Chalk Surface Research Articles

The surface reactivity of chalk (biogenic calcite) with hydrophilic and hydrophobic functional groups

The surface properties of calcium carbonate minerals play an important role in a number of industrial and biological processes. Properties such as wettability and adsorption control liquid–solid interface behaviour and thus have a strong influence on processes such as biomineralisation, remediation of aquifers and oil recovery. We investigated how two model molecules of different polarity, namely water and ethanol, interact with reservoir and outcrop chalk samples and we compared their behaviour with that of pure, inorganically precipitated calcite. Thermodynamic quantities, such as the work of wetting, surface energy and isosteric adsorption enthalpy, were determined from vapour adsorption isotherms. The chalks were studied fresh and after extraction of organic residues that were originally present in these samples. The work of wetting correlates with the amount of organic matter present in the chalk samples but we observed a fundamental difference between the adsorption properties of chalk and pure, inorganically precipitated calcite toward the less polar, ethanol molecule. Further analysis of the chemical composition of the organic matter extracted from the chalk samples was made by gas chromatography (GC–MS). Monitoring surface composition by X-ray photoelectron spectroscopy (XPS) before and after extraction of the organic material, and with atomic force microscopy (AFM), showed that nanometer sized clay crystals observed on the chalk particle surfaces could be an important part of the reason for the differences. Removal of the extractable portion of the hydrocarbons liberates adsorption sites that have different wetting properties than the rest of the chalk and these have an energy distribution that is similar to clays. Thus, the results exemplify the complexity of biogenic calcite adsorption behaviour and demonstrate that chalk wetting in drinking water aquifers as well as oil reservoirs is controlled partly by the nanoparticles of clay that have grown on the chalk surfaces and partly by adsorbed organic material.

Buried pockmarks on the Top Chalk surface of the Danish North Sea and their potential significance for interpreting palaeocirculation patterns

Three-dimensional (3D) seismic data from the southern Danish North Sea were used to analyse the morphology and spatial distribution of depressions in the Danian Chalk deposit. Previously, these depressions were either interpreted as karst structures or pockmarks. The observed depressions occur in an interval from 25 ms below to 12 ms above the Top Chalk surface. Three types of depressions were differentiated based on their plan-view geometry and their degree of symmetry: Type 1, comprising sub-circular and symmetrical depressions, is the dominant group (ca. 70 %). Type 2, elongated and symmetrical depressions, represents only a small fraction (ca. 5 %). The elongated and asymmetrical depressions of Type 3 compose ca. 25 %. In cross section, each depression type can be either characterised by a V- or a U-shape. The maximum size of the depressions ranges from 50 to 580 m, with an average internal depth of 10 m. We interpret the depressions as pockmarks formed by the expulsion of biogenic or thermogenic fluids at the Danian seafloor. Likely, the initial form of the pockmarks has been circular (Type 1) and was subsequently modified for Types 2 and 3 to an elongated form by currents. The long axis of the pockmarks is interpreted to represent the effective current direction. The inferred direction is sub-parallel to the palaeobathymetric contours. The 3D seismic interpretation of pockmarks presented in this paper contributes to the understanding of fluid migration and palaeocirculation patterns during the sedimentation of the terminal Chalk Group in the southern Danish North Sea.

The effect on chalk recrystallization after treatment with oxidizing agents

Chalk consists of the fossilized remains of the hard parts from some species of unicellular algae. These algae use polysaccharides to control the morphology of their shields, called coccoliths, which are made of from 20 to 60 individual, sub-micrometer size calcite crystals. This biogenic calcite has resisted recrystallization during the more than 65 million years of diagenesis. In this study, we examined the effect of oxidizing agents on the ability of the chalk to recrystallize. Samples from a North Sea drill core that had never been exposed to oil and Rørdal chalk from the Ålborg Portland quarry, north Jutland, Denmark, were treated with oxidizing agents to remove organic material and the recrystallization rates were determined. We treated the chalk with solutions of NaOCl and H 2O 2 with and without 0.1 M NaCl and monitored recrystallization in CaCO 3 saturated 0.1 M NaCl solutions using H 14CO 3 − as a tracer. X-ray photoelectron spectroscopy (XPS) showed that initially, the North Sea chalk had a higher surface concentration of organic carbon than the Ålborg sample, but it decreased significantly after the oxidizing treatment. Loss of C O correlated with higher recrystallization rates; rate increase was highest (~ 50%) after H 2O 2 treatment. Our results demonstrate that through geological time, miniscule amounts of organic material remain associated with the chalk surface and act as an inhibitor for chalk recrystallization. This implies that the polysaccharides originally made by the algae to control coccolith morphology have contributed to hindering chalk diagenesis to limestone, keeping the biogenic particles much smaller, even after 65 million years, than inorganic calcite grown in the lab for a few days.

