Clay Dissolution Research Articles

Effects of kaolinite and montmorillonite calcined clays on the sulfate balance, early hydration, and artificial pore solution of limestone calcined clay cements (LC3)

This study investigated the physicochemical effects of kaolinite (CK) and montmorillonite (CM) calcined clays on the sulfate balance, early hydration, and artificial pore solution of limestone calcined clay cement (LC3). The effects of fineness, clay dissolution, and ion-adsorption capacity were evaluated by isothermal calorimetry, compressive strength, ICP-OES, and zeta potential within 72 h, respectively. Increasing the fineness of both calcined clays did not significantly affect the sulfate depletion kinetics or the compressive strength and the adsorption of Ca2+ ions onto the calcined clay’s surface is not the main factor responsible for differences in sulfate demand. The higher dissolution of ions Al in CK provided an intensified and accelerated formation of ettringite that competes for the available sulfate. We demonstrate that the chemical effects have a significant impact on the sulfate balance of LC3, revealing the lesser impact of alternative clays like montmorillonite compared to metakaolin (MK) which can minimize the problem of accelerated sulfate depletion of LC3 mixes with MK.

Geochemical impact of biomethane and natural gas blend injection in deep aquifer storage

Biomethane is one of the main renewable gases available today to offset fossil gas and decarbonize the energy system. The EU is seeking to rapidly replace part of the natural gas usually stored in deep saline aquifers by biomethane which contains up to 10000 ppm of O2. This study presents for the first-time assessment of deep aquifers’ resilience to biomethane and natural gas blend injection into two sandstone reservoirs with different mineral paragenesis from the Paris Basin (France). Multiphase reactive transport modeling allowed to evaluate changes in gas and water quality, mineralogy and moreover, to identify major resilience properties of storage facilities necessary to buffer the oxygen reactivity and induced acidification coming from gas–water–rock interactions. In the presence of pyrite, the injected oxygen is consumed during pyrite oxidation and contributes to the acidification of the formation water close to the injection point. However, the injection of 1% mole fraction of CO2(g) contained in the gas blend is found to be the main acidification factor. The analysis of reservoir resilience showed three protection levels of pH buffering capacity, which can effectively mitigate an increase in oxygen content up to 1000 ppm: (i) calcite dissolution, (ii) plagioclase and clay dissolution and (iii) clay sorption capacity. Models predict that the gas blend injection could induce minimal but long-term change in the mineralogy without significantly impacting the global porosity. Overall, both sandstones can preserve the quality of the gas plume despite partial CO2(g) exsolution induced by the geochemical perturbation of the formations. The developed models will be used as a basis for assessment of renewable natural gas storage facilities in the long-term.

Research on organic polyacid system suitable for water injection wells in low permeability reservoirs

Abstract Currently, the pollution and clogging problems of injection wells in low-permeability reservoirs are becoming more and more serious, leading to a series of issues, such as the decline of fluid production in wells and the increase of pressure in injection wells. To cope with these challenges, SY organic polyacid solution was developed in this paper and its dissolution efficiency was thoroughly investigated. The experimental results show that SY organic polyacid solution has a high dissolution rate of 88.7%, which exhibits excellent scale dissolution effect, and the solvent's dissolution has a good slow-release performance to ensure the long-lasting action time. At the same time, the corrosion rate of the solvent on the N80 steel sheet is only 0.6291 g/m²-h, much lower than the industry standard, which effectively protects the safety of the construction pipeline and production column. In addition, the dissolution rate of SY organic polyacid on clay reaches 10.14% in five minutes, indicating that it prevents the ground collapse caused by excessive dissolution of clay and has good profound penetration ability. In summary, using SY organic polyacid solution can effectively solve multiple problems such as reservoir mud, inorganic scale, organic scale and water injection fouling oil, which provides an effective solution for the contamination and clogging of water injection wells in low permeability reservoirs.

