due to high flow velocities, the sensors measure dynamic pressures (Nermoen et al., 2010). In natural underground reservoirs pressures are dynamic in the near-well surroundings, while further out pressures may be more static. Critical pressures measured in the basal pressure chamber and the lower reservoir layer of our experiments might be a suitable proxy for the critical pressure required to uplift or fluidize the cover layer. Thus, critical pressures were compared to the lithostatic pressure of the entire material column (PPCcrit/σN(tot.)) and of the cover layer directly above the needle valve (PRLcrit/σN(CL)), respectively (Fig. 10a and b). In most of the experiments shown here, critical pressures are lower than respective lithostatic pressures. However, in some experiments it exceeded the lithostatic pressure. These were experiments, in which air was injected only in the basal pressure chamber and not through the needle valve. It has to be noted that air pressure in the material decreases linearly from the base to the top of a porous layer (Cobbold et al., 2001). The tip of the pressure sensor in the lower reservoir layer was positioned only 2 cm above its base (Fig. 2). However, the actual critical pressure acting on the base of the cover layer is located roughly 3 cm above the tip of the sensor in case of a modelled anticlinal structure (e.g. Fig. 4). Due to the linear decrease, the actual critical pressure should be lower so that the ratio to the lithostatic pressure is also lower. In experiments in which the needle valve was active, however, the air flow from the needle spread rather radially away from the tip of the needle valve (Mourgues et al., 2012). Thus, the pressure measured at the tip of the sensor is similar to that acting at the base of the cover layer. Overall, it can be stated that critical pressures are close to lithostatic, but more likely sub-lithostatic. Further experiments carried out in the frame of this project showed that critical pressures for the onset of deformation are systematically higher, if the thickness or the density of the cover layer and, therefore, the lithostatic pressure is larger (see Warsitzka and Kukowski (2019) for more information). As the lithostatic pressure in natural settings also depends on thickness and bulk density of the sediments, it can be inferred that the onset of SSR occurs at higher critical pressures, if the overpressured reservoir is buried under a thicker or denser sediment cover. These are important implications for transferring experimental findings to nature, even though quantitative upscaling should not be applied (cf also 4.4). Critical pressures for the reactivation of existing deformation structures are lower compared to critical pressures necessary to initiate them (Fig. 10c and d). In experiments with a high-cohesion cover layer, the critical pressure required for reactivation of SSD was up to 40% lower than the critical pressure to initiate such deformation, as the cover layer had already been fractured and fluidized reservoir material could easily move into these fractures. In experiments with a low-cohesion cover layer, fluidization structures were reactivated at critical pressures up to 22% lower than the initial critical pressures. Presumably, the higher reactivation pressure may be due to fluidized material of the low-cohesion cover layer having settled to a rather dense grain package after the flow of air was stopped, so that the material partly regained its strength. Due to the formation of fractures in and fluidization of the analogue materials, bulk permeability of the entire material column increased by ˜16% in experiments with a high-cohesion cover material and by ˜20%