Salinity on chalk cliffs and its influence on maritime vegetation

AbstractThe chalk cliffs of south-eastern England are liable to catastrophic cliff falls, giving edaphic conditions very variable in space and time. Chalk has ∼50% porosity, holding much water between 100 kPa and 1,000 kPa, but its availability to plants is restricted by salinity. The electrical conductivity of extracts of saturated pastes (ECs), as commonly used for agricultural soils, was used to measure the salinity of chalk and chalky soils. Studies along cliff transects between Folkestone and Dover showed that salinity thus measured was a good predictor of the distribution of both individual plant species and associations. Cliff geomorphology was far more important than distance from the sea in explaining salinity and plant distribution. We suggest that moist chalk surfaces are effectively impermeable, causing salt spray to run off during storms and concentrate in crevices, ledges and the topmost part of talus slopes, even at 400 m from the sea, which then support halophytes. We suggest that ECs coul...

Initial Wetting Properties of Carbonate Oil Reservoirs: Effect of the Temperature and Presence of Sulfate in Formation Water

Water-based enhanced oil recovery (EOR) from carbonates is usually restricted by initial wetting properties, especially in naturally fractured carbonates. Carboxylic material in the crude oil, quantified by the acid number (AN), is regarded as the most important wetting parameter. In this paper, it is shown experimentally that, for a given AN, the temperature and the amount of sulfate present in the formation water will affect the wetting condition significantly. The concentration of sulfate in the formation water is usually low because of a high amount of Ca2+, anhydrite, CaSO4(s), is precipitated at high temperatures. The interaction between small amounts of sulfate dissolved in the formation water and the rock surface was studied using chalk cores. The relative amount of sulfate dissolved in the pore water, SO42–(aq), and sulfate adsorbed onto the chalk surface, SO42–(ad), was quantified at different temperatures of 20, 50, 90, and 130 °C. Below 50 °C, the relative amount of SO42–(aq) and SO42–(ad) was...

Distinguishing between calcites formed by different mechanisms using infrared spectrometry: archaeological applications

Infrared spectrometry is a well-established method for the identification of minerals. Due to its simplicity and the short time required to obtain a result, it can be practiced on-site during excavation using portable infrared spectrometers. However, the identification of a mineral may not be sufficient. For example, a lime plaster floor and a crushed chalk surface have a similar appearance and are composed of the same mineral – calcite. Here we exploit differences in the infrared spectra of geogenic, biogenic and pyrogenic calcites for the identification of each calcite type. The infrared calcite spectrum has three characteristic peaks in the region of 400–4000 cm −1, designated ν 2, ν 3, and ν 4. When a calcite sample is ground, as part of the measurement preparation procedure, some grinding dependent changes will be revealed in the infrared spectrum. With additional grinding, the ν 3 peak narrows and the heights of the ν 2 and ν 4 peaks decrease, when both are normalized to the ν 3 height. By plotting the normalized heights of the ν 2 versus the ν 4 of several grindings of the same sample, a characteristic trend line is formed for each calcite type. The trend lines of geogenic calcites have the shallowest slopes and highest ν 4 values when compared to pyrogenic calcites, which can be further divided to ash and plaster/mortar samples. This method can assist in distinguishing between the various calcites, and provide insights into homogeneity and preservation state of the calcitic materials in question.

“Smart Water” as a Wettability Modifier in Chalk: The Effect of Salinity and Ionic Composition