Influence of Different Types of Clay Minerals on the Shape and Form of Silica-Carbonates (Biomorphs) of Ca(II), Ba(II), and Sr(II)

The array of atmospheric and chemical conditions that prevailed during the Precambrian era favored the origin of life in our planet. The minerals and rocks that existed in that era catalyzed the synthesis of the first biomolecules. It has been shown that mineral clays possess the best-suited mineral surface to study the polymerization of protobiomolecules because they catalyze the binding of monomers to form peptides and nucleic acids and foster the binding between them. Inorganic structures exist formed by silica-carbonates of calcium, barium, or strontium, called biomorphs, which emulate the morphological structure of diverse organisms. Some of these morphologies resemble several of the microfossils found in the Precambrian cherts. Notwithstanding, it has not been documented yet whether biomorphs are a consequence of the dissolution of clays in alkaline environments (rich in alkaline earth metals) during the initial stages of the Earth, which would favor the hypothesis that the biomorphs or cherts existed since that time. To know whether biomorphs could have been embedded within minerals or in diverse types of clays and contributed to the synthesis of protobiomolecules, we aimed at synthesizing biomorphs in the presence of different clays of natural origin in two synthesis conditions that resemble both the atmosphere of the Precambrian era and that of the current time, regarding the CO2 concentration. Our results revealed that in the condition emulating the Precambrian, polymorphs of calcite, aragonite, and vaterite were formed. The barium and strontium biomorphs adopted the morphology of radiolarians, diatoms, and foraminifers in this same condition, indicating that they could be the first structure of these organisms. Results also revealed that the clays present in the Precambrian together with the prevailing environmental conditions played a preponderant role as catalyzers in the structural morphology, chemical composition, and crystalline habit adopted by the first organisms. Biomorphs could have been the first inorganic structures with a structural order and characteristic shape (preserving biomolecules in their interior when they were formed) that were synthetized in the early era of our planet, but they have not been dated as a fossil registry.

Investigating the formation of zeolitic compounds from dissolution of clays through vibrational spectroscopy

The dissolution of clays in alkaline media is one of the important pathways in the hydrothermal formation of zeolites; despite this it is not fully understood. In this work we investigate the dissolution of kaolinite, halloysite and metakaolin and the subsequent re-precipitation of zeolites via infrared spectroscopy. Specifically, the behaviour in Bayer synthetic liquor was focused on. It was found that the appearance of a broad H-bonding absorbance suggests dissolution for all these clays begins by ingress of OH− into the interlayer region. In addition, the calcined kaolinite (metakaolin) appears to reform surface or accessible Al-OH species as shown by the sharp peaks observed at ∼3620 and ∼3680 cm−1. All clay dissolution leads to observable silicate species in the solution at early times and water dominates at later times. The rate at which the clays dissolve was found to be in the order kaolinite < halloysite = calcined kaolinite. The dissolution of halloysite and kaolinite showed no subsequent zeolite Linde-type A (LTA) formation but dissolution of calcined kaolinite showed that desilication product formation followed an LTA > sodalite transformation pathway in caustic while this was not observed in Bayer liquor. Finally, infrared can be used to monitor the silicate in solution (provided samples have not aged and an appropriate control is used) as well as monitoring the solids formed through a peak ratio method. Having said this, LTA is harder to monitor than sodalite where other supporting evidence was required.

Multimodal study of the impact of stimulation pH on shale pore structure, with an emphasis on organics behavior in alkaline environments

