Experimental fracture sealing in reservoir sandstones and its relation to rock texture

https://doi.org/10.1016/j.jsg.2021.104447Get rights and content

Highlights

  • Detrital grain size and rock composition affect fracture cementation.

  • Positive correlation between substrate grain size and overgrowth c-axis length.

  • Additional precipitate forms where host rock is quartz-poor.

  • Sheet silicate formation is sourced from fluid-rock interactions.

Abstract

Faults and fractures are important fluid pathways in subsurface energy reservoirs. Especially in geothermal energy production, hydrocarbon production, and energy storage in the subsurface, fractures can enhance reservoir quality and production or storage potential. However, mineral precipitations often reduce available fracture apertures, and thus fracture porosity and permeability. Hydrothermal experimental setups, natural samples, and numerical simulations have been studied to great extent. In fractured sandstones, the main focus has been on only some controlling factors, e.g. fracture opening rate and the orientation of crystallographic axes in substrates. Furthermore, substrates have mostly been fairly homogeneous or sediment textures have not been explicitly considered. Here, fracture sealing experiments are performed on a homogeneous, massive marine sandstone and heterogeneous, laminated fluvial sandstone. Hydrothermal flow-through experiments are performed at 421 ± 1 °C and 30.5 ± 0.5 MPa for 72 h to compare resulting precipitated quartz cement textures on fracture surfaces on natural sandstones. Results indicate a strong impact of grain size variations associated with lamination on observed syntaxial quartz crystal dimensions on fracture surfaces. In layers of finer grain sizes, smaller cement overgrowths develop, as opposed to coarser grained laminae in the same sample. In homogeneous sandstones, the overgrowths appear more uniform, apart from the differences induced by varying c-axis orientations. Some open fracture porosity may be preserved in areas of finer grained laminae, as opposed to coarser grained laminae. Additionally, the potential to stabilize fractures by cement growth spanning the aperture could preserve fracture porosity and permeability in generally unfavorable stress regimes. Furthermore, the relative abundance of suitable syntaxial precipitation sites and additional mineral dissolution as a function of varying detrital compositions appears to influence the mineralogy of the precipitate. If samples are rich in quartz grains, the largest quantity of precipitate will be quartz, due to the large supersaturation in the input solution. If samples contain less quartz by volume, the same solution additionally precipitates phyllosilicates, implying that some structural-diagenetic mineral phases are a function of heterogeneity of the host rock, rather than a result of variable external fluid compositions entering the formation. The phyllosilicates additionally act as nucleation discontinuities, reducing the size of surfaces available for syntaxial precipitation of quartz. Results may be applicable in fractured sandstone lithologies, which have been in the focus of energy storage and production.

Introduction

Open fractures enhance fluid flow and are thus important structures in many applications utilizing the subsurface as energy source, such as hydrocarbons or geothermal, or for storage such as heat, hydrogen or carbon capture (Becker et al., 2018; Busch et al., 2019; Gale et al., 2014). On the other hand, they provide a risk for seal and barrier integrity for subsurface waste disposal (Tirén et al., 1999). Fractures are known to host a wide range of mineral deposits, which affect the utilization of the subsurface (e.g., Laubach et al., 2019). Hence, mineralogical alterations like dissolution or precipitation over geological timescales (e.g., Laubach et al., 2019) may profoundly affect fracture strength, openness, and the capacity to fractures to conduct fluids, with ramifications for engineering management of reservoirs and storage systems (Laubach et al., 2004). Fluid flow of supersaturated solutions in fractures may result in fracture sealing as syntaxial overgrowth on the fracture wall, depending on the present fluid chemistries, and fluid pathways may be completely blocked (Hilgers et al., 2004; Hilgers and Tenthorey, 2004). Even small cement volumes within fracture systems connected by narrow fracture segments were shown to affect fluid flow (Philip et al., 2005). However, where opening rates are larger than the mineral growth perpendicular to the fracture surface, open porosities may be preserved (Gale et al., 2010; Hilgers et al., 2001; Lander and Laubach, 2015; Laubach, 2003; Prajapati et al., 2018a; Urai et al., 1991). This is largely influenced by the orientation of the fastest growing crystallographic axis in relation to the fracture surface (Cox and Etheridge, 1983; Hilgers and Urai, 2002; Lander and Laubach, 2015; Okamoto and Sekine, 2011). The bridging of local fracture cements may keep a fracture open even in unfavorable stress regimes and positively impacts fracture permeability (Laubach et al., 2004; Okamoto and Sekine, 2011).

