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10.4-2 Understanding reactions and transport in porous and fractured media - from rock analytics to predictive modelling
4:15pm - 5:45pm
Session Chair: Benjamin Busch, Karlsruher Institut für Technologie Session Chair: Michael Kühn, GFZ German Research Centre for Geosciences
The interaction of fluid and rock, and the properties of pores and their connectivity are among the main controls on the production and storage potential in clastic and carbonate rocks. At least as much as in the hydrocarbon sector, the exploration of geothermal resources or storage sites for CO2 or hydrogen, require knowledge of processes on the pore scale. Compaction, cementation, dissolution, and alteration, control the evolution of reservoir quality and are also key to understanding the risk of formation damage. All may occur on geological time scales or happen rather quickly during production or storage as the composition and/or pressure of the pore fluids is changed dramatically within a short period of time. The process-based understanding of diagenesis controlling reservoir behavior will aid in future utilization of the subsurface in outlining possibilities for better assessment, prediction chances and risks of classic and renewable energy. Numerical simulations are the only way to connect micro-scale processes, which may significantly alter the internal rock structure, with the macro-scale, which consequently affect the hydraulic behaviour of the system.This session aims to showcase recent developments in reservoir petrography, petrographic modelling, and prediction utilizing multiple geological, geochemical, and geophysical methods and approaches like reactive transport modelling. We invite submissions presenting e.g., case studies, integrations of novel methodologies, and new interpretations of legacy data for future energy challenges.
4:15pm - 4:45pm Session Keynote
3D Digital Sedimentary Petrology Models
Robert Lander1, Linda Bonnell1, James Guilkey2
1Geocosm; 2Laird Avenue Consulting
“Digital sedimentary petrology” models represent the microstructure of clastic rocks in 3D and use forward process models to simulate diagenesis in response to evolving burial conditions. This modeling approach predicts textures and morphologies that can be readily compared with natural samples and laboratory experiments. These models are useful tools for studying diagenetic processes and also are designed to predict rock microstructure in undrilled areas of the subsurface.
Digital petrology models are natural counterparts to “digital rock physics” models that use rock microstructure as input when simulating a broad array of fluid transport and geomechanical properties. Linking these models extends digital rock physics models beyond assessment of rock properties based on scans of physical samples to predicting rock properties in undrilled portions of the subsurface. Applications of this coupled modeling approach includes hydrocarbon and geothermal energy exploration and production, CO2 sequestration, hydrogen and compressed air storage, wastewater injection, and groundwater studies.
Our work to date on the development of the Cyberstone™ digital sedimentary petrology model involves simulation of sediment deposition, grain rearrangement, mechanical compaction, chemical compaction (pressure solution as well as temperature dependent contact dissolution resulting from chemical corrosion), and growth of various cement types with various morphologies. Although the system was developed for clastic sedimentary rocks, we also have found it to be a useful tool for simulation of the evolution in fluid flow and geomechanical properties of evaporite rubble associated with the collapse of a chamber in a salt dome that is being used for nuclear waste disposal.
4:45pm - 5:00pm
Time-dependent fracture permeability induced by fluid-rock interactions under intermittent and continuous flow
Chaojie Cheng, Harald Milsch
GFZ German Research Centre for Geosciences, Germany
Fractures are the predominant flow pathways in low-permeability rocks. Understanding the fluid-rock interactions that occur in rock fractures and their effects on fracture aperture variations is important for assessing the sustainability of reservoir productivity. This study presents two long-term flow-through experiments with fractured pure quartz sandstones to investigate how fluid composition affects fracture changes over time. One sample was continuously flowed through with fluids (DI or Si-rich fluid), while the other sample was subjected to intermittent flow (DI) at certain time intervals. The results show that the hydraulic aperture of the sample with intermittent flow maintains relatively constant, and the pore fluid is enriched with Si that is higher than the corresponding quartz solubility. On the contrary, hydraulic aperture reduces by 50% of its initial value in the case of continuous flow. The pore fluid Si concentrations are far below the quartz solubility. Based on the microstructure variations of contact asperities and the fluid concentration changes, we demonstrate that pressure solution plays a dominant role in rock fracture deformation and permeability changes. The pore fluid composition has a remarkable effect on the permeability decay process. The cumulative Si in the pore fluid without flow would mitigate fracture closure by limiting pressure solution. In contrast, the continuous injection of DI would lead to the continuous mass transfer between the contact asperities and the pore fluid. The permeability evolutions in the two cases are likely governed by the Si precipitation process and the stress-driving dissolution process, respectively.