The tight nature of shale formations calls for hydraulic fracturing techniques able of altering the pore architecture to facilitate the production of hydrocarbon resources. The composition, pH, salinity, density and chemistry of Hydraulic Fracturing Fluids (HFF) vary substantially depending on reservoir characteristics. pH is among the most important stimulation fluid properties, ranging from very acidic, for carbonate reservoirs, to basic, in the case of clay-rich formations. pH regulates what geochemical reactions take place between the stimulation fluid and shale, as well as pore architecture alteration. The dissolution of carbonates and pyrite, and the precipitation of common minerals such as barite, gypsum and iron oxides in acidic environments have been extensively documented. In contrast, alkali stimulation environments and the role of organic components have received less attention. This research provides insight into the role of both minerals and organic components during alkaline stimulation, and the resulting pore architecture alterations. A set of reactive experiments are performed using three different shales, with varying organic matter (OM) content, at different pH (3, 6, 8, 10, 12). The analysis of pore architecture alteration induced by the reaction was performed via nitrogen (N2) gas adsorption and Time-Domain Nuclear Magnetic Resonance (TD-NMR). Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) was used to measure the composition of the stimulation fluid post-reaction. The system with a larger impact on the pore architecture (pH 12) underwent a more extensive analysis through the application of Focused Ion Beam Scanning Electron Microscopy (FIB-SEM) and x-ray Diffraction (XRD). Results show that mineral dissolution contributed to the creation of secondary porosity, mainly through dissolution of clays and silicates. We show that the abundance and distribution of organic matter both play a significant role in changes in pore architecture. Most samples showed an increase in inter-organic porosity, likely due to the organic acids within kerogen reacting with the stimulation fluid. Samples with a larger amount and more widespread distribution of organic matter show the most significant alterations. Finally, we also highlight the dissolution process of iron-rich minerals, including framboidal pyrite, as another important source of secondary porosity.

Geochemical processes and mechanisms for cesium enrichment in a hot-spring system

Abstract Geothermal systems in Tibet, a crucial geothermal region in China, belong to the Mediterranean-Himalayan geothermal belt and are characterized by a broad distribution of cesium (Cs) bearing geyserite deposits. Targejia, one of the largest Cs-bearing geyserites in southern-western Tibet, contains 1.446 × 104 tons of Cs. The highest ore grade reaches 2.89 wt%, and the ore-forming process can be subdivided into mineralization stages I to V. Cs is heterogeneously distributed in geyserites. Herein, two Cs-bearing ores are investigated, with distinct characteristics of (1) low-Cs-bearing ore (amorphous silica opal-A and opal-CT type) with low Cs (average of ~0.2 wt%), Na, K, Al, and Ca contents, and (2) high-Cs-bearing ore (clay type) with high-Cs (average of ~1.40 wt%), -Na, -K, -Al, and -Ca contents. It is reported for the first time that Cs primarily exists in clay rather than in amorphous silica opal. The Cs-enrichment mechanisms are different for the above two Cs-bearing geyserite types: (1) The deprotonated –OH, surrounded by water molecules, controls the amount of Cs absorbed on the geyserite surface (Si–OH) in the low-Cs-bearing ore. (2) The variable Cs content depends on the Al content because Al substitutes Si, yielding more negative charges to absorb Cs in the high-Cs-bearing ore. Geothermal fluid loading-mass elements, such as Cs and SiO2, precipitate as amorphous silica (opal) with clay minerals. Mineral saturation index modeling was used to predict the most applicable physical parameters for ore formation. The results confirm that the ore forms at ~85 °C and a pH of ~8.5 in the Na-Cl system at stage V. The degree of Cs enrichment reduces from the latest stage V (0–4 ka) to the early stage IV (4–17 ka), and is controlled by clay dissolution, which might further relate to the climate change in Tibet’s Holocene. Fluid-rock interaction modeling shows that dissolution–reprecipitation induces a higher order of amorphous silica formation and clay dissolution at &amp;gt;40 °C and pH of 5–9 at stages V and III, excluding Cs from the ore.