Seawater can improve the water wetness of chalk at high temperatures, which improves the oil displacement by spontaneous imbibition of water. It is experimentally verified that the interaction between Ca2+, Mg2+, and SO42− at the chalk surface will displace adsorbed carboxylic acids and increase the water wetness. In this work, the effect of salinity and ionic composition of smart water on oil recovery was studied at different temperatures, 100, 110, and 120 °C. The ultimate oil recovery was compared using seawater as the base fluid. When NaCl was removed from seawater, both the imbibition rate and oil recovery increased in comparison to seawater at the temperatures tested. At 110 and 120 °C, the oil recovery from seawater depleted in NaCl increased by about 10% of original oil in place (OOIP) compared to seawater. A decrease in oil recovery of about 5% of OOIP was observed when increasing the amount of NaCl in seawater 4 times. A systematic decrease in oil recovery was observed when using seawater diluted with distilled water as imbibing fluid. Imbibition tests at 110 °C showed that the water-wet fraction increased 29% for seawater depleted in NaCl compared to 11% for ordinary seawater. Diluted seawater to 10 000 ppm did not change wetting conditions at 110 °C. The results confirmed that not only is the concentration of the active ions Ca2+, Mg2+, and SO42− important for wettability alteration to take place but also the amount of non-active salt, such as NaCl, has an impact on the wettability alteration process, which is discussed as a double-layer effect at the chalk surface. No significant improvement in the ultimate oil recovery was observed during forced displacement by modified seawater.

Coinjection of Seawater and Produced Water to Improve Oil Recovery from Fractured North Sea Chalk Oil Reservoirs

Carbonate oil reservoirs are often fractured with moderate water-wet conditions, which prevent spontaneous imbibition of water into the matrix blocks. Enhanced oil recovery by water flooding is therefore seldom successful, and the average oil recovery from carbonates is usually much less than 30%. Hence, the improved oil recovery potential is very high in these types of reservoirs. Recent studies on chalk cores from the North Sea formations have shown that seawater is able to change the wettability toward a more water-wet condition at high temperatures, >100 °C. The successful injection of seawater into the Ekofisk formation, where the oil recovery is now estimated to reach 50%, is a good example. Seawater contains favorable concentrations of the potential determining ions Ca2+, Mg2+, and SO42− that are active in the displacement of strongly adsorbed carboxylic material from the chalk surface. The initial formation water will partly mix with seawater. Therefore, the amount and composition of the produced ...

Spontaneous imbibition of seawater into preferentially oil-wet chalk cores — Experiments and simulations

Spontaneous imbibition is an important IOR process, especially for fractured carbonate reservoirs with low permeability matrix blocks. If the chalk is oil-wet, the process will not take place. Previous studies have shown that seawater may increase the water-wetness. The sulphate ions in seawater may alter the wetting conditions of the chalk surface, especially at high temperatures. One-dimensional imbibition tests of water into vertically placed, preferentially oil-wet chalk cores were performed, with non-sulphate formation water as a reference. The cores were sealed and only open to flow at one or both end faces. For core plugs with both ends open, a delay period was observed if the core initially was 100% oil saturated, and the difference in oil recovery from top and bottom was about 2–4% of OOIP. For core plugs with initial water saturation, the difference was increased to 14% of OOIP with higher oil production from the top. For cores with only the upper end face open to flow, only countercurrent imbibition takes place. Higher oil recovery was observed with seawater than with formation water as imbibing brine. Cleaned core plugs can be more easily rendered partially oil-wet. A numerical model was developed to describe the seawater imbibition process, including the effect of wettability alteration, and used to simulate the experiments. The model includes molecular diffusion and adsorption of salts (sulphate), and gravitational and capillary forces. The salts in the seawater diffuse into the formation water initially present in the core, absorb onto the rock surface and induce wettability alteration. Two measured capillary pressure curves are used in the simulation. The curve with seawater is taken as the water-wet extreme, and the curve with non-sulphate formation water is taken as the oil-wet extreme. The capillary pressure curve is dynamically shifted from oil-wet to water-wet conditions proportionally to the absorb amount of salt. The simulation results match the experimental data well. The inclusion of the dynamic shift of the wettability condition controlled by molecular diffusion results in delayed oil recovery, in line with the experimental results. The model can easily be extended to include different types of ion concentration, e.g. of magnesium and calcium, to include more of the chemical reactions taking place.

The earthquake that shook central Sjælland, Denmark, November 6, 2001

Earthquakes on Sjælland are in general small and seldom felt. The largest earthquakes in the Danish region occur in Skagerrak and Kattegat, and they are felt in NW Jylland (Thy) and in North Sjælland on average several years apart. A small earthquake measuring just 2.8 on the Richter Scale was felt and heard over a surprisingly large area of Sjælland, Denmark on November 6, 2001. The earthquake caused people to abruptly leave their houses near the epicenter, and minor damage to several buildings was observed. The felt area is oriented strongly asymmetrically with respect to the epicenter, but it correlates well with the local geology. Specifically the shaking was felt in a region where the depth to the Top Chalk surface is small, and the thickness of the Quaternary sediments is less than 50 m. In 1869 an earthquake was felt strongly in the exact same area, and contours separating the felt area from the area where nothing was felt coincide almost exactly for the two earthquakes. This supports that geology and not human subjectivity is the determining factor in delineating the felt area for this earthquake.

