Chalk Cores Research Articles

Radionuclide transport in fractured chalk under abrupt changes in salinity

Internationally, it has been agreed that geologic repositories for spent fuel and radioactive waste are considered the internationally agreed upon solution for intermediate and long-term disposal. In countries where traditional nuclear waste repository host rocks (e.g., clay, salt, granite) are not available, other low permeability lithologies must be studied. Here, chalk is considered to determine its viability for disposal. Despite chalk's low bulk permeability, it may contain fracture networks that can facilitate radionuclide transport. In arid areas, groundwater salinity may change seasonally due to the mixing between brackish groundwater and fresh meteoric water. Such salinity changes may impact the radionuclides' mobility. In this study, radioactive U(VI) and radionuclide simulant tracers (Sr, Ce and Re) were injected into a naturally fractured chalk core. The mobility of tracers was investigated under abrupt salinity variations. Two solutions were used: a low ionic strength (IS) artificial rainwater (ARW; IS ∼0.002) and a high IS artificial groundwater (AGW; IS ∼0.2). During the experiments, the tracers were added to ARW, then the carrier was changed to AGW, and vice versa. Ce was mobile only in colloidal form, while Re was transported as a conservative tracer. Both Re and Ce demonstrated no change in mobility due to salinity changes. In contrast, U and Sr showed increased mobility when AGW was introduced and decreased mobility when ARW was introduced into the core. These experimental results, supported by reactive transport modeling, suggest that saline groundwater solutions promote U and Sr release via ion-exchange and enhance their migration in fractured chalk. The study emphasizes the impact of salinity variations near spent fuel repositories and their possible impact on radionuclide mobility.

Flow-Through Experiments of Reactive Ba-Sr-Mg Brines in Mons Chalk at North Sea Reservoir Temperature at Different Injection Rates

Summary North Sea Chalk reservoirs in Norway are potential candidates for enhanced hydrocarbon recovery by modifying the injected brine composition. This work investigates how barium (Ba), strontium (Sr), and magnesium (Mg) brines interact when injected into chalk. Ba and Sr are often associated with mineral precipitation and occur in formation water, while Mg is present in seawater, commonly injected in chalk. Relatively clean (>99% calcite) outcrop chalk cores from Mons, Belgium, were flooded at 130°C in triaxial cells with four brines containing 0.12 mol/L divalent cations, either 0.06 mol/L Sr and Ba, 0.06 mol/L Sr and Mg, or 0.12 mol/L Ba or Sr. Each brine was injected in a separate core, with 100–150 pore volumes (PV). The injection rate varied between 0.5 and 8 PV/D. Produced brine was analyzed continuously and compared with the injected composition. After flooding, the cores flooded with only Ba or only Sr were cut into slices and analyzed locally in terms of scanning electron microscopy (SEM), matrix density, specific surface area (SSA), and X-ray diffraction (XRD). In all experiments, the produced divalent cation concentration was reduced compared with the injected value. The total reduction of injected cation concentration closely equaled the produced Ca concentration (from calcite dissolution). When flooding 0.12 mol/L Sr, the Sr concentration depleted 55%, while when flooding 0.12 mol/L Ba, 15% Ba depleted. When injecting equal concentrations of Ba and Sr, 40% Sr and 7% Ba depleted, while with equal concentrations of Mg and Sr injected, ~50% Sr was retained and almost no Mg depleted. Sr appeared to dominate and suppress other reactions. There was less sensitivity in steady-state concentrations with variation in injection rate. The similar modification of the brine regardless of residence time suggests the reactions reached equilibrium. Cutting the cores revealed a visually clear front a few centimeters from the inlet. The material past the front was indistinguishable from unflooded chalk in terms of density, SSA, microscale structure, porosity, and composition [XRD and SEM-energy-dispersive spectroscopy (EDS)]. The material near the inlet was clearly altered. Images, XRD, SEM-EDS, and geochemical simulations indicated that BaCO3 and SrCO3 formed during BaCl2 and SrCl2 flooding, respectively. Geochemical simulations also predicted an equal exchange of cations to occur. The matrix densities, porosities, and the distance traveled by the front corresponded with these minerals and suggested that the chalk was completely converted to these minerals behind the front. It was demonstrated that Ba, Sr, and Mg brines and their mixtures can be highly reactive in chalk without clogging the core, even after 100 + PV. This is because the precipitation of minerals bearing these ions is associated with simultaneous dissolution of calcite. The Ca-, Ba-, and Sr-mineral reactions are effectively in equilibrium. Previous investigations with MgCl2 (in pure and less pure chalk, at 130°C) show injection rate-dependent results (Andersen et al. 2022) and smoother alterations [Mg precipitation was seen from inlet to outlet (Zimmerman et al. 2015)], indicating that Mg-mineral reactions at same conditions have a longer time scale. The limited distance mineral alteration has occurred, suggesting that adsorption processes, happening in parallel, can explain previous observations (Korsnes and Madland 2017) of Ba and Sr injection strengthening chalk. Flushing out formation water with these ions during injection may be a new water-weakening mechanism.