The risk of scaling in Danish geothermal plants and its effect on the reservoir properties predicted by hydrogeochemical modelling

Injectivity problems have been observed in two of three Danish geothermal plants. The utilized geothermal reservoirs comprise sandstones of Lower Triassic and Upper Triassic – Lower Jurassic. The great variations in the formation mineralogy and the chemical composition of their brines allow for a thorough analysis of the differences in the potential risks of scaling. The key scaling processes upon injection of the cooled brines into the reservoirs were identified and changes in the reservoir porosity assessed by hydrogeochemical modelling. Scaling was predicted to be of importance only in one of the geothermal reservoirs, where barite precipitation in the near field of the injection well was identified as a potential risk and could potentially alter the reservoir porosity. Scaling induced by barite nucleation in the injection well, and precipitation and dissolution of carbonates, silicates and clays in the geothermal reservoirs are all of minor importance.

CO2 sequestration in sandstone reservoirs: How does reactive flow alter trapping mechanisms?

Numerous studies have attempted to assess the possible changes in the petrophysical parameters of sandstone reservoirs for the geological storage of CO2. However, the results have been highly variable and hardly conclusive, highlighting mineral alteration, precipitation, dissolution, and fine migration as the main drivers of changes in porosity, permeability, and surface wettability. In this paper, a series of systematic experimental studies were carried out on the Berea sandstone samples to carefully analyse the changes in porosity, relative permeability, capillary pressure, and surface wettability after CO2 treatment. The results obtained indicated that porosity increased in the samples due to dissolution of minerals (calcite and clay) and there was no sign of pore throat closure due to mineral precipitation. It was found that the surface wettability can be shifted towards a CO2 wet system, probably because of calcite precipitation and clay dissolution or reduction in the hydrophilicity of quartz, leading to a decrease in capillary entry pressure. It appears that the mineral and capillary trapping in sandstone make it a suitable storage site for CO2, but that require further research to fully understand the changes posed by mineral dissolution in the long term.

Characteristics of Lithofacies Combinations and Reservoir Property of Carbonate-Rich Shale in Dongying Depression, Eastern China

Drilling has demonstrated that oil can be produced from the lacustrine shale in the upper part of the fourth member of the Shahejie Formation (Es4s) in Dongying Depression. These wells are often drilled in multiple lithofacies combinations, rather than a single lithofacies. Thus, it is necessary to classify the shale lithofacies combinations to optimize favorable exploration targets. The basic characteristics of carbonate mineral-rich shales in the Es4s were determined by core description, optical microscopy of thin sections, mineral analysis by X-ray diffraction, and scanning electron microscope analysis of argon ion-polished samples, combined with organic geochemical analysis and reservoir property analysis. The depositional setting was determined, and the lithofacies combinations of the shale were divided based on the depositional setting and vertical changes in lithofacies combination. Lithofacies combinations include 1) laminated argillaceous limestone interbedded with the calcareous mudstone (LLM); 2) laminated argillaceous limestone interbedded with dolomite (LLD); and 3) calcareous mudstone intercalated with argillaceous limestone (BML). LLM formed in a semi-humid climate, with limited detrital provenance, while LLD was mainly the product of an arid climate, little detrital input, semi-deep water, and a strongly reducing saltwater environment. In addition to the basal strata in the center of the depression, the LLD lithofacies combination was also widely developed on the gentle slope and the slope area transitional to deep depression in the south of the basin. BML was mainly formed in a semi-humid climate, brackish, deep water, and a reducing environment with many detrital provenances. The porosity in LLM and LLD consists of various types. Intercrystalline pores in clay minerals, intercrystalline pores and dissolution pores in carbonate minerals, interbedded fractures, and ultra-pressure fractures are well developed. The pore width distribution range of both LLM and LLD is large. The porosity of LLM is slightly higher than that of LLD. Interlayers with higher dolomite content have higher permeability. The types of pores in BML with no significant laminae characteristics are less abundant, and it is difficult to form an effective reservoir porosity network from the pores and microfractures, and hence, the reservoir physical properties are relatively poor. Oil saturation index indicates that both LLD and LLM of medium–low degree of evolution have good oil content.