Weathering rates from top to bottom in a carbonate environment

This study investigates U-series, Sr isotopes, major and trace elements in a chalk aquifer system located in Eastern France. Soil and rock samples were collected along depth profiles down to 45 m in four localities as an attempt to investigate the weathering processes in the soil, the unsaturated zone and the saturated zone of the aquifer. Interstitial water was extracted from soils and rocks by a centrifugation technique. U-series offer a powerful tool to calculate weathering rates because the relative mobility of the U- and Th-isotopes can be precisely measured and it does not require the determination of a reference state as in other approaches. As expected, the data show very large mobile element depletion in the soil with large 230Th excess relative to 238U, while the rocks show more limited but not insignificant mobile element depletion. The U-series data have been used to constrain weathering rates based on a 1-D reactive transport model. Weathering rates in the near surface are about 10–100 times faster than at depth. However, when integrated over the depth of the cores, including the unsaturated and the saturated zones, this underground weathering represents more than 30% of the total weathering flux, assuming congruent dissolution of carbonates. The ( 234U/ 238U) ratios in interstitial water are consistent with solid samples showing 234U depletion near the surface and an excess 234U at depth. A leaching experiment performed on chalk shows that the excess 234U in natural waters percolating through carbonate rocks results both from preferential 234U leaching and direct recoil in the interstitial water. A new approach was used to derive the recoil ejection factor based on BET measurements and the fractal dimension of chalk surface. Consideration of preferential leaching and recoil allows a more accurate modeling of weathering rates.

Injection of seawater and mixtures with produced water into North Sea chalk formation: Impact of fluid–rock interactions on wettability and scale formation

Seawater has been injected into high temperature and natural fractured North Sea chalk reservoirs to improve oil recovery with great success. Previous studies have shown that seawater will improve the water wetness and cause enhanced compaction of the matrix. The composition of the produced water will be quite similar to initial formation water at the start of the water injection. The formation water contains various amounts of divalent cations like Mg 2+, Ca 2+, Sr 2+, and Ba 2+. Later on, the composition will change due to the interaction between seawater and the chalk formation and some mixing with formation water. The fluid–rock interaction will involve dissolution of CaCO 3, substitution of Ca 2+ by Mg 2+ at the chalk surface, precipitation of CaSO 4, SrSO 4 and BaSO 4 depending on the reservoir temperature. Because of environmental reasons, it is desirable to re-inject produced water together with seawater. In the forthcoming research, we will study experimentally the effect of this re-injection on oil recovery and chalk compaction by using mixtures of produced water and seawater as injection fluid. Based on model studies using the OLI software package, the compatibility of mixtures of produced water and seawater has been studied at different temperatures by looking at the precipitation of CaSO 4, SrSO 4 and BaSO 4. Also the impact of changes in the concentrations of Ca 2+ and Mg 2+ due to surface substitution is modelled. The results are discussed in terms of possible scale formation in the producer and injector. In addition, actual chemical equilibrium reactions in the chalk matrix are discussed in relation to variation in temperature during continuous injection of seawater. Special focus is made on wettability modification, irreversible thermodynamics, and impact on the mechanical strength of the chalk matrix.

Effect of Temperature on Enhanced Oil Recovery from Mixed-Wet Chalk Cores by Spontaneous Imbibition and Forced Displacement Using Seawater

Seawater, as wettability modifier in chalk, has been studied at different temperatures and flooding conditions. Without sulfate initially present in the formation water, previous studies by spontaneous imbibition have shown that a temperature of about 100 °C is needed for the wettability modification to take place. The temperature plays a very important role for the chemical reactions taking place at the chalk surface and for the diffusion of active ions (Ca2+, Mg2+, and SO42−) into the chalk matrix. In the present study, the enhanced oil recovery using seawater was studied by both spontaneous imbibition and viscous flooding at 90, 110, and 120 °C. A crude oil of high acid number (AN = 1.90 mg of KOH/g) was used to ensure a low, initial water wetness. At 90 °C, no difference in oil recovery using seawater and formation water in a spontaneous imbibition process was observed, but in the following viscous flooding process, seawater enhanced the oil recovery by 14% compared to formation water. At 110 and 120 °C, the impact of wettability modification using seawater became even more significant regarding oil recovery, but still, the viscous flood by seawater improved the oil recovery significantly compared to formation water. As a conclusion, improvements in oil recovery using seawater as a wettability modifier are obtained even at 90 °C, when displacing the oil by a viscous flood at low flooding rate.