Direct visualization of colloid transport over natural heterogeneous and artificial smooth rock surfaces

Colloid transport in fractured rock formations is an important process impacting the fate of pollutants in the subsurface. Despite intensive and outstanding research on their transport phenomena, the impact of small-scale surface heterogeneity on colloid behavior at the fracture scale remains difficult to assess. In particular, there is relatively little direct experimental evidence on the impact of natural fracture surface heterogeneity on colloid transport. To investigate this, we developed an experimental setup allowing the direct visualization of fluorescent colloid transport in a flow cell containing a natural chalk rock sample while simultaneously monitoring effluent colloid concentrations. We used samples containing both a natural fracture surface and an artificially made smooth surface from the same chalk core. We characterized the roughness and chemical composition of both surface types and numerically calculated each surface's velocity field. From the experiments, we obtained direct images of colloid transport over the surfaces, from which we calculated their dispersion coefficients and quantified the residual deposition of colloids on the rock surface. We also measured the colloid breakthrough curves by collecting eluent samples from the flow cell outlet. The natural fracture surface exhibited larger physical and chemical heterogeneity than the smooth, artificially generated surface. The aperture variability across the natural surface led to preferential flow and colloid transport which was qualitatively apparent in the fluorescent images. The colloid transport patterns matched the calculated velocity fields well, directly linking the surface topography and aperture variation to colloid transport. Compared to the artificially made surface, the natural surface also showed higher dispersion coefficients, which corresponded to the colloids' earlier breakthrough from the flow cell. While we found differences between the elemental composition of the natural and artificially smooth surfaces, we could not observe their impact on the colloids' surface attachment and retention. The main novelty in this work is the coupling of direct colloid transport imaging, breakthrough curve measurements, and colloid surface deposition analyses, in a flow cell containing a natural carbonate rock sample. Our experimental setup can be used to further investigate the link between surface heterogeneity, both chemical and physical, and colloid transport and deposition in natural rock fractures.

Coreflooding and Steady-State Principles Determine Reaction Kinetics

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 209380, “Reaction Kinetics Determined From Coreflooding and Steady-State Principles for Stevns Klint and Kansas Chalk Injected With MgCl2 Brine at Reservoir Temperature,” by Pål Østebø Andersen, Reidar Inge Korsnes, and Andre Tvedt Olsen, University of Stavanger, et al. The paper has not been peer reviewed. A methodology is presented for determining reaction kinetics from coreflooding. A core is flooded with reactive brine at different compositions with injection rates varied systematically. Each combination is performed until steady state, when effluent concentrations no longer change significantly with time. The authors also propose shut-in tests where brine reacts statically with the core for a defined period and then is flushed out. The residence time and produced brine composition is compared with the flooding experiments. This design allows characterization of the reaction kinetics from a single core. The steady-state method allows computationally efficient matching even with complex reaction kinetics. Introduction The complete paper presents a methodology to interpret reactive flow experiments. The main principle is to focus on the steady-state data collected when effluent concentrations do not change further with time for given injected composition, injection rate, core properties, temperature, and pressure. Such measurements depend only on the dynamic equilibrium between reactions, advection, and to some extent dispersion, while transient events and mechanisms can be ignored. The authors hypothesize that, by measuring such data under different injected compositions, injected rates, and temperatures, characterization of reaction kinetics is possible with great detail. This is demonstrated by performance of reactive flow experiments on chalk cores at varied conditions with the purpose of collecting steady-state data. A key advantage from the interpretation perspective is that complicated reactive transport equations can be simplified by ignoring the changes in time and only simulating and matching the steady state. The experimental setup is detailed in the complete paper, as is a general description of advection-dispersion-reaction systems, a description of steady states in an advection-dispersion-reaction system and an advection-reaction system, and a theoretical description of a magnesium chloride (MgCl2)/calcite system.

