Sedimentary Porous Rock Research Articles

Geochemical effects of carbonated water on reservoir and caprock minerals for carbon capture and storage

CO2 can be safely stored in geological formations, such as deep saline aquifers and depleted oil or gas reservoirs. However, knowledge about caprock and storing bed integrity is essential for safe storage in sedimentary porous rocks. In this study typical carbonate and sandstone minerals present in the reservoir and caprocks have been exposed to carbonated water above supercritical conditions of CO2 for short time periods (7 days) to investigate the CO2-brine-rock interactions and their effects on mineral stability. After exposure, the equilibrated brine composition was determined and any changes to the mineral samples were examined by SEM, EDX and XRD analyses. The solubility of CO2 in brines was low, increasing with pressure and decreasing with salinity, and no significant increase in CO2 solubility was observed above critical pressure and temperature for CO2. Quartz and clay minerals were the least reactive sandstone minerals, while ion exchange reactions were observed for the feldspar minerals. For calcite minerals, calcium ion liberation was observed accompanied by a pH increase, which is linked to mineral dissolution. SEM, EDX and XRD analyses did not detect any changes on the surfaces of silicate and carbonate minerals exposed to the carbonated water. The overall results confirm minimal effects on silicate and carbonate minerals after short-term exposure to carbonated water above supercritical conditions of CO2.

A visco-plastic framework for interface processes in sedimentary reservoir rocks at HPHT conditions

As energy operations face the challenge of reservoirs at ever-increasing depths, modelling the response of reservoir rocks at high pressure and high temperature (HPHT) conditions is a crucial step for successfully unlocking new resources. At these conditions, rocks can experience thermal and pressure sensitivity, as well as admit internal phase changes at the pore-solid interfaces that determine their macroscopic response to external loading. It is therefore expected that constitutive laws of Geomechanics should be enriched to accommodate such effects. In this work, a multi-physics constitutive theory for sedimentary rocks is proposed within the general framework of viscoplasticity. The viscosity of the material is assumed to be a function of the temperature, pore-pressure and energy required to alter the inter-granular interfaces. This energy is expressed through the chemical potentials of the phases involved during processes like chemical dissolution/precipitation of the interfaces or mechanical debonding. The resulting flow law and corresponding stress equilibrium are coupled to the energy and mass conservation laws, constituting a closed system of equations, which is solved using the Finite Element simulator REDBACK. A series of numerical tests for performance and calibration is then successfully performed against different types of reservoir rocks, confining pressures and temperatures (from room temperature to over 800K). We show that the mechanical response of sedimentary porous rocks at strains usually achieved in laboratory testing can be explained by accounting for the interface processes taking place at the cementitious material bonding the grains, a process that can also determine the brittle-to-ductile transition of sedimentary rocks.

Effects of aligned fractures on the dielectric properties of synthetic porous sandstones

Understanding the dielectric properties of reservoir rocks has important applications in various aspects in the petroleum industry. Fractures are common features in sedimentary porous rocks that not only provide space for the hydrocarbon to accumulate and reside but also are important pathways for the hydrocarbon to flow and therefore have a controlling impact on the formation and distribution of the hydrocarbon reservoirs. However, the dielectric properties of fractured reservoir rocks are still poorly understood. We investigated, through dedicated laboratory experiments, the dielectric behaviors of synthetic porous sandstones with aligned fractures and their relationships with the salinity of the electrolyte saturating the samples, the measurement frequency of the applied electrical field and the directions relative to the fractures. It showed that the fractures have significant effects on the dielectric properties of porous sandstones and their dependence on the brine salinity and measurement frequency reveals the various polarization mechanisms occurring at different frequency ranges. The results offer a preliminary basis for the characterization of fractured rocks using the dielectric properties, which has great potential applicability in oil and gas exploration and exploitation.

Numerical modeling of the geomechanical behavior of Ghawar Arab-D carbonate petroleum reservoir undergoing CO2 injection

Sedimentary porous rocks can be used for long-term subsurface containment of CO2. Before injecting CO2 to sedimentary reservoirs, it is necessary to perform stability analysis of the reservoir and to estimate the maximum sustainable pore fluid pressures. In order to avoid the reservoir damage during the CO2 injection, the effective stresses in the reservoir should be evaluated. In this paper, numerical modeling techniques are used for the evaluation of stresses and deformations in a naturally fractured carbonate sedimentary reservoir. The developed numerical modeling scheme couples the behavior of the CO2 injection and the change in the geomechanical behavior of the sedimentary carbonate reservoir for a case study from Saudi Arabia. The present investigation extends the previous studies by considering the sorption-based deformation during the injection of the compressed CO2 fluid into the Arab-D naturally fractured carbonate reservoir. The change in permeability during the injection of CO2 is evaluated. The adopted CO2 injection scenario was shown to be within the safe maximum occupancy, and that the increase in the pore pressure does not result in the reservoir failure.