The growth rate of syntaxial cement on detrital grains is controlled by the reactive surface area of the grain fabric (e.g., Lander et al., 2008). Smaller available surface area for syntaxial cementation in the pore space, i.e. smaller grains or sub-grains, results in smaller volumes of quartz cement formed over geological timescales (Lander et al., 2008; Prajapati et al., 2018b). This process has been experimentally shown to be effective for different substrate sizes exposed to the same thermal and pressure conditions over time (Lander et al., 2008). This should thus have an impact on observed cement textures in cemented fractures hosted in lithologies with varying substrate sizes (e.g. reflected by the detrital grain size).

Additionally, the detrital composition (i.e. rock fragments and mineral grains present at the deposition of the sediment) of host rocks needs to be considered. The lower the amount of quartz or quartz-containing grains in the host rock, the less surface area is available for syntaxial precipitation from silica solution, resulting in different amounts of formed syntaxial cements (see also Lander and Walderhaug, 1999). Such multi-mineral rock composition may also act as nucleation sites for additional precipitates such as clay minerals affecting reactive flow (Deng et al., 2018; Steefel, 2019).

The reactive surface area for quartz precipitation decreases, if the sandstone contains clay mineral grain coatings covering the quartz grains (Heald and Larese, 1974; Pittmann et al., 1992). Such clay mineral grain coatings prevent syntaxial overgrowth on detrital quartz grains (Busch et al., 2017), while a fractured quartz grain exposes a reactive surface.

We want to test if the different grain sizes in individual laminae affect syntaxial fracture cement precipitation as is observed in porous sandstones (e.g., Lander et al., 2008) and if the mineralogical composition of the host rock affects syntaxial cement precipitation as in metamorphic samples (e.g., Okamoto et al., 2008).

Here we show that the varying substrate grain size and detrital composition in the studied sandstone samples containing laminations of finer and coarser grains affect the volumes of syntaxially precipitated quartz cement on the fracture surface. A homogeneous sandstone sample containing little grain size variation, without prominent lamination, and above 95% quartz grain content experiencing the same hydrothermal experimental conditions are substrates to larger cement volumes than laminated sandstones containing grain size variations and having a less quartz-rich (avg. 50% quartz grains) composition.

Section snippets

Samples

Homogeneous, massive sandstone samples were taken from Gildehaus Quarry in Bad Bentheim, Lower Saxony, Germany. The Bentheim sandstone is a shallow marine sandstone, deposited during the Lower Cretaceous Valanginian (Mutterlose and Bornemann, 2000). The Bentheim sandstone is a reservoir unit and also often used for geomechanical tests due to its homogeneous texture and quartz-rich composition (Klein et al., 2001; Stanchits et al., 2009; Vajdova et al., 2004). The samples are classified as

Saturations and fluid composition

The pressure and temperature data for all experimental runs show consistent experimental conditions within the reaction vessel (Fig. 3 a-d, supplementary materials, temperatures and pressures). The measured average temperature gradient between T2 and T3 was between 0.5 and 0.85 °C over a distance of 5 cm to maintain similar precipitation conditions for all samples. The resulting calculated Si solubility at average temperatures throughout the whole run at the precipitation conditions in the

Quartz precipitation

The generated syntaxial quartz precipitates are euhedral and match overgrowths on natural samples previously synthesized in similar experimental approaches on metachert, granite, and sandstones (Okamoto and Sekine, 2011; Wendler et al., 2016). As in previous approaches, amorphous silica phases are absent (Okamoto and Sekine, 2011; Wendler et al., 2016). The c/a-axis ratio of ∼3:1 is in agreement with other experimental studies (see also Lander et al., 2008). However, as the c-axis growth rate

Conclusions

Reactive-flow precipitation experiments of quartz in sandstones show decreasing growth downstream. At constant flow rates and similar supersaturations only 4–32% of the amount precipitated on the upstream sample is precipitated on the downstream sample.