The nature, formation and engineering significance of sinkholes related to dissolution of chalk in SE Hampshire, England

An area around Horndean in SE Hampshire, England, has been studied to determine the reasons for an unusually high density of sinkholes related to dissolution of the underlying chalk. Ground investigations and excavations for a variety of engineering and construction projects in this area have provided detailed information that gives an insight into their formation and development. Contouring of the chalk surface indicates the presence of a north–south-trending palaeovalley, beneath Tertiary cover deposits, to the south of Horndean. The significance of this feature to chalk karstification and continuing sinkhole development is discussed, along with the significance of other geological factors, such as fracture zones in the chalk and the nature and distribution of cover deposits. This forms the basis of a risk assessment of ground subsidence in this area, with particular reference to collapse. The engineering significance of the sinkholes and the design of ground investigations are discussed in the light of experience gained from past projects in this area and the established risk of pollution of the chalk aquifer

“Smart Water” for Oil Recovery from Fractured Limestone: A Preliminary Study

Seawater is characterized as an enhanced oil recovery (EOR) fluid for hot, fractured chalk oil reservoirs because it is able to modify the wetting conditions and improve the displacement of oil. The chemical mechanism for the wettability alteration has been described previously, and it was verified that Ca2+, Mg2+, and SO42− played an important role because of their reactivity toward the chalk surface. Chalk, which is purely biogenic CaCO3, consists of fragmentary parts of calcite skeletons produced by plankton algae known as coccolithophorids, and it is believed to have a more reactive surface than ordinary limestone. To validate seawater as an EOR fluid also for limestone and dolomite, the affinities of these ions toward the rock surfaces must be evaluated. The present paper describes some preliminary experimental studies of the affinity of SO42−, Ca2+, and Mg2+ toward the surface of reservoir limestone cores at temperatures ranging from room temperature to 130 °C. The results confirmed that the ions interacted with the rock surface, and that the established chemical equilibrium was sensitive to the relative concentrations of the ions. It was also observed that the adsorption of Ca2+ and Mg2+ from a NaCl solution onto the limestone surface was quite similar at room temperature but that Mg2+ adsorbed more strongly at higher temperatures. At high temperatures, T = 130 °C, Mg2+ in seawater was able to substitute Ca2+ on the surface but the reactivity was less than for chalk. These findings indicate that seawater will act as an EOR fluid in limestone as well but its potential is probably smaller than for chalk. This was also confirmed by spontaneous imbibition tests performed at 120 °C.

New Method To Prepare Outcrop Chalk Cores for Wettability and Oil Recovery Studies at Low Initial Water Saturation

The initial water saturation in North Sea chalk reservoirs is extremely low, often in the range of 5–7%. Outcrop material is often used to perform parametric studies on wettability and secondary oil recovery by water injection. To relate the experiments to a certain reservoir, it is desirable to mimic the reservoir conditions as closely as possible. This is done using outcrop cores. The cores are usually drilled in the same direction from the same block, dried at 120 °C, evacuated, and saturated with the actual brine. Homogeneous water saturation can be obtained by draining the initial brine to a saturation of less than 10% by using water-saturated nitrogen gas and a porous plate. During this process, the water saturation is decreased from 100% to less than 10%. Potential determining ions like Mg2+, Ca2+, CO32−, and SO42− present in the brine or as salt initially present in the core will adsorb onto the chalk surface. Thus, during the drainage process, the potential determining ions are increased in concentration inside the core, and the apparent concentration in the residual water becomes different from the initial brine. The wetting condition obtained during an aging period at 90 °C is very sensitive to the concentration of SO42− in the initial brine. Outcrop chalk cores that were preflushed with 4 pore volumes of distilled water to remove SO42− appeared to be much less water-wet compared to cores not preflushed in the same way. The amount of SO42− present before and after flooding with distilled water was quantified. A new standard procedure to prepare outcrop chalk cores for wettability studies is suggested: (1) The cores must be flooded with distilled water or formation brine without SO42− to remove SO42− that may be present initially. (2) The initial saturation brine must not contain SO42−.