Water Weakening of Artificially Fractured Chalk, Fracture Modification and Mineral Precipitation during Water Injection—An Experimental Study

This experiment was designed to study the water-weakening effect of artificially fractured chalk caused by the injection of different compositions of brines under reservoir conditions replicating giant hydrocarbon reservoirs at the Norwegian Continental Shelf (NCS). NaCl, synthetic seawater (SSW), and MgCl2, with same ionic strength, were used to flood triaxial cell tests for approximately two months. The chalk cores used in this experiment originate from the Mons basin, close to Obourg, Belgium (Saint Vast Formation, Upper Cretaceous). Three artificially fractured chalk cores had a drilled central hole parallel to the flooding direction to imitate fractured chalk with an aperture of 2.25 (±0.05) mm. Two additional unfractured cores from the same sample set were tested for comparison. The unfractured samples exposed a more rapid onset of the water-weakening effect than the artificially fractured samples, when surface active ions such as Ca2+, Mg2+ and SO42− were introduced. This instant increase was more prominent for SSW-flooded samples compared to MgCl2-flooded samples. The unfractured samples experienced axial strains of 1.12% and 1.49% caused by MgCl2 and SSW, respectively. The artificially fractured cores injected by MgCl2 and SSW exhibited a strain of 1.35% and 1.50%, while NaCl showed the least compaction, at 0.27%, as expected. Extrapolation of the creep curves suggested, however, that artificially fractured cores may show a weaker mechanical resilience than unfractured cores over time. The fracture aperture diameters were reduced by 84%, 76%, and 44% for the SSW, MgCl2, and NaCl tests, respectively. Permeable fractures are important for an effective oil production; however, constant modification through compaction, dissolution, and precipitation will complicate reservoir simulation models. An increased understanding of these processes can contribute to the smarter planning of fluid injection, which is a key factor for successful improved oil recovery. This is an approach to deciphering dynamic fracture behaviours.

Impact of hydro-chemical conditions on structural and hydro-mechanical properties of chalk samples during dissolution experiments

The importance of karst reservoirs for water resources, and their complexity in terms of structural properties and hydraulic responses, require a better understanding of the formation and location of conduits in these systems, in particular for chalk reservoirs. For this purpose, we conduct laboratory experiments of acid solutions injection in chalk core samples using a homemade experimental device. Three different flow rates and two acid concentrations are applied on six samples at atmospheric conditions with Peclet numbers ranging from 0.1 to 1.7 and Damköhler numbers from 10−6 to 10−3. Hydraulic and chemical measurements are carried out during the experiments, while petrophysical characterization is conducted on the samples before and after the reactive percolation experiments. The analysis of these data shows an increase in permeability of the rock samples due to calcite dissolution, which is associated with the creation of preferential conduits. We also observe that the initial structural properties control the dissolution regime with the following main conclusions. For homogeneous systems, the overall dissolution rate and percolation time are directly linked to the considered flow rate and injected acid concentration. For heterogeneous systems, (i) pre-existing large-scale structural heterogeneities control the location of newly created paths while reducing the value of the dissolution rate, and (ii) micro-scale heterogeneities result in highly heterogeneous general structures, which are responsible for channels formation while applying low Damköhler numbers.

Experimental investigation of low salinity water-flooding in tight chalk oil reservoirs

Chalk reservoirs, due to their high storage capacity and very low permeability, are one of the most interesting cases for reservoir engineering research on carbonates. They exhibit complex fluid-rock interactions because of their chemically active porous media. This study investigates the effect of brine composition, injection scenario, and temperature on oil recovery by low salinity water-flooding in chalk core samples from a Danish North Sea reservoir. The mechanisms governing oil-brine-rock interactions were also explored. Actual reservoir chalk core samples, without any open fractures, were selected using computed tomography analyses. These cores were saturated with representative fluids (crude oil and synthetic formation brine) and aged at reservoir conditions for approximately three weeks. The role of brine chemistry has been investigated through effluent analysis by ion chromatography, and results indicate that low salinity diluted seawater promotes rock-surface reactions if left to incubate for at least 48 h. Rock dissolution, observed through the monitoring of effluent ions, increased both with increase in temperature and decrease in brine salinity. The recovery curves show that formation water and diluted seawater produce significantly more oil (of the order of 10 % more) at the secondary stage compared to seawater. Additionally, there is also some indication of an effect of low salinity brine at the tertiary stage. These experiments were performed on reservoir materials and corresponding crude oil samples and provide new data on the low salinity flooding potential for chalk, and provide further evidence for the applicability of the low salinity effect in carbonates.

