Wettability Index Research Articles

Integration of torrefaction and in-situ pelletization for biodried products derived from municipal organic wastes: The influences of temperature on fuel properties and combustion behaviours

Biodried products (BPs), derived from municipal organic wastes (MOWs), are generally accepted as carbon-neutral and renewable energy sources. To upgrade their combustion utilization as solid fuels, pelletization subsequent to torrefaction was conducted for BPs by activating the inherent binders using in-situ heat. In this study, the effects of temperature on the fuel properties and combustion characteristics of torrefied BP (TBP) pellets were comprehensively investigated. The results indicated that torrefaction promoted the emissions of O-containing volatiles, including CO2, CO, water and some tar (acids, esters, phenols, etc.), and dehydration might be dominant with higher ratios of deoxygenation and dehydrogenation. High torrefaction temperatures improved the energy densities (1.03–1.27) of TBPs, but decreased the energy yields (99.61%–68.46%). Meanwhile, high temperatures (above 200 °C) for pelletization facilitated the porosity structures and energy consumption of TBP pellets, which also upgraded the pellet properties, especially for TBP250, which exhibited a low wettability index (5.08%), high hardness (10.89 N/mm2) and durability (>96%). Moreover, the TBP pellets were found to retard moisture absorption and volume expansion significantly. In addition, torrefaction enhanced the decomposition of residual lignocellulose and char during the combustion of TBPs. Compared with lignite, TBPs were more flammable with a lower average activation energy. The integration of torrefaction and pelletization effectively decreased the emissions of CO, NO and SO2 during combustion of TBP pellets, and the combustion slags also exhibited favourable potentials for soil regulation. By integrating torrefaction with in-situ pelletization for BP pretreatment, this study provided an efficient alternative for MOW management.

Wettability of Carbonate Reservoir Rocks: A Comparative Analysis

Various methods have been proposed for the evaluation of reservoir rock wettability. Among them, Amott–Harvey and USBM are the most commonly used approaches in industry. Some other methods, such as the Lak and modified Lak indices, the normalized water fractional flow curve, Craig’s triple rules of thumb, and the modified Craig’s second rule are based on relative permeability data. In this study, a set of capillary pressure curves and relative permeability experiments was conducted on 19 core plug samples from a carbonate reservoir to evaluate and compare different quantitative and qualitative wettability indicators. We found that the results of relative permeability-based approaches were consistent with those of Amott–Harvey and USBM methods. We also investigated the relationship between wettability indices and rock quality indicators RQI, FZI, and Winland R35. Results showed that as the rock quality indicators increased, the samples became more oil-wet.

Analysis of the bounded and unbounded forms of USBM wettability index

Understanding wettability is important in many processes where porous media and fluids are in contact. The U.S. Bureau of Mines (USBM) is one of the industry standard techniques for measuring wettability. This index is calculated as USBM=lg(A1A2), where A1 and A2 are the areas under capillary pressure curves of oil-drive and water-drive processes, respectively. Usually, the USBM is mistakenly assumed to vary over the range of −1 to 1 and compared with other indices. In this study we indicate that the lower and upper bounds of this index are not fully known in practice. As a result, comparison between USBM and other indices may cause erroneous interpretations due to dissimilar ranges of variation. In addition, even during examining the USBM of a sample it may not be possible to accurately interpret its wettability. We highlight the bounded form of the USBM index (denoted as USBM∗=A1−A2A1+A2), which varies over the range of −1 to 1, and suggest that it should replace the traditional form of USBM index. Twenty limestone core-plugs were collected from Asmari and Fahlian formations in two Iranian fields. These samples were used for performing primary imbibition relative permeability measurements, as well as primary imbibition and secondary drainage capillary pressure tests. These experiments are used to show the differences between USBM and USBM∗ in comparative studies and compare them against other indices of Amott-Harvey, Lak and modified Lak.

Characterization of physicochemical, packing and microstructural properties of beet, blueberry, carrot and cranberry powders: The effect of drying methods

Physicochemical, packing and microstructural properties of beet, blueberry, carrot, and cranberry powders produced by hot-air drying (HAD), freeze drying (FD), and a film drying method (InfriDri® Process (IFD)) were evaluated. Higher bulk and tapped densities were recorded in the IFD powders than other powders. The IFD powders had a lower Hausner ratio and compressibility index, a higher wettability index, and were less hygroscopic compared to other powders. The IFD powders had a smooth and flaky appearance while other powders exhibited a more irregular and porous morphology. Total phenolic content (TPC) of the IFD beet and carrot powders was 20.95% and 30.43% higher than that produced by HAD, and 3.42% and 7.96% higher than those from FD, respectively. Similarly, the beet and carrot powders from IFD exhibited an antioxidant activity 21.45% and 11.52% higher than that from the HAD method, and 14.84% and 14.43% higher than the FD counterparts, respectively.