4.3. Comparison of laboratory experiments with natural SSRs Our physical laboratory experiments of SSR processes resulted in structures similar to features observed in sedimentary basins and outcrops associated with natural fluid migration, underground fluid storage, and even volcanic environments. Karstens and Berndt (2015) distinguished different types of chimneys from seismic data in the central North Sea and from outcrop studies on the Colorado Plateau. Type A probably was generated by fluidization of sediments, penetration of the overburden, and rapid expulsion (blow-outs) of large amounts of natural gas, formation water and fluidized sediment. Type B is rather comprised by a network of fractures that could have been pathways for more persistent, less vigorous leakage of natural gas from reservoirs below. They argue that the availability and type of fluids may be one of the reasons for the appearance of the two types of chimneys. According to our experiments, cohesion of unconsolidated sediments may be another reason for different styles of SSR and types of chimneys observed. In our experiments we observed v-shaped structures in the fluidization zones in analogue material of low cohesion. Downward bending or v-shaped reflections within chimneys and upward bending of bedding have been observed in seismic images of chimneys (Karstens, 2015). The origin of the v-shaped reflections is a matter of debate. Pockmark stacking, or collapse and compaction of fluidized sediment are possible explanations for the origin of such features. Similarly, inwards dipping layers have been observed in volcanic diatremes and in the root zones of maar craters formed by phreatomagmatic eruptions (e.g. Lorenz, 2003; Lorenz and Kurszlaukis, 2007). Another similarity between structures observed in our experiments and maar craters are circular or curved faults formed around the vents. During and after eruptions the vents of such relatively small volcanoes fill with erupted material as well as material of collapsed side walls or fault blocks of country rock displaced down into the craters. The sliding of sidewall material into the conical zones of fluidized material resulted in vshaped layers observed in our experiments (Fig. 6). Thus, gravitational instabilities, collapse and slumping of wall rock material appears to be a plausible explanation for the processes generating the v-shaped layers in various geological environments. Material contrasts generated this way could explain seismic v-shaped reflections in sedimentary chimneys as well. However, the scaling of our experiments and sedimentary chimneys does not match in all dimensions and thus, apart from the fluidization observed in our experiments, further processes may be involved in the generation of sedimentary chimneys in nature. In our experiments the growth of fractures through cohesive material could be analyzed. Fractures growing vertically branched laterally at shallower depth (Fig. 3). These fractures served as pathways for fluidized reservoir material, creating wing-shaped fractures intruded by material of the reservoir layer, which resemble natural sand injections known from seismic images of sedimentary basins and from outcrops (e.g. Cartwright et al., 2008; Hurst et al., 2016). For instance, highresolution 3D seismic data from the North Sea region (Fig. 1) show that wing-like sand intrusions typically originate from up domed (‘jackedup’) sandy source layer (e.g. Duranti, 2007; Huuse et al., 2007; Jackson et al., 2011). The source layers can be deposited sand bodies as well as 12

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Fig. 10. (a) Ratio of critical pressures in the basal chamber (PPCcrit (init).) to the lithostatic pressure (σN(tot.)) of the entire material column within the experiments in which air was only injected into the basal chamber. (b) Ratio of critical pressures in the lower reservoir layer (PRLcrit (init).) to the lithostatic pressure of the cover layer (σN(CL); including the upper reservoir layer) in the material column directly above the needle valve. (c) Ratio of the critical pressure in the basal chamber in the reactivated experiments (PPCcrit (React.)) and the corresponding main experiment (PPCcrit (Main)). (d) Ratio of the critical pressure in the reservoir layer in the reactivated experiments (PRLcrit (React.)) and the corresponding main experiment (PRLcrit (Main)). The data shown here are compiled from all experiments carried out in the frame of this project (see Warsitzka and Kukowski (2019) for detailed information).

concluded that flow rates of formation fluids must be low at this fracture. This field case and our experiments thus infer that high flow rates in nature may be mostly associated with pipes, blow-outs, large pockmarks or mud volcanoes. The basic similarity between SSR structures evolving in the experiments and those observed in seismic data and in nature at the earth surface suggests that the scaling of the analogue experiments was appropriate. Thus, properties of the analogue materials and deformation processes of fracturing and fluidization may be transferred from model to nature to roughly estimate critical dynamic conditions for CO2 storage complexes, e.g. critical fluid pressures or minimum overburden thicknesses.

intrusive sand laccoliths. Such up domed sand bodies and wing-like intrusions are similar to structures observed in our experiments with high-cohesion cover layers (Figs. 3 and 4). This analogy indicates a sequence for the formation of the natural features. Initial anticlinal uplift of sealing caprocks above the sand bodies was driven by fluid overpressure, followed by sand intrusions into the bent fractures at the flanks of the anticlines. Uplift and doming of cohesive cover layers followed by caprock fracturing can also have an impact on fluid flow processes. Methane bearing formation water, venting along a fracture in unconsolidated sediments down to 550 m depth, has been observed at the Hugin Fracture (Norwegian North Sea; Lichtschlag et al., 2018). The Hugin Fracture is a 3 km long bent fracture, similar to the initial crestal fractures occurring during dome-shaped uplift in our 3D experiment (Fig. 5). Fracture networks developed in the experiments at higher flow rates (Fig. 4). In the neighbourhood of the Hugin Fracture, however, a fracture network is absent and methane concentrations in the sea water along the Hugin Fracture are low. Thus, Lichtschlag et al. (2018)

4.4. Implications for underground CO2 storage and subsurface fluid injection As physical analogue experiments, although usually having highly idealized set-ups, allow e.g. for high-resolution observation, they do not 13