Crude Oil–Brine–Rock Interactions in Tight Chalk Reservoirs: An Experimental Study

We present a systematic study of crude oil–brine–rock interactions in tight chalk cores at reservoir conditions. Flooding experiments are performed on outcrops (Stevns Klint) as well as on reservoir core plugs from Dan field, the Ekofisk and Tor formations. These studies are carried out in core plugs with reduced pore volumes, i.e., short core samples and aged with a dynamic ageing method. The method was evaluated by three different oil compositions. A series of synthetic multicomponent brines and designed fluid injection scenarios are investigated; injection flow rates are optimized to ensure that a capillary-dominant regime is maintained. Changes in brine compositions and fluid distribution in the core plugs are characterized using ion chromatography and X-ray computed tomography, respectively. First, we show that polar components in the oil phase play a major role in wettability alteration during ageing; this controls the oil production behavior. We also show that, compared to seawater, both formation water and ten-times-diluted seawater are better candidates for enhanced oil recovery in the Dan field. Finally, we show that the modified flow zone indicator, a measure of rock quality, is likely the main variable responsible for the higher oil recoveries observed in Tor core samples.

Performance prediction of ion-engineered water injection (EWI) in chalk reservoirs using Response Surface Methodology (RSM)

Ion-engineered water injection (EWI) is one of the advantageous enhanced oil recovery (EOR) techniques that gained considerable attention in recent years. Many mechanisms have been proposed for additional oil recovery with modified seawater composition when injected into carbonate formations. However, the prediction of EWI is a big challenge due to complex rock/fluid interactions at pore level and related mechanisms. Thus, an efficient design method for water composition is required. This study investigates the optimization of EWI through the concentration of the active and non-active ions in injection water at laboratory scale for chalk core samples by considering the most dominant design parameters. Data from many imbibition tests of chalk core samples were selected. The Response Surface Methodology (RSM) was used to screen the significant contributing parameters. The sensitivity analysis showed that the four most important design parameters are crude oil acid number (TAN), crude oil base number (TBN), temperature, and injected water salinity. An optimum EWI design was suggested, and the predicted responses were validated using other experimental results that have not been used for model development with high accuracy. The model can be applied for experimental design and optimum water composition.

Modeling interaction between CO2, brine and chalk reservoir rock including temperature effect on petrophysical properties

Carbon dioxide (CO2) capture and sequestration through CO2 enhanced oil recovery (EOR) in oil reservoirs is one of the approaches considered to reduce CO2 emission into the atmosphere. The injection of CO2 into a subsurface geological formation may lead to chemical reactions that may affect the formation pore structure and characteristics. In this study, the effect of CO2–brine–rock interaction on the rock petrophysical properties and mineral volume fraction was numerically investigated during CO2 injection into a chalk reservoir rock. A 3D numerical modeling and simulation were conducted using COMSOL® Multiphysics commercial software of computational fluid dynamics (CFD) to simulate CO2–brine core flooding process in a chalk core. The model was validated against a core–scale experimental data from literature. Simulation differential pressure data matched the literature experimental data closely and consistently indicating good agreement between them. Temperature effect on the performance of CO2–brine–chalk sequestration was also evaluated in the present study. Results indicated that porosity was only slightly affected by temperature increase during CO2 injection in contrast to permeability that was substantially affected by temperature. Moreover, chemical reactions enhanced as temperature increased leading to significant increase in permeability. Thus, carbonated brine sequestration excelled at elevated temperature due to increased acidity which governs the sequestration process. The developed model maybe considered as a reliable tool to optimize various operating parameters of CO2–brine sequestration.

Spectroscopic Fingerprinting of Organic Material Extracted from Tight Chalk Core Samples of the North Sea.

The detailed chemical composition of crude oil in subsurface reservoirs provides important information about reservoir connectivity and can potentially play a very important role for the understanding of recovery processes. Relying on studying produced oil samples alone to understand the rock-fluid and fluid–fluid interactions is insufficient as the heavier polar components may be retained by tight reservoirs and not produced. These heavy and polar compounds that constitute the resin and asphaltene fractions of crude oil are typically present in low concentrations and yet are determining for the physical–chemical properties of the oil because of their polarity. In order to obtain a fingerprint analysis of oils including polar compounds from different wells, the oil content of drill cores has been extracted and analyzed. Infrared spectroscopy has been used to perform chemical fingerprinting of the oil extracted from drill cores sampled in different geographical locations of the Danish North Sea. Statistical analysis has been employed to identify the chemical differences within the sample set and explore the link between chemical composition and geographic location of the sample. A principal component analysis, based on spectral peak fitting in the 1800–1400 cm–1 range, has allowed for statistical grouping of the samples and identified the primary chemical feature characteristic of these groups. Statistically significant differences in the quantities of polar oxygen- and nitrogen-containing compounds were found between the oil wells. The results of this analysis have been used as guidelines and reference to establish an express statistical approach based on the full-range infrared spectra for a further expansion of the sample set. The chemical information presented in this work is discussed in relation to oil fingerprinting and geochemical analysis.