Geochemical and physical alteration of clay-rich shales under supercritical CO2 conditions

The injection of Carbon Dioxide (CO2) into shale gas formations is a promising approach that not only reduces the impact of greenhouse gas on climate change but also enhances the gas recovery due to geochemical interactions between CO2 and clay minerals. However, these geochemical interactions have not been fully explored and changes in the petrophysical properties of shales have been reported over time. This paper evaluates the geochemical reactions and physical changes caused by the injection of supercritical carbon dioxide (ScCO2) on the properties of clay-rich shales. Samples from the Eagle Ford formation were collected and exposed to ScCO2 at different pressures ranging from 10 to 24 MPa at 70 °C. Analytical methods such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and thin-section microscopy were used to characterize the ScCO2-treated samples. The results showed dissolution of clays and precipitation of quartz with increasing ScCO2 treatment pressure. The content of carbonate minerals was also reduced at high pressure, which can be attributed to the reactions of the dolomite with H+ to form magnesium carbonate. The percentage of absorption of aromatic hydrocarbons and oxygenated groups gradually increased with increasing pressure, which can be attributed to the increase in CO2 adsorption. On the other hand, the absorption of aliphatic and hydroxyl groups decreased after treatment. ScCO2 treatment pressure is an important factor to evaluate the CO2 adsorption capacity of clay-rich shales. The presented results enrich the understanding of the interactions between CO2 and shale under different pressures, which may be helpful to determine the feasibility of long-term injection and storage in shale.

Reactivity of clay minerals of the Eocene Esmeraldas Formation rocks of the Middle Valley Magdalena Basin (Colombia) in brines and alkaline solutions

The present work focused on understanding the interaction between clay minerals with alkaline fluids used in the oil industry. For this, a comparative study was conducted where the reaction of three rocks belonging to the Eocene Esmeraldas Formation was observed under two experiments. One of these tests consisted of a hydrothermal treatment at static conditions solutions with different concentrations of sodium hydroxide at different interaction times and constant temperature. In parallel, core-flooding tests were carried out, where the original conditions to which these rocks are subjected in the well were considered. Both the original and treated samples were examined by X-ray powder diffraction (XRPD) and Scanning Electron Microscopy (SEM) and the standard test method for Methylene Blue Index to determine the changes that these minerals underwent upon contact with high pH alkaline fluids. As a result, zeolite Linde Type A (LTA), sodalite (SOD) and cancrinite (CAN) formed after alkaline hydrothermal treatment under static conditions, which coincided with the dissolution of kaolinite and smectite. In contrast, core-flooding tests did not show the appearance of zeolites; but dissolution of smectite and kaolinite was observed, in addition to the appearance of an unidentified crystalline phase. The concentration of the alkaline solution and reaction time influenced the dissolution of clays. On the other hand, the cation exchange capacity (CEC) increased at lower concentrations and short reaction times in hydrothermal treatments, which is explained by the appearance of zeolite LTA. Core-flooding tests did not yield the same results as alkaline hydrothermal treatments. It is due to the continuous flow of the alkaline solution through the core plugs that did not allow the polymerization of dissolved aluminosilicate material.

Evaluation of the Accumulation Conditions and Favorable Areas of Shale Gas in the Upper Palaeozoic Marine-Continental Transitional Facies in the Daning-Jixian Area, Ordos Basin