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constitute a considerable and underestimated risk scenario for storage sites. Our experiments with high-cohesion cover layers indicate that fractures at the flanks of uplifted anticlines lead to localization of fluid flow off-site from the location of the needle valve. Transferring this observation to nature would imply that CO2 might migrate upwards through a permeable branched fracture network formed by sand injections. In the experiments, such a network usually widens in its upper parts (cf. Fig. 4). If such a behavior would be also present in nature, we cannot rule out that shallower CO2 leakage could occur in far distance from the injection point. Therefore, we suggest, that monitoring the surface or shallow depths should cover a wide region around a storage site in regions, where SSR structures have been identified. Overall, the detection of SSR features in the vicinity of a potential CO2 storage site would require additional efforts for exploring, monitoring and safety assessment. Given the important influence of cohesion of the cover layer on the evolution of SSR structures and having in mind, that not much is known about cohesion of natural sediments (cf. section 2), we would propose to take effort to gain some quantitative insight in the level and variability of the cohesion of natural sediments. Our study demonstrated that critical pressures for reactivating preexisting SSR structures are lower than their initiation pressures and probably well below the lithostatic pressure of the cover layer (Fig. 10). Studies addressing the formation of natural pockmarks, mud volcanoes and sand injection also revealed their episodic activity, for instance by identifying stacked pockmarks in seismic data or multiple, cross-cutting generations of sand injections in outcrop data (e.g. Andresen et al., 2010; Vetel and Cartwright, 2010Vetel and Cartwright, 2010; Løseth et al., 2011). This clearly shows the potential for reactivation of SRR structures. Especially, if a pre-exiting pore fluid overpressure is present within or below fossil SSR structures, a local-scale increase in fluid pressure, e.g. due to CO2 injection might trigger their reactivation. For CO2 storage sites, this implies that thresholds for the maximum safe injection pressure might be lower in order to avoid reactivation of SRR features. Consequently, the geotechnically feasible volume for safe storage could be lower than conventional assessments estimated, which often focus only on reservoir properties and distribution of caprock formations. Compared to critical reactivation pressures, pressures for triggering new SSR processes might be close to, but still below lithostatic as shown by most of our experiments and by other experimental studies (e.g. Mourgues et al., 2012). In contrast, Cartwright (2010) argued that supra-lithostatic pressures are required for the initiation of large-scale (in the range of several hundreds of meters wide) natural sand intrusions. As CO2 storage projects have to avoid excessive fluid pressure built-up in the storage formations (e.g. Chadwick et al., 2008), critical conditions for initiating new SSRs should only occur under special circumstances. One such example is the CO2 injection project at Krechba (In Salah) in Algeria (Ringrose et al., 2011; Rutqvist, 2012). There, CO2 was injected into lithified Carboniferous sediments in a depth of roughly 2 km. Monitoring surface deformation using InSAR measurements showed that the injected CO2 plume forced upward movement of blocks on different sides of a fault with a maximum amount of ˜2 cm. A similar process of differential block tilting and the opening of fractures was observed in our 3D experiment with a highcohesion cover layer (Fig. 5). Assuming, CO2 injection took place into shallower, unconsolidated, submarine sediments, which are much weaker, we would foresee that this could result in larger amounts of uplift and, therefore, to the incipient formation of bending fractures as observed in our experiments (cf. Fig. 4). Another example for SSR resulting from geotechnical operations is the Sleipner storage site located in the Central North Sea, where CO2 was injected into the unconsolidated Utsira Formation (Miocene - Pliocene) (Arts et al., 2004; Chadwick et al., 2004). Repeated acquisition of seismic data revealed that CO2 rapidly ascended to the top of the Utsira Formation passing intra-formational, low-permeability layers of clay (Chadwick et al.,

Fig. 11. Relative increase of the overall permeability (κtot) resulting from the deformation structures in the experiments. In average, the permeability increased by ˜16% in experiments with a high -cohesion cover layer due to the formation of fractures and by ˜10 to 40% in experiments with a low-cohesive cover layer due to fluidization structures. The data shown here are compiled from all experiments carried out in the frame of this project (see Warsitzka and Kukowski (2019) for detailed information).