Validation study of water weakening research from outcrop chalks performed on Eldfisk reservoir cores

Seawater injection for Enhanced Oil Recovery (EOR) purposes can increase the hydrocarbon recovery factor in carbonate reservoirs but are also responsible for weakening their mechanical strength. This study investigates the geomechanical behavior of oil-bearing reservoir chalk subject to reactive flow at reservoir conditions. The obtained results are compared with previous results from experiments on water-saturated outcrop chalks.Two unwashed oil-bearing reservoir chalk cores from the Eldfisk Field in the North Sea are mechanically tested in a triaxial cell. The cores are saturated with NaCl brine prior to testing, in addition to residual oil. The cores’ axial and radial deformations were monitored during hydrostatic stress loading and creep under constant 50 MPa stress, at 130 °C. During the test, the cores were flooded with 0.657 M NaCl, 0.219 M MgCl2, 0.219 M MgCl2 + 0.130 M CaCl2 and synthetic seawater (SSW). The average creep strain increased from 0.03% to 0.04% per day radially and from 0.03% to 0.06% axially after changing from the NaCl brine to SSW brine. Flooding MgCl2 brine after NaCl increased the average compaction rate from 0.05% to 0.09% per day radially and from 0.04% to 0.07% per day axially. Adding CaCl2 to the MgCl2 brine reduced the average compaction rate from 0.05% per day during MgCl2 injection to 0.02% per day both radially and axially, comparable to reports from outcrop chalk experiments. The brine composition-dependent creep compaction in the core flooding experiments was explained by dissolution of primary calcite, confirmed by Ion Chromatography, and precipitation of secondary Mg-bearing minerals, seen by Scanning Electron Microscopy (SEM) analytics.Generally, the aforementioned results describing geomechanical behaviors of oil-bearing reservoir chalk cores under hydrostatic stress and thermochemical influence in this study are comparable to those from previous studies on outcrop chalk, thus supporting many years of laboratory research as applicable in the reservoir chalk context.

Colloid-facilitated transport of 238Pu, 233U and 137Cs through fractured chalk: Laboratory experiments, modelling, and implications for nuclear waste disposal

The influence of montmorillonite colloids on the mobility of 238Pu, 233U and 137Cs through a chalk fracture was investigated to assess the transport potential for radioactive waste. Radioisotopes of each element, along with the conservative tracer tritium, were injected in the presence and absence of montmorillonite colloids into a naturally fractured chalk core. In parallel, batch experiments were conducted to obtain experimental sorption coefficients (Kd, mL/g) for both montmorillonite colloids and the chalk fracture material. Breakthrough curves were modelled to determine diffusivity and sorption of each radionuclide to the chalk and the colloids under advective conditions. Uranium sorbed sparingly to chalk (log Kd = 0.7 ± 0.2) in batch sorption experiments. 233U(VI) breakthrough was controlled primarily by the matrix diffusion and sorption to chalk (15 and 25% recovery with and without colloids, respectively). Cesium, in contrast, sorbed strongly to both the montmorillonite colloids and chalk (batch log Kd = 3.2 ± 0.01 and 3.9 ± 0.01, respectively). The high affinity to chalk and low colloid concentrations overwhelmed any colloidal Cs transport, resulting in very low 137Cs breakthrough (1.1–5.5% mass recovery). Batch and fracture transport results, and the associated modelling revealed that Pu migrates both as Pu (IV) sorbed to montmorillonite colloids and as dissolved Pu(V) (7% recovery). Transport experiments revealed differences in Pu(IV) and Pu(V) transport behavior that could not be quantified in simple batch experiments but are critical to effectively predict transport behavior of redox-sensitive radionuclides. Finally, a brackish groundwater solution was injected after completion of the fracture flow experiments and resulted in remobilization and recovery of 2.2% of the total sorbed radionuclides which remained in the core from previous experiments. In general, our study demonstrates consistency in sorption behavior between batch and advective fracture transport. The results suggest that colloid-facilitated radionuclide transport will enhance radionuclide migration in fractured chalk for those radionuclides with exceedingly high affinity for colloids.