The microstructural composition of two different sandstones’ detrital quartz grain size and reactive surface area have a profound effect on the quartz precipitation and coprecipitates. The relative abundance of available precipitation sites and

Author statement

Benjamin Busch: Conceptualization, Methodology, Formal analysis, Investigation, Writing – Original Draft, Visualization.

Atsushi Okamoto: Conceptualization, Methodology, Resources, Writing – Original Draft.

Krassimir Garbev: Investigation, Writing – Original Draft, Visualization.

Christoph Hilgers: Conceptualization, Writing – Original Draft.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

BB thankfully acknowledges travel funding by the HeKKSaGOn Strategy Fund of the Executive Board of KIT. AO thankfully acknowledges grants by the Japan Society for Promotion of Sciences (18KK0376 and 17H02981). The authors are grateful for ICP-OES analyses by Shinichi Yamasaki, for assistance during experimental procedures and SEM analyses by Takamasa Niibe, and for assistance during SEM-EDX analyses by Armin Zeh. The constructive comments by three anonymous reviewers and editorial handling by

References (71)

  • C. Hilgers et al.

    Experimental study of syntaxial vein growth during lateral fluid flow in transmitted light: first results

    J. Struct. Geol.

    (2002)
  • E. Klein et al.

    Mechanical behaviour and failure mode of Bentheim sandstone under triaxial compression

    Phys. Chem. Earth

    (2001)
  • S.E. Laubach et al.

    Fault core and damage zone fracture attributes vary along strike owing to interaction of fracture growth, quartz accumulation, and differing sandstone composition

    J. Struct. Geol.

    (2014)
  • S.E. Laubach et al.

    Structural diagenesis

    J. Struct. Geol.

    (2010)
  • S.E. Laubach et al.

    Are open fractures necessarily aligned with maximum horizontal stress?

    Earth Planet Sci. Lett.

    (2004)
  • H. Li et al.

    The correlation between Raman spectra and the mineral composition of muscovite and phengite

    Ultrahigh-Pressure Metamorphism

    (2011)
  • L. Li et al.

    Upscaling geochemical reaction rates using pore-scale network modeling

    Adv. Water Resour.

    (2006)
  • C.E. Manning

    The solubility of quartz in H2O in the lower crust and upper mantle

    Geochem. Cosmochim. Acta

    (1994)
  • A.C. Monsees et al.

    Rock typing of diagenetically induced heterogeneities – a case study from a deeply-buried clastic Rotliegend reservoir of the Northern German Basin

    Mar. Petrol. Geol.

    (2020)
  • J. Mutterlose et al.

    Distribution and facies patterns of Lower Cretaceous sediments in northern Germany: a review

    Cretac. Res.

    (2000)
  • A. Okamoto et al.

    Mineralogical and textural variation of silica minerals in hydrothermal flow-through experiments: implications for quartz vein formation

    Geochem. Cosmochim. Acta

    (2010)
  • A. Okamoto et al.

    Textures of syntaxial quartz veins synthesized by hydrothermal experiments

    J. Struct. Geol.

    (2011)
  • A. Okamoto et al.

    Porosity and permeability evolution induced by precipitation of silica under hydrothermal conditions

    Proc. Earth Planetary Sci.

    (2017)
  • S.A. Tirén et al.

    Geologic site characterization for deep nuclear waste disposal in fractured rock based on 3D data visualization

    Eng. Geol.

    (1999)
  • J.L. Urai et al.

    Kinematics of crystal growth in syntectonic fibrous veins

    J. Struct. Geol.

    (1991)
  • P. Wüstefeld et al.