This study focuses on the organic-rich mud shale in the Upper Palaeozoic transitional facies in the southeastern margin of the Ordos Basin. It systematically analyzes the shale gas accumulation conditions of the organic-rich mud shale in the Lower Permian Shanxi-Taiyuan Formation, including the thickness, distribution, organic matter type and content, thermal maturity, reservoir space, gas-bearing property, and rock brittleness. The results show that the thick dark mud shale contains a high organic matter content, is a suitable kerogen type for gas generation, and exhibits moderate thermal evolution, providing excellent conditions for hydrocarbon accumulation. Residual primary pores formed by shale compaction, secondary pores formed by organic matter hydrocarbon generation, clay mineral transformation and dissolution, and fractures provide suitable reservoir spaces for shale gas. The shale in the study area has a higher gas content than the shale strata in the marine basins of the United States. In addition, the content of brittle minerals such as quartz is higher, and Poisson’s ratio is lower, facilitating the subsequent transformation. The accumulation conditions indicate the high potential of the study area for shale gas exploration and development. The geological analogy method is used to compare the study area with five major shale gas basins in the United States. The results indicate that the shale gas resources of the Shanxi-Taiyuan Formation in the study area are in the range of 2800– 3200 × 10 8 m3. The primary controlling factors affecting shale gas reservoirs in this area are the abundance of organic matter, thermal maturity, shale thickness, and quartz content. Favorable areas are predicted based on these factors.

Effects of Carbonate Minerals on Shale-Hydraulic Fracturing Fluid Interactions in the Marcellus Shale

Natural gas extracted from tight shale formations, such as the Marcellus Shale, represents a significant and developing front in energy exploration. By fracturing these formations using pressurized fracturing fluid, previously unobtainable hydrocarbon reserves may be tapped. While pursuing this resource, hydraulic fracturing operations leave chemically complex fluids in the shale formation for at least two weeks. This provides a substantial opportunity for the hydraulic fracturing fluid (HFF) to react with the shale formation at reservoir temperature and pressure. In this study, we investigated the effects of the carbonates on shale-HFF reactions with a focus on the Marcellus Shale. We performed autoclave experiments at high temperature and pressure reservoir conditions using a carbonate-rich and a decarbonated or carbonate-free version of the same shale sample. We observed that carbonate minerals buffer the pH of the solution, which in turn prevents clay dissolution. Carbonate and bicarbonate ions also scavenge reactive oxidizing species (ROS), which prevents oxidation of shale organic matter and volatile organic compounds (VOCs). Carbonate-free samples also show higher pyrite dissolution compared to the carbonate-rich sample due to chelation reactions. This study demonstrates how carbonate minerals (keeping all other variables constant) affect shale-HFF reactions that can potentially impact porosity, microfracture integrity, and the release of heavy metals and volatile organic contaminants in the produced water.

Use of chelators and hydrofluoric acid for the improvement of well productivity

The paper presents the results of microscopic analysis of the content of elements on the surface of clay samples after treatment with an acidic mixture of HV:HF. The analysis showed that as a result of the dissolution of clay and quartz, quartz dissolves faster, the Si/Al ratio on the sample surface decreased, which indicated the formation of a protective film of aluminum phosphate silicate. The studies of the ability of chemical reagents to dissolve clay by the method of weight loss showed that in the HV:HF system, the dissolution rate of clays is much lower due to the formation of a coating layer. The stage of the increase in the rate of dissolution occurs when a part of this layer is dissolved. The studies of the ability of the HV:HF chemical reagent system to dissolve silicon showed that the amount of dissolved silicon varied linearly with the exposure time and the rate of silicon dissolution was proportional to the HF concentration in the system. The dissolution rate of silicon in the HV:HF system is much higher than in the HCl:HF system at the same HF concentration. This is a significant advantage of the proposed system of chemical reagents HV: HF in comparison with the traditional HCl:HF system. HV: HF system dissolves calcium carbonate much more efficiently than traditional hydrochloric or acetic acid systems.