suffer from limitations seismic imaging usually does because of its inherent limited resolution. Therefore, physical experiments offer some insight into the sub-seismic scale. Further, experiments are an efficient means to test the impact of various physical properties, which are difficult to estimate in situ in natural settings. Thus, keeping in mind the methodological limitations (cf 4.1) and idealized experiment set-ups, the results of experiments like those we have presented here, may contribute to the actual debate about storage safety, as they offer to explore tendencies for critical pressures as well as a better understanding of deformation processes in a more general manner. However, it should be emphasized, that deriving specific predictive insights into certain given examples of natural sites, which are thought to be potential storage sites for gases like CO2, is well beyond to possibilities of such experiments. In the North Sea Basin, where several potential storage sites have been extensively studied, SSRs are widespread in Cenozoic sediment formations (Fig. 1). Our experiments imply that SSR structures potentially increase the overall permeability of sediments (cf. Fig. 11), and may act as ‘seal bypass systems’ due to fracturing and sand injections penetrating potential sealing caprocks (Cartwright et al., 2007). In contrast, it was suggested that intrusive sand bodies can provide hydrocarbon traps, in case they were buried by fine-grained sediments or their pore space became cemented after the end of their activity (e.g. Duranti and Mazzini, 2005; Hurst and Cartwright, 2007). Thus, consequences for CO2 storage are difficult to evaluate and might be ambiguous. Further, most probably, it is difficult to test if fossil SSR structures have been completely sealed by diagenetic processes after their phase of activity. Our experiments reveal that even relatively small branched fractures and fluidization structures of SSRs might be of considerable importance for the overall integrity of a potential storage site. Similarly, outcrop studies (Vetel and Cartwright, 2010) pointed to the importance of small SSR structures. Thus, it may be that such natural structures in the subsurface are well below seismic resolution and thus, fracture networks in caprocks could remain undetected. If so, leakage of CO2 through unrecognized or insufficiently explored SSR structures above presumptively sealed reservoir formations might

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formations. Beneath unconsolidated sediments, older consolidated strata of indurated sedimentary rocks, such as the Lower Triassic Sandstones may offer storage opportunities as well. Increased storage safety through avoiding the hazards associated with SSD, may offset the disadvantages of lower injectivity and storage capacity and higher cost for drilling compared to the Cenozoic formations.

2010). This might be the result of uplifting cohesive clay layers by CO2 plumes that accumulated below, followed by fracturing and upward migration of the CO2. Furthermore, at the edges of discontinuous clay layers low-cohesion sandy sediments may have been mobilized. Geological processes, such as seismic events or rapid sediment loading have been suggested to trigger the deformation of slightly overpressured unconsolidated sediments (e.g. Jolly and Lonergan, 2002). Such mechanisms could induce SSR processes in CO2-bearing reservoirs as well. Karstens and Berndt (2015), for instance, suggested that naturally formed chimneys resulted from regional pressurization and that compaction of natural gas reservoirs was reactivated due to the loading by thick ice shields during Quaternary glaciations. As CO2 storage has to be permanent (according to EU law), geological and climatic events, such as future glaciations, have to be considered as a potential cause for excessive fluid overpressures in long term safety assessments, and therefore might constitute a larger risk for reservoirs in unconsolidated sediments than for those in lithified formations. Because of the limited resolution of geophysical imaging and the sparse occurrence of vents at the seafloor, certain fluid migration pathways, e.g. abandoned, non-tight wells, faults, or fossil SSR features, connecting deeper reservoirs in consolidated sediments with shallower, unconsolidated sedimentary layers, may remain undetected and therefore pose a potential risk for CO2 storages. In the North Sea region, for instance, consolidated strata of indurated sedimentary rocks, such as Lower Triassic sandstones (e.g. Bense and Jähne-Klingberg, 2017; Heinemann et al., 2012; Norwegian Petroleum Directorate, 2014), beneath partly consolidated Cenozoic sediments offer storage opportunities. Especially, non-tight wells penetrating such deep reservoirs for hydrocarbon exploitation can serve as leakage pathways (e.g. U.S. Department of Energy, 2017; Landrø et al., 2019), since they have not been built for the purpose of permanent CO2 storage. An illustrative example is the Bad Lauchstädt leakage incident (Katzung et al., 1996), where a failed pipe connection at about 100 m depth enabled leakage of ethane that migrated along a fault to the shallow subsurface. There, it accumulated and spread underneath a till layer that was uplifted by the gas below an elongated area extended for roughly 2 km. As soon as the till capping failed to hold the pressure, gas and sediment escaped through vents along the axis of the uplift, similar to what we observed in experiment with a high-cohesive cover layers (Figs. 4 and 5). This example implies that CO2 leaking through wells, faults or fossil SSR structures could also accumulate in shallow reservoirs underneath less permeable, but unconsolidated sediments. Besides an increased pore fluid pressure, the strong expansion of the CO2 phase in depths around ˜800 m and resulting density decrease might cause additional buoyancy forces on the sealing layer. If such accumulations remain undetected, sudden sediment fluidization or uplift of the ground surface could occur. Such near-surface sediment fluidization was observed during CO2 injection experiments conducted by Cevatoglu et al. (2015) off the Scottish west coast. Here, CO2 was injected into a few tens of meter deep soft sediments. After one day, a vertical chimney structure was observed within the muddy sedimentary formation, which was suggested to result from micro-scale fracturing during upward fluid migration through the muddy sediments. However, fluidization due to rapid fluid flow as shown in our experiments (e.g. A2D4, Fig. 6) could also be a potential deformation process in such near-surface sediments. In summary, our experiments, natural SSR features, observations from CO2 injection projects, and other technical analogues described here demonstrate the importance of research on fluid injection into unconsolidated sediments. Such sediments provide potential for CO2 storage owing to their high porosity, storage capacity and injectivity. However, risk management has to address potential leakage scenarios through pre-existing, activated, or reactivated SSR structures. In the North Sea Basin for example, SSDs are widespread (Fig. 1) and the geotechnically feasible volume for safe storage may be lower than traditional assessments have indicated, which were merely focusing on reservoir properties and the distribution of caprock

