Experimental fracture sealing in reservoir sandstones and its relation to rock texture
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
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2022, Marine and Petroleum GeologyCitation 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.
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