The effects of temperature on the water weakening of chalk by seawater

Seawater is injected into the high temperature North Sea chalk reservoirs to improve oil recovery with great success. Increased compaction of the rock, which also is a significant contribution to the oil displacement, is observed in the water-flooded area. Very recent laboratory studies at high temperature documented that enhanced compaction took place when chalk cores were flooded by seawater under stress. The special composition of seawater, regarding reactive ions towards the chalk surface, appeared to be the reason. In this paper, two different water-weakening mechanisms are discussed: (1) Substitution of Ca 2+ at the inter-granular contacts by Mg 2+ and (2) Chemical dissolution. The effect of temperature is opposite for the two mechanisms. The impact of temperature on chalk compaction is studied both in a static (cores aged in water) and dynamic (cores flooded with water) way by performing hydrostatic tests by use of standard triaxial cells. Distilled water was used as a reference fluid in the static tests at 80–130 °C. The mechanical strength of chalk containing seawater was opposite to chalk containing distilled water. With seawater, the chalk became weaker as the temperature increased, while using distilled water, the chalk became stronger as the temperature increased. In the dynamic creep tests, at 50–130 °C, the cores flooded with seawater were significantly weaker than cores flooded with seawater without sulfate, but the difference in mechanical strength decreased as the temperature decreased. The mechanism for enhanced chemical induced weakening of chalk by seawater is discussed.

Wettability alteration and improved oil recovery by spontaneous imbibition of seawater into chalk: Impact of the potential determining ions Ca 2+, Mg 2+, and SO 42−

Carbonate wettability is dictated by the surface chemistry related to stability of the water film between the oil phase and the rock surface. It has been verified, both in the field and laboratory, that seawater is an excellent injection fluid to enhance the oil recovery from fractured chalk. The objective of different papers in this series has been to understand the chemistry for improved spontaneous imbibition of seawater into low permeable chalk at low water wetness. Improved spontaneous imbibition of water will take place if the chalk becomes more water-wet during the production phase. The potential determining ions present in seawater, Ca 2+ and SO 4 2−, have great influence on the surface charge of chalk, which can modify the wettability during water injection. In the present study, it was verified that Mg 2+ is another strong potential determining ion towards chalk, which can increase the positive surface charge density. At high temperatures, Mg 2+ can even substitute Ca 2+ from the chalk surface, and the degree of substitution increased as the temperature increased. The interplay between the three potential determining ions: Ca 2+, Mg 2+ and SO 4 2− and the chalk surface with the aim to improve the water wetness of biogenic chalk, was studied from a spontaneous imbibition point of view. To improve water wetness, SO 4 2− must act together with either Ca 2+ or Mg 2+. In both cases, the efficiency increased as the temperature increased. The water wetness of chalk can be improved if some of the carboxylic material adsorbed onto the chalk surface is displaced. A chemical mechanism discussing the mutual interaction between the potential determining ions and the chalk surface is proposed.

Fluviokarst in the top of the Maastrichtian chalk at Rørdal, Northern Jutland, Denmark

Construction work and planning operations in the second half of the 20th century in northern Jutland supplied important detail to the geological background for the topographic development of the Rørdal area, eastern Aalborg. The inversion and uplift of the Sorgenfrei-Tornquist Zone in end-Cretaceous – Paleocene time initiated some 60 million years of erosion that removed the uppermost Cretaceous and Cenozoic deposits. Detailed mapping of the pre-Pleistocene chalk surface supplemented with aerial photographs, quarry sections and excavated sections document the existence of karst phenomena and demonstrate that fluviokarst processes played an important role in formation of the topography. Live and palaeokarst features are supposed to be widespread in Danish limestone areas and the possible existence in a given area may be important in relation to areal planning, including raw materials extraction and water supply and protection.

Wettability Alteration and Improved Oil Recovery in Chalk: The Effect of Calcium in the Presence of Sulfate

It was previously documented that seawater was able to change the wettability of oil-saturated chalk toward more water-wet conditions and that enhanced spontaneous imbibition of water was observed. The efficiency of the imbibition process was improved by increasing the sulfate concentration in seawater. Both calcium and sulfate present in seawater are potential determining ions toward the chalk surface; it is therefore expected that both of the ions are involved in the wettability modifying process, and the symbiotic effects between the ions are studied in this paper with the aim of improving oil recovery from moderate water-wet chalk. Outcrop chalk samples were aged at 90 °C in an acidic crude oil for at least 4 weeks. The concentration of Ca2+ was varied both in the imbibing seawater and in the initial brine. Chromatographic wettability tests showed that the initial wetting condition of the chalk was not significantly affected by changing the concentration of Ca2+ in the initial brine. Increased oil rec...