Evaluating porous media wettability from changes in Helmholtz free energy using spontaneous imbibition profiles

Spontaneous imbibition profiles are widely used for wettability evaluation of porous media such as rocks. However, mostly the equilibrium imbibed volume is the basis for wettability evaluation. Here, we model the relationship between the shape of imbibition profile and wettability of a medium. We develop a wettability evaluation criterion based on the change in Helmholtz free energy of the system during the imbibition process. The model relates the Helmholtz free energy to the area under the profile, the slope of the imbibition profile, equilibrium imbibed volume, and equilibrium time. We propose a modified form of Lucas-Washburn equation to model the capillary-driven flow of a viscous wetting phase into a porous medium saturated with a viscous non-wetting phase. We introduce a wettability index using the volume-normalized Helmholtz free energy. Finally, the model is tested on imbibition data of eight twin rock samples, and the wettability results show moderate to strong correlations with rock properties. The results show that wettability indices predicted by the proposed technique exhibit more accurate correlations compared with those obtained by the volume-based method.

CO2 – brine – sandstone wettability evaluation at reservoir conditions via Nuclear Magnetic Resonance measurements

CO2-rock wettability is a key parameter which governs CO2 trapping capacities and containment security in the context of CO2 geo-sequestration schemes. However, significant uncertainties still exist in terms of predicting CO2 rock wettability at true reservoir conditions. This study thus reports on wettability measurements via independent Nuclear Magnetic Resonance (NMR) experiments on sandstone (CO2–brine systems) to quantify Wettability Indices (WI) using the United States Bureau of Mines (USBM) scale. The results show that CO2 (either molecularly dissolved or as a separate supercritical phase) significantly reduced the hydrophilicity of the sandstone from strongly water-wet (WI ≈ 1) to weakly water-wet (WI = 0.26), and associated with that the water-wetness of the rock for the two-phase systems. This was caused by additional protonation of surface silanol groups on the quartz, induced by carbonic acid. Capillary pressure and relative permeability curves and residual CO2 saturation were also measured; these results were compared with literature data, and general consistency was found. NMR T2 distribution measurements also demonstrated preferential water displacement in large pores (r > 1 µm) following scCO2 flooding, while no change was observed for smaller pores (r < 1 µm). These insights add confidence to the assessments of CO2-rock wettability and therefore reduce project risk. This work thus aids in the implementation of large-scale CO2 sequestration.

Influence Mechanism of Surfactants on Wettability of Coal with Different Metamorphic Degrees Based on Infrared Spectrum Experiments.

Based on experiments, a numerical analysis is used to quantitatively explore the influence of coal and surfactant microstructures on wettability. First, based on an infrared spectrum experiment, the distribution of oxygen-containing functional groups, aromatic hydrocarbons, and aliphatic hydrocarbons of coal and surfactants was obtained. Second, the wettability relationship between coal and different surfactants was determined by optical titration, and the coal dust wettability and surfaces were optimized. The key factors of the active agent wetting ability affecting lignite wetting mainly depend on the carbonyl, ether, and carboxyl groups in the surfactant. The factors affecting non-stick coal and gas coal wetting mainly depend on the ether group and aromatic amine in the surfactant. The factors affecting fat coal wetting mainly depend on the ether group and hydroxyl group in the surfactant. Finally, the factors affecting coking coal and anthracite wetting mainly depend on the surfactant ether group, aliphatic amine, and aromatic amine. Then, combining the structural parameters with the coal wetting results, the quantitative mathematical relationship between coal dust wettability, the important influencing factors of the surfactant, and the wettability index was established. Finally, a perfect and reasonable wettability evaluation model between coal and the surfactants was established. The relative activity of methyl ether and aromatic ether is greater than that of methyl ether, and the influence on the lignite, coking coal, and anthracite wettability conforms to the model . The influence on the non-caking coal and fat coal wettability conforms to the model , while the influence on the gas coal wettability conforms to the model Z = A + B1X1 + B2X2. In general, this study provides scientific guidance for the compounding of high-efficiency and environmentally protective composite dust suppressors to realize clean mine production.