5. Conclusions Following the main aims of our study, (i) to employ several series of 2D and 3D scaled physical analogue laboratory experiments to decipher first order parameters influencing SSR processes, and (ii) to use our findings to discuss the potential role of SSR processes for geotechnical applications, i.e. safe subsurface CO2 storage, we summarize our conclusions in two paragraphs. (1) From our experiments, we derive the following findings on the consequences of fluid overpressuring and formation of SSR structures: (2) Cohesion of the analogue cover layer representing a natural caprock is a first order factor controlling deformation styles. Deformation in a high-cohesion cover layer is governed by layer bending and fracturing, whereas deformation in a low-cohesion cover layer is dominated by fluidization. (3) The structures resulting from the deformation processes vary systematically according to geometrical properties of the experiments, such as thickness of layers, shape of anticlines and fault throw. (4) SSR-like structures can be triggered by both, regional fluid overpressure and fluid overpressure generated by a local source of fluid pressure. The pressures required and the size of the structures differ between local and regional overpressure conditions. (5) Critical pressures required for the reactivation of SSR structures are lower than critical pressures needed for the initial deformation of the analogue materials. (6) With regard to the initially introduced questions about the possible relevance of SSR processes and structures for CO2 storage (cf. Section 1), the results of our experiments, when transferred to natural settings, suggest that: (7) The permeability of reservoirs and their overburden bearing structures created by SSR is higher than the permeability of the same undeformed sediments. (8) Existing structures in deformed unconsolidated sediments could be reactivated at injection pressures considerably lower than those required for the initial formation of such structures. (9) Deformation of unconsolidated sediments can be triggered by fluid injection, especially in formations with a regional fluid overpressure. Deformation of unconsolidated sediments can commence at pressures lower than the lithostatic pressure. Acknowledgements This study and the employment of M. Warsitzka were supported by the Federal Institute for Geosciences and Natural Resources (BGR, Germany) under research contracts to project no. A-0602028.A. Laboratory experiments and measurements of physical properties of analogue materials were carried out at the University of Jena. Technical assistance in the laboratory came from S. Neugebauer-Semsch. J. Rätz helped with the artwork. We equally thank A. Goepel and D. Rebscher for discussion and three anonymous reviewers for their helpful and critical comments, all of which lead to improving our manuscript. References Adam, J., Urai, J.L., Wieneke, B., Oncken, O., Pfeiffer, K., Kukowski, N., Lohrmann, J., Hoth, S., Van der Zee, W., Schmatz, J., 2005. Shear localisation and strain distribution during tectonic faulting—new insights from granular-flow experiments and high-

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