    Kilometer-scale fault-related thermal anomalies in tight gas sandstones

    Mar. Petrol. Geol.

    (2017)
  • T. Amagai et al.

    Silica nanoparticles produced by explosive flash vaporization during earthquakes

    Sci. Rep.

    (2019)
  • C. Bossennec et al.

    Diagenetic features of Buntsandstein Sandstones of the eastern border of the Upper Rhine Graben and implications in terms of burial history, 15E Congrès Français de Sédimentologie, Chambéry, France

    (2015)
  • B. Busch et al.

    Reservoir Quality Modeling in Deeply-Buried Permian Rotliegendes Sandstones, N-Germany. Impact of Illite Textures, EAGE Conference and Exhibition

    (2018)
  • B. Busch et al.

    Cementation and structural diagenesis of fluvio-aeolian Rotliegend sandstones, northern England

    J. Geol. Soc.

    (2017)
  • C. Cheng et al.

    Permeability variations in illite‐bearing sandstone: effects of temperature and NaCl fluid salinity

    J. Geophys. Res.

    (2020)
  • H. Deng et al.

    Fracture evolution in multimineral systems: the role of mineral composition, flow rate, and fracture aperture heterogeneity

    ACS Earth Space Chem.

    (2018)
  • P. Dijk et al.

    Precipitation and dissolution of reactive solutes in fractures

    Water Resour. Res.

    (1998)
  • J.F.W. Gale et al.

    Natural Fractures in shale: a review and new observations

    AAPG (Am. Assoc. Pet. Geol.) Bull.

    (2014)
  • S. Haffen et al.

    Geothermal, Structural and Petrophysical Characteristics of Buntsandstein Sandstone Reservoir (Upper Rhine Graben, France)

    (2015)
  • Cited by (8)

    • The significance of outcrop analog data for reservoir quality assessment: A comparative case study of Lower Triassic Buntsandstein sandstones in the Upper Rhine Graben

      2022, Marine and Petroleum Geology
      Citation Excerpt :

      The increased quartz cement content is thus not necessarily related to higher thermal exposure but to the available reactive surface areas (cp. Lander et al., 2008; Busch et al., 2021). As quartz growth rates are slower on euhedral quartz faces opposed to non-euhedral faces (Lander et al., 2008; Prajapati et al., 2020), a large difference may not be visible, as the sample series from subsurface and outcrop material both exhibit euhedral faces.

    • Formation of radiator structures in quartz veins - Phase-field modeling of multi-crack sealing

      2022, Journal of Structural Geology
      Citation Excerpt :

      Fig. 2 illustrates different crystal morphologies in a quartz microstructure from Portugal near Carrapateira, where different radiator microstructures are present. Besides microscopic analysis of natural veins hydrothermal flow-through experiments of quartz or in quartz rich rocks (e.g. Thomas et al. (1949); Cecil and Heald (1971); Lander et al. (2008); Okamoto et al. (2010); Okamoto and Sekine (2011); Busch et al. (2021)) enable a better understanding of how crystals form (e.g. facets, overgrowth of nucleation discontinuities) and how they interact during the precipitation under various conditions in polycrystalline environments. Okamoto and Sekine (2011) for example investigated the syntaxial quartz growth in open fractures and observed growth competition between differently oriented crystals and a growth termination against crystals from the opposite fracture wall.

    • Controls on compactional behavior and reservoir quality in a Triassic Buntsandstein reservoir, Upper Rhine Graben, SW Germany

      2022, Marine and Petroleum Geology
      Citation Excerpt :

      Mineral phases within the fractures are thus precipitated during or following the Eocene. Alternatively, small volumes of syntaxial quartz precipitates in the observed fracture may be related to the fine sand substrate sizes being substrates to small overgrowths, analogously to results by Lander et al. (2008) and Busch et al. (2021). Both of these experimental studies show that smaller sizes of precipitation sites correlate with smaller volumes of precipitated quartz cement.

    View all citing articles on Scopus
    View full text