5:00pm - 5:15pm
Clay and basic understanding of burial diagenesis
Jūratė Vaznytė, Nicolaas Molenaar
Science research center, Lithuania
Clay cements can occur pervasively throughout larger volumes of sandstone, thereby affecting the reservoir properties significantly. They affect irreducible water content and pore surface roughness. Moreover, any induced fluid or heat flow, as a consequence of hydrocarbon and geothermal production or CO2 sequestration, may have unwanted effects because of the clay minerals present. The effects will be dependent on the mineralogy, texture and distribution of the clay minerals. Clay minerals replacing detrital components (feldspars and rock fragments) have limited effect because of their dispersed and isolated occurrence. In this study diagenesis of intragranular clay in siliciclastic sandstones is evaluated, using Rotliegend deposits in the Southern Permian Basin and Lower Triassic Buntsandstein deposits as examples. This study clearly shows that large-scale fluid flow does not play a significant role and that much of the mass involved in diagenesis is retained more or less in situ. In the sandstone proper, clay occurs in various ways: as detrital laminae and beds, as patches related to burrows, and as grain coatings through clay infiltration. In addition, clay occurs as cements rimming grains and replacing detrital feldspars and rock fragments. The apparent detrital clay is partly or largely modified during burial diagenesis and much of the clay is authigenic. Not only the mineralogy is changed but also the location and distribution of the authigenic clays. In conclusion: authigenic clays in reservoir sandstones, including clay rim cement, are genetically associated with and directly linked to infiltrated or bioturbated clay.
5:15pm - 5:30pm
Ternary porosity systems: New perspectives for Buntsandstein geothermal reservoirs in the Upper Rhine Graben, SW Germany.
The clastic Lower Triassic Buntsandstein Formation in the Upper Rhine Graben of SW Germany and NE France has been identified as an attractive geothermal reservoir due to its fracture density and exceptionally high matrix porosity at depth levels of economic geothermal energy extraction. New petrophysical data from deep exploration wells reveal the existence of ternary porosity systems evolved during a multi-phase subsidence history and diagenesis especially at intra-graben structural highs. Primary elements of these porosity systems are high-permeability faults and fractures which can be utilized as technical fluid conduits connecting geothermal injectors and producers. Second component is the primary matrix porosity, controlled by pure mechanical compaction. Third component is an interconnected system of secondary pores and micropores. Secondary porosity originates from diagenetic dissolution of chemically and mechanically unstable framework grains like feldspars and rock fragments. At depths of about 2.300 m secondary porosity and microporosity can exceed the compaction-controlled primary porosity of around 7 %, causing high total pore volumes of up to 21 %. All matrix pore types are linked to form an interconnected pore network hosting significant connate brine volumes. These brine volumes don´t contribute to technical hydrogeothermal fluid cycling but increase the thermal capacity of the reservoir and favour heat conduction. Although this phenomenon has been described from hydrocarbon pools, their quantitative significance in geothermal reservoirs is still poorly understood. Micro-scale reservoir simulations may help to upgrade geothermal prospects.
5:30pm - 5:45pm
Geochemical control of hydraulic and mechanical reservoir sandstone properties
Maria Wetzel1, Thomas Kempka1,2, Michael Kühn1,2
1GFZ German Research Centre for Geosciences, Fluid Systems Modelling; 2University of Potsdam, Institute of Geosciences
Geochemical processes such as mineral dissolution and precipitation alter the microstructure of rocks, and thereby affect their hydraulic and mechanical behaviour. Quantifying and considering these property changes in reservoir simulations substantially supports risk assessments related to geological subsurface utilization.
In our virtual laboratory, 3D pore-scale models of typical reservoir sandstones are applied to determine the effective hydraulic and elastic properties of sandstones. In order to adequately depict characteristic distributions of secondary minerals, the virtual samples are systematically altered, and the resulting changes in geometric, hydraulic, and mechanical rock properties are quantified. Characteristic pore space alterations for a reaction- and a transport-limited precipitation regime are approximated by correlating precipitation with fluid flow velocity magnitudes. A purely surface reaction-limited regime is represented by a uniform modification of the pore space, whereas transport-limited precipitation is characterised by the successive clogging of pore throats and a drastic decrease in permeability. It is demonstrated that the location of mineral growth within the pore space strongly affects the magnitude of permeability reduction. The presented digital pore-scale simulations enable to quantify changes in permeability and stiffness resulting from geochemical processes, and thus are relevant for a wide range of natural and engineered subsurface applications.