Alteration effect on petrophysical properties of reservoirs rocks by drilling fluid emulsifiers

Surfactants formulated into oil-based mud can adsorb to pore surfaces causing changes in rock wettability, which are widely believed to impair formation permeability when oil-based mud invades the near-wellbore region. In the present study, the effects of oil-based mud with Versawet and Versacoat surfactants, on the petrophysical properties and wettability of drilling cutting samples from sandstone reservoirs were investigated through relative permeability, wettability index, and adsorption measurements at standard conditions in a micromodel device. The X-ray analysis and flame photometer were used to characterize the drilling cuttings samples, and the functional groups of surfactants were identified by the infrared spectroscopy. Relative permeability was studied using the unsteady state method, and the wettability index was measured by the Amott method after the aging with drilling formulation for 24 to 720 h and then the adsorption behavior of surfactants using Langmuir isotherm. The results show that drilling cutting is mainly composed of quartz 71.9%. Surfactant in oil-based mud reduces permeability and causes a change in wettability to a more oil-wet state, and these effects become more pronounced as the contact period with the surfactant increases. The wettability of a reservoir rock was altered due to surfactant adsorption and the formation of a hydrophobic layer on the surface of drilling cutting samples. The adsorption and retention of polar part of the surfactants -NH2 for Versawet and -COOH for Versacoat on the silanol groups and the calcium atoms included in the structure of the reservoir rocks. They are the key interactions in the wettability alteration process.

Distinguishing the Effect of Rock Wettability from Residual Oil on Foam Generation and Propagation in Porous Media

One of the common challenges of applying foam for enhanced oil recovery is the foam instability in the presence of crude oil and nonwater-wet surfaces. In this experimental study, we systematically distinguish the effect of rock surface wettability from that of crude oil saturation on foam rheology under reservoir conditions. Neutral-wet Berea and reservoir sandstone cores are prepared by aging with crude oil, followed by the wettability index measurements. Transient foam generation and steady-state foam quality scans are conducted in neutral-wet cores, with/without water-flood residual oil. Nuclear magnetic resonance imaging is also utilized to measure the remaining oil saturation at the end of the foam-flood. It is shown that strong foam can be generated in a neutral-wet core with no residual oil because of the solubilization of the adsorbed crude oil components and the wettability alteration toward more water-wet conditions. However, in a neutral-wet core containing residual oil, foam generation is initially hindered. Foam generation occurs after injecting several pore volumes of surfactant solution and increasing the superficial velocity to overcome the minimum pressure gradient required for in situ foam generation. The findings from this study suggest that surface wettability in the presence of bulk oil saturation significantly affects transient foam generation. The final steady-state foam strength becomes comparable to the water-wet and oil-free case once the residual oil saturation is adequately reduced.

Wettability of organic-rich source rocks: case study on Bazhenov Formation (Abalak-Bazhenov group)

Abstract. In this work, we have investigated the wettability of Bazhenov Formation rock samples using a nuclear magnetic resonance (NMR), the methods of vapor adsorption, and wetting contact angle. We have conducted the petrographic description of rocks using ultra-thin sections and scanning electron microscopy (SEM). In addition, we used data on the organic content (TOC) obtained by the Rock-Eval method and the results of lithological typing on thin sections for detailed analysis of NMR and contact angle methods results. Thus, for target rock, the groups with a similar rock wettability were highlighted by the lithological description of thin sections. The calculation of the wetting angle provided an initial assessment of the surface wettability of the rock and made it possible to establish the relationship between the wetting angle and the TOC, which is relevant for rocks of the Abalak-Bazhenov group. The NMR method described the core plug wettability proved to be less sensitive to the rock lithotype and organic matter (OM) texture in the rock and, therefore, used for the integral characterization of core plugs. Correlations of calculated wetting angle and adsorption wettability index vs. TOC and OM texture illustrated the dependence of rock wettability behavior on both the lithological specifics and the OM properties.

Physicochemical, techno-functional, and fat melting properties of spray-dried camel and bovine milk powders.

In this study, three skimmed and one whole-fat spray-dried camel milk powders were produced and their characteristics were compared to those of bovine milk powders. The physicochemical analysis of the produced powders indicated that camel milk powders (whether skimmed or not) presented higher ash and whey protein contents as compared to those of bovine milk powders. Our results indicated that the investigated camel and bovine milk powders exhibited a high solubility index (>99%) with poor dispersibility and wettability indexes due to their small particles size (d50 ≤ 12µm) and their narrow size distribution (span ≤ 2). In addition, although camel and bovine milk powders presented the same total fat content, lower free fat content was measured for camel milk powders. Besides, the whey protein nitrogen index and the SDS-PAGE electrophoresis underlined that camel and bovine milk proteins remained intact after drying with low denaturation extent. It is worth noticed that camel milk proteins were less denaturized due to the absence of the heat-sensitive β-lactoglobulin in camel milk. Moreover, the low denaturation extent participated in the enhancing of the foaming capacity and stability of camel and bovine milk powders. Finally, the calorimetric analysis showed that