Core wettability reproduction: A new solvent cleaning and core restoration strategy for chalk cores

Laboratory core restoration procedures include different core cleaning solvents, establishment of initial water saturation (Swi) by diverse techniques and processes of crude oil exposure that influence the final core wetting properties.In carbonate reservoirs, the acidic Polar Organic Components (POC) in crude oil are the main components dictating the wettability of the carbonate surface. Negatively charged carboxylates strongly attach to the positively charged CaCO3 surface, thus acting as anchor molecules for the oil phase.During core cleaning with organic solvents, some of the adsorbed POC will be detached from the mineral surfaces, thereby changing the wetting conditions. However, cores are not expected to become completely water-wet even when using strong solvents.The main aim of this paper is to develop core cleaning and core restoration procedures to reproduce initial core wettability.Fractional-wet outcrop chalk cores were exposed to two different core cleaning procedures, (1) mild kerosene-heptane cleaning, or (2) standard toluene-methanol cleaning. Chromatographic wettability tests showed that both cleaning methods increased the fraction of water-wet surface area to a similar extent, but neither of them removed all adsorbed POC from the rock surface. Wettability tests by spontaneous imbibition showed that the kerosene-heptane cleaned core behaved slightly water-wet, while the toluene-methanol cleaned core behaved very water-wet, even though the cores had similar fractions of water-wet surface areas. Increased amount of crude oil exposure during core restoration reduced the water wetness below the initial fractional core wetting.Initial core wettability was successfully reproduced by combining mild cleaning solvents preserving initial adsorbed POC, and minimizing the amount of crude oil exposure during core restoration. Only a 1.5–3.6 %OOIP variation in ultimate recovery during spontaneous imbibition in four different core systems was observed.

Modeling of Permeability and Strain Evolution in Chemical Creep Compaction Experiments with Fractured and Unfractured Chalk Cores Conducted at Reservoir Conditions

Summary Understanding the effect of typical water-related improved oil recovery techniques is fundamental to the development of chalk reservoirs on the Norwegian Continental Shelf (NCS). We investigate the contribution and interplay of key parameters influencing the reservoir's flow and storativity properties, such as effective stresses, injecting fluid chemistry, and geomechanical deformation. This is done by developing a mathematical model that is applied to systematically interpret experimental data. The gained understanding is useful for improved prediction of permeability development during field life. The model we present is for a fractured chalk core whereby fluids can flow through the matrix and fracture domains in parallel. The core is subject to a constant effective stress above the yield, resulting in time-dependent compaction (creep) of the matrix, while the fracture does not compact. Reactive brine injection causes enhanced compaction but also permeability alteration. This again causes a redistribution of injected flow between the two domains. A previous version of the model parameterizing the relation between chemistry and compaction is here extended to quantify the effect on permeability and see the effect of flow in a fracture-matrix geometry. A vast set of experimental data were used to quantify the relations in the model and demonstrate its usefulness to interpret experimental data. Two outcrop chalk types (Aalborg and Liège) being tested at 130°C and various concentrations of Ca-Mg-Na-Cl brines are considered. However, assumptions were required, especially regarding the fracture behavior because directly representative data were not available. The tests with inert injecting brine were used to quantify the effect of matrix and fracture mechanical compaction on permeability trends. To be able to explain the tests with reactive brine, an important finding is that permeability not only decreased because of enhanced porosity reduction but also because of a quantifiable chemistry-related process (dissolution/precipitation). Sensitivity analyses were performed regarding varying fracture width, injection rate, and chemistry concentration to evaluate the effect on chemical creep compaction and permeability evolution in fractured cores. The model can be used to highlight parameters with great influence on the experimental results. An accurate quantification of such parameters will contribute to refining laboratory experiments and will provide valuable data for upscaling and field application.

Permeability Evolution of Shear Failing Chalk Cores under Thermochemical Influence.