Combined determination analysis of surface properties evolution towards bentonite by pH treatments

Bentonite possesses multiple physicochemical properties because of a large number of charges on the surface. Understanding charges-related surface characteristics at various aqueous solution conditions is critical for bentonite application in various fields, including material modification and environmental restoration. In this study, the evolution of surface properties (surface potential, surface electric field strength, specific surface area, and surface charge numbers) for bentonite samples was investigated using simple ion-exchange experiments and extended combined determination method at different pH conditions. Zeta potential, X-ray diffraction (XRD), and attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) techniques were used to examine changes in bentonite structure and surface functional groups. The results demonstrated that surface potential was considerably negative (larger in absolute value) and more sensitive to pH variation than the zeta potential of bentonite under identical conditions. At multiple pH settings, the surface electric field strength and the numbers of bentonite’s surface charges were inversely proportional to hydrogen ion concentrations. Cations covalently bonded to clay’s surface might reduce negative surface charges and diminish surface electric field strength, particularly as ionic strength increased. Although moderate pH variation has no discernible effect on bentonite interlayer structural alterations, moderate clay dissolution and changes in mineralogical properties may increase the specific surface area of bentonite, particularly when the solvent chemical conditions are acidic. This study comprehensively sheds light on the evolution of surface properties of bentonite clay under acidic, neutral and alkaline conditions, which is helpful in understanding and optimizing its surface performance for material and environmental applications.

Integrated sedimentological and petrophysical characterization of the Lower Cenomanian clastic Bahariya reservoir in Abu Gharadig Basin, Western Desert, Egypt

The Lower Cenomanian siliciclastic Bahariya Formation represents one of the main producing reservoirs in the north Western Egyptian Desert. A few previous subsurface studies exist that integrated core, petrographic, scanning electron microscope (SEM), and X-ray diffraction (XRD) analyses to study the reservoir quality of this formation. Consequently, the present study is carried out to fill this gap and outline the sedimentological, petrophysical, and digenetic characteristics of this siliciclastic reservoir in addition to the impact of these characteristics on reservoir quality. Detailed microscopic examination, XRD, SEM, and conventional core analyses were applied on the available cores. Five distinct microfacies types were petrographically identified, belonging to the arenite and wacke facies. These microfacies are represented by quartz arenite, subarkose, argillaceous quartz wacke, calcareous quartz wacke, and calcareous feldspathic greywacke. The formation was subjected to compaction and was affected by four types of cement; silica, calcite, kaolinite, and pyrite cementation. It has three types of porosities: primary intergranular and secondary porosity (microporosity and dissolution). The studied core samples of the Bahariya Formation are discriminated into two reservoir rock types (RRTs) and exhibit extreme heterogeneity. RRT1 comprises mainly quartz arenite, whereas RRT2 comprises quartz wacke. RRT1 has a reservoir quality of tight to poor due to porosity/permeability occluding by the different cementations.In contrast, RRT2 has a reservoir quality of tight to fair, despite the presence of the clay matrix. The tight to fair quality is possibly attributed to the dissolution of feldspars and authigenic clays. The Lower Cenomanian reservoir is considered a second cycle deposition and represents a tight to a fair quality reservoir in the studied wells.

Surface wettability alteration of shales exposed to CO2: Implication for long-term integrity of geological storage sites

Surface wettability is a key factor controlling the CO2 seal capacity and defines the CO2 storage potential. Limited studies have addressed the shale/water wettability behavior during CO2 injection, thus considerable attention is needed to understand this concept. In this paper, an ample number of supercritical CO2 exposure experiments were conducted to evaluate the alteration of shale/water contact angles. Different types of shales with various mineralogy from Eagle Ford, Wolfcamp, and Mancos fields, were exposed to SCCO2 at different durations, pressures, and temperatures. Shale mineralogy and surface were analyzed using X-ray diffraction and scanning electron microscope. The results indicated a strong relationship between mineral composition and the alteration in shale/water wettability. Clay-rich shales displayed a possible turn in wetting behavior to CO2-wet with extending the SCCO2 treatment time and increasing the treatment pressure, caused by SCCO2 dissolution of clay and carbonate minerals. While the wettability of high-quartz contents shales remained strongly hydrophilic after various SCCO2 treatment conditions. Increasing the temperature accelerated the CO2/shale interactions, and a minor effect was observed on the shale hydrophilicity. Increasing the cohesive energy density of CO2 promotes a favorable CO2 wetting environment, which reduced the hydrophilicity of the surface and reduces the surface energy. In conclusion, shales with high quartz contents exhibit strong water wetting behavior after SCCO2 treatment, which leads to better sealing capacity, more efficient integrity of geological storage sites, and higher potential for CO2 containment.