Calibration of a Novel Impedance Sensor for Water Content Measurement in Rocks

The water content (θ) of the subsoil is an important parameter affecting all the processes that occur in the vadose zone, playing a key role in the infiltration, aquifer recharge, and flow and transport of water and substances, as well as groundwater storage. Although the vadose zone is usually considered constituted of soil, it may also be comprised of rock. In the latter case, θ cannot yet be operationally measured despite its unquestionable importance. The aim of the present work was therefore to investigate the potential of a novel electrical impedance sensor for θ measurement in rocks and to evaluate the effects of frequency and salinity on the calibration of this device. In this study, only the resistance (R), i.e., the real part of the measured impedance, was investigated because of its direct correlation with θ. Calcarenite, a sedimentary porous rock, was used for the calibration in the laboratory via the installation of penetration‐type probes. The independence of the measured R from the different frequencies set on the device was checked using a statistical approach. The θ–R calibration functions obtained for the different saturation solutions have a power‐law dependence with a good degree of correlation. The results highlight both the strong reproducibility of the experimental data (by using the penetration‐type probes) and the suitability of the device for θ measurement in calcarenite. The method could be advantageous for field applications that involve rocks.

Compaction of porous rock by dissolution on discrete stylolites: A one‐dimensional model

Compaction of sedimentary porous rock by dissolution and precipitation is a complex deformation mechanism, that is often localized on stylolites and pressure solution seams. We consider a one‐dimensional model of compaction near a thin clay‐rich stylolite embedded in a porous rock. Under the assumption that the clay enhances solubility, the model predicts a reactive transport away from the clay layer followed by pore cementation. The evolution of the porosity, reactant transport, and compaction rate are studied as functions of model parameters and shown to reach a stationary state. We find good agreement between the porosity distribution predicted by the model and previously reported field measurements. The model provides quantitative estimates for compaction rates on stylolitic surfaces.

Modeling the Flow Characteristics of a Fractured Medium Overlaid by a Sedimentary Porous Medium: Application to the Borehole RCF 3 Pumping Test in Sellafield (UK)

A numerical model was developed in order to produce more realistic simulations of the flow phenomena in fractured rock overlaid by porous sedimentary rock. This model combines the discrete fracture network (DFN) model and the equivalent porous medium (EPM) model. This coupled model was then applied to the interpretation of the RCF 3 pumping test performed in the Sellafield area of West Cumbria (UK), where crystalline fractured rock (Borrowdale Volcanic Group, BVG) is overlaid by sedimentary porous rock (Sherwood Sandstone Group, SSG).

Drying of a porous spherical rock for compressed air energy storage

A model for compressed hot air storage in a sedimentary porous rock composed of spherical rock is presented. During charging, the rock loses moisture and a dry spherical shell develops around a moist zone. The transient heat conduction, convection and mass transfer that take place during charging and discharging are simulated using a moving-grid numerical methodology for both the wet and dry zones. A process of evaporation/condensation moves the boundary between the two zones. The model is verified using data from a concrete drying experiment. A parametric study is conducted to demonstrate the sensitivity of the system to various parameters.

Modeling flow characteristics of a fractured medium overlaid by a sedimentary porous medium: Application to borhole RCF 3 pump test in sellafield area (UK)

A numerical tool was developed in order to make a more realistic simulation of the flow phenomenon in a fractured rock overlaid by porous sedimentary rock. Such tool is a combination of the discrete fracture network (DFN) model and the equivalent porous medium (EPM) model. This coupled model was verified in two test cases with relatively simple geometries for the fractured medium. Results were then compared with those obtained from the other coupled model CONNECT-FLOW and the analytical solution. This coupled model was applied to the RCF 3 Pump Test performed in the Sellafield area of West Cumbria (UK), where the crystalline fractured rock (Borrowdale Volcanic Group, BVG) is overlaid by sedimentary porous rock (Sherwood Sandstone Group, SSG).

Fractals in Rock Physics

The concept of fractals, introduced by Mandelbrot ( 1 982), has proven to be a highly useful way to describe the statistics of naturally occurring geometries. Fluid flow, whether it be the rise and fall of rivers (Bridge & Jarvis 1 982), turbulence in air or water (Nelkin 1989), or precipitation (Mandelbrot & Wallis 1 968), is found to follow self-similar patterns in time and space. Natural objects, from mountains and coastlines (Mandel­ brot 1982) to the perimeters of forests (Loehle 1 983), are found to have boundaries best described as self-similar, appearing the same on all length scales and having a measured size that depends on the scale ofthe measure­ ment. Microscopic processes of diffusion (Orbach 1 989) and chemical reaction kinetics (Kopelman 1988) can lead to fractal structures, while the scale independence of natural processes can lead to the ubiquitous I/! noise and stretched exponential relaxation (West & Shlesinger 1 990). Many review articles and books have been written on fractals [see Hurd (1 988) for a bibliography], and reviews (Wong 1 988) and conference proceedings (Scholz & Mandelbrot 1 989, Aharony & Feder 1 989, Fleischmann et al 1 989, Avnir 1 989) have appeared on various aspects of fractal applications in geophysics. I do not attempt to review here basic fractal concepts as they are covered in the cited literature but rather concentrate on fractal structure at the microscopic scale in sedimentary porous rock and on the geometry of flowing fluids. One objective of this work is to develop models for rock pore geometry that will distinguish rocks from other porous materials. The length scales of interest include the submicron structure on pore walls,