Development of petroleum reservoirs, including primary depletion of the pore pressure and repressurization during water injection naturally, leads to changes in effective stresses of the formations. These changes impose mechanical deformation of the rock mass with subsequent altering of its petrophysical properties. Besides mechanical compaction, chalk reservoirs on the Norwegian Continental Shelf also seem susceptible to mineralogical and textural changes as an effect of the injecting fluid’s chemical composition and temperature. Understanding such chemical and thermal effects and how they interplay with the mechanical response to changes in effective stresses could contribute to improved prediction of permeability development during field life. This article presents results from mechanical testing of chalk cores of medium-porosity (32%) outcrop chalk (Niobrara Formation, Kansas) in triaxial cells. The experimental setup allows systematic combinations of fluctuating deviatoric stress, temperature (50 and 130 °C), and injecting fluid (calcite-equilibrated sodium chloride, calcite-equilibrated sodium sulfate, and reactive synthetic seawater) intended to replicate in situ processes, relevant to the North Sea chalk reservoirs. Deviatoric loading above yield resulted in a shear failure with a steeply dipping fracture of the core and a simultaneous increase in permeability. This occurred regardless of the brine composition. The second and third deviatoric loadings above yield did not have the same strong effect on permeability. During creep and unloading, the permeability changes were minor such that the end permeability remained higher than the initial values. However, sodium sulfate-injected cores retained most of the permeability gain after shear fracturing compared to sodium chloride and synthetic seawater series at both temperatures. Synthetic seawater-injected cores registered the most permeability loss compared to the other brines at 130 °C. The results indicate that repulsive forces generated by sulfate adsorption contribute to maintain the fracture permeability.

Explicit tracking of CO2-flow at the core scale using micro-Positron Emission Tomography (μPET)

Safe subsurface sequestration of carbon dioxide (CO2) is becoming increasingly important to meet climate goals and curb atmospheric CO2 concentrations. The world-wide CO2 storage capacity in carbonate formations is significant; within deep, saline aquifers and through several CO2-enhanced oil recovery projects, with associated CO2 storage. Carbonates are complex, both in terms of heterogeneity and reactivity, and improved core scale and sub core-scale analysis of CO2 flow phenomena is necessary input to simulators, aiming to establish large-scale behavior. This paper presents a recent advancement in in-situ imaging of CO2 flow, utilizing high-resolution micro-Positron Emission Tomography and radioactive tracer [11C]arbon dioxide to explicitly track CO2 during dynamic flow and subsequent trapping at the core scale. Unsteady state water injection (imbibition) and CO2 injection (drainage) were performed in a low-permeable chalk core at elevated pressure conditions. Short-lived radioisotopes were used to label water and CO2, respectively, and facilitated explicit tracking of each phase separately during single phase injection. Local flow patterns and dynamic spatial fluid saturations were determined from in-situ imaging during each experimental step. Initial miscible displacement revealed displacement heterogeneities in the chalk core, and dynamic image data was used to disclose and quantify local permeability variations. Radial permeability variations influenced subsequent flow patterns, where CO2 predominantly flooded the higher-permeability outer part of the core, leaving a higher water saturation in the inner core volume. Injection of water after CO2 flooding is proposed to be the most rapid and effective way to ensure safe storage, by promoting capillary trapping of CO2. PET imaging showed that presence of CO2 reduced the flow of water in higher-permeability areas, improving sweep efficiency and promoting a nearly ideal core-scale displacement. Alternate injections of water and gas is also expected to improve sweep efficiency and contribute to improved oil recovery and CO2 storage on larger scales. Sub-core analysis showed that residually trapped CO2 was evenly distributed in the chalk core, occupying 40% of the pore volume after ended water injection. Micro-Positron Emission Tomography yielded excellent small-scale resolution of both water and CO2 flow, and may contribute to unlocking fluid flow dynamics and determining mechanisms on the millimeter scale; presenting a unique opportunity in experimental core-scale evaluations of CO2 storage and security.

Impact of rock properties and wettability on Tertiary-CO2 flooding in a fractured composite chalk reservoir