The Evolution of Diagenetic Fluids and Accumulation Characteristics of Tight Sandstone Reservoir in Upper Paleozoic, Southwestern Ordos Basin

The Upper Paleozoic in the southwestern Ordos Basin has significant potential for natural gas exploration. This study investigated the diagenetic fluid evolution and hydrocarbon accumulation characteristics of He 8 section from Permian Lower Shihezi formation and Shan 1 section from Shanxi formation tight sandstone reservoirs by petrographic observation, scanning electron microscope imaging, fluid inclusion study, and laser Raman spectrum analysis. The results show that He 8 section and Shan 1 section reservoirs are mainly composed of quartz sandstone, subordinate arkose quartz sandstone, and lithic quartz sandstone, with minor lithic sandstone and lithic arkose sandstone. The major pores are intergranular dissolved pores. The main diagenetic minerals include quartz overgrowth, siliceous cement, carbonate cement, illite, montmorillonite, and mixed-layer clay minerals. The overall diagenetic features show strong compaction, multistage siliceous and calcareous cements, and abundant clay minerals, strong dissolution, and well-developed fractures. Two stages of fluid inclusions developed in the He 8 and Shan 1 sections recorded the migration and accumulation of the early-stage and late-stage natural gas, respectively. The reservoir in the study area experienced early and late diagenetic stages, and its formation was simultaneous with or after its densification. The diagenetic environment changed from alkaline to acidic and again into alkaline. There are two stages of fluid activities in the study area, namely, the early diagenetic stage corresponding to hydrocarbon generation and migration and the late diagenetic stage corresponding to hydrocarbon accumulation. This study suggests that Upper Paleozoic natural gas migrated into the reservoir in Weibei Uplift, Yishan Slope, and Tianhuan Depression tectonic units during 220-197 Ma, and the large-scale migration and accumulation occurred in these tectonic units at different times. No natural gas was generated in the west margin of the basin because the temperatures of the hydrocarbon source rocks in the Upper Paleozoic were below the gas window.

Insight into cyanobacterial preservation in shallow marine environments from experimental simulation of cyanobacteria-clay co-aggregation

Clay minerals associated with microbial body and trace fossils in rocks deposited in shallow marine environments have importance in understanding microbial preservation. We show experimentally that the presence of suspended detrital clays in oceanic environments is a crucial control on the formation of co-aggregates between filamentous cyanobacteria and clay minerals and is thus important in the sedimentation and subsequent preservation of cyanobacteria. Strong cyanobacteria-clay co-aggregation occurs over short time periods (3–7 days) in the presence of 10 mg/L of suspended clay minerals (illite, kaolinite and montmorillonite), and the occurrence of co-aggregation is not affected by the type of clay minerals present. The cyanobacteria-clay co-aggregation shows a pronounced effect on preserving the biomass of cyanobacteria. The surface properties of clay minerals and cyanobacteria, specifically the electrostatic attraction between the oppositely charged surfaces of cyanobacteria and clay minerals, are the main drivers of cyanobacteria-clay co-aggregation. Chemical effects, such as the deprotonation of COOH groups in active organic compounds secreted by cyanobacteria and therefore the release of H+, result in local dissolution of clay and slight structure changes in the bulk of clay minerals. Compared with the suspended clay, dissolved ions (Si, Al or Fe ions) are of secondary importance in the formation of cyanobacteria-clay co-aggregates, although these ions further promote this process. Overall, the cyanobacteria-clay co-aggregation not only substantially affects the preservation of cyanobacteria, but also may have potentially important and profound impacts on related geochemical processes such as microbial dissolution of clay minerals and the biogeochemical Si cycle in marine environments.