This study presents the experimental and numerical evaluation of tertiary-CO2 flooding (CF) using reservoir and outcrop composite chalk at reservoir conditions. We were able to reproduce the results of composite core flooding experiment and our findings show the considerable effect of rock properties and wettability on the tertiary-CO2 flooding.In our experiments, we place the composite reservoir chalk core vertically in the core-holder with the total length of 45 cm and average diameter of 3.74 cm. The composite core consists of six core plugs of 7.5 cm each and include a centralized axial hole that represents the single fracture with the diameter of 0.6 cm. The whole core plugs are sampled from the reservoir formation in North-Sea-Chalk-Field (NSCF) and are saturated with NSCF stock tank oil (STO) and synthetic connate water. Once reservoir conditions are established, the sea water is injected from the bottom of the core holder and the STO is produced from the top. After no additional produced oil is observed, water flooding (WF) is stopped and CO2 is then injected from the top and the hydrocarbon streams are produced from the bottom of the fracture. The whole core flooding is operated at constant reservoir conditions of 258 bara and 110 °C, representative of NSCF reservoir conditions. Two different chalks, one from the reservoir formation (Exp-3C) and the other from outcrop (Exp-2C) are used in this study to investigate the effect of wettability during tertiary CO2 flooding. A comprehensive compositional numerical simulation with a tuned equation of state (EOS) was developed to model the experiments. Best match for the WF experiments was achieved mainly through tuning the oil-water capillary pressure and reducing the oil relative permeability data. Although the final tuned capillary pressure and relative permeability data were different, we observe similar water saturation at the end WF experiments for Exp-3C and Exp-2C.The tertiary CF lab results were reproduced by numerical model mainly by tuning the multi-component diffusion coefficients. The produced water during CF was successfully modeled by taking the hysteresis effect into account in water-oil capillary pressure data. We also observe that although the final water saturation at the end of WF is comparable, performance of tertiary CO2 flooding for each experiment is different. The reservoir chalk experiment shows higher oil recovery during the tertiary CO2 injection.

The Role of Stabilizer Concentration in the Mobility of Carbon‐Supported Nanozerovalent Iron (nZVI) in Fractured Media

AbstractThe mobility of nanozerovalent iron (nZVI) particles is of major importance when assessing their effectiveness for use in contaminated sites. Here, the influence of the stabilizer loading on nZVI mobility was investigated in fractured media. nZVI in the form of Carbo‐Iron Colloids (CIC), a composite material of activated carbon as a carrier for nZVI, was used for the tests. CIC was supplemented with carboxymethyl cellulose stabilizer, at three different loadings: 5% wt., 20% wt., and 80% wt., in a moderately saline solution of 250 mM. Transport experiments were conducted in a fractured chalk core, and the characteristics of the solutions and particles were analyzed. The effect of ionic strength was investigated by application of the 80% loading, also in a lower salinity solution of ~3 mM. Transport experiments were modeled, and all results were compared with especially high 1,600% wt. stabilizer loading, from a previous study. Among the different carboxymethyl cellulose loadings, significant differences in mobility and a correlation between mobility and stability were observed. Though, differences in stability were found insignificant. Higher stabilizer concentration and, to a greater extent, a higher number of free stabilizer molecules in solution enhance particle stability and, hence, mobility. A good fit was obtained between the transport experiment results and a model of solute and colloid transport in a single fracture. The model attachment parameters showed a good correlation with the decay rate constants of the stability tests, indicating yet again the significant role played by stability in CIC mobility and the possibility for mobility prediction.

Modeling of Creep-Compacting Outcrop Chalks Injected with Ca-Mg-Na-Cl Brines at Reservoir Conditions

Summary Numerical and analytical 1D solutions are presented to interpret the link between geochemical alterations and creep compaction (compaction under constant effective stress) in chalk cores. Chemically reactive flow enhancing chalk compaction is of significant importance for enhanced oil recovery (EOR), compaction, and subsidence in North Sea chalk reservoirs. The focus of this study is on Ca-, Mg-, and NaCl brines that interact with the chalk by the dissolution of calcite and the precipitation of magnesite. An explicit analytical solution is derived for the steady–state ion and dissolution–rate distributions at a given injected composition and injection rate. A mathematical description of creep compaction is proposed on the basis of applied affective stresses and rock ability to carry these stresses as a function of porosity. The reaction and compaction models are then coupled as follows: The compaction rate is assumed, which is enhanced by the dissolution rate, which can vary spatially. Furthermore, the solid volume changes by mineral dissolution and precipitation. Brine–dependent and nonuniform compaction is hence built into the model by means of the dissolution–rate distribution. The model is validated and parameterized against data from a total of 22 core samples from two chalk types (Åalborg and Liege) where reactive and inert brines were injected from ambient to Ekofisk–reservoir conditions (130°C). Experimentally measured effluent concentrations, distributions in mineralogy after flooding, and creep–compaction behavior were matched. Our model is the first to link a vast set of data on this subject and predict performance under new experimental conditions. This also represents a first step in upscaling such results from the laboratory toward the field. Our interpretations indicate that the two chalk types would respond differently chemically and by compaction to changes in the concentration and injection rate. Brines injected through Liege chalk appeared to approach stable oversaturation, while in Åalborg, the equilibrium condition was in agreement with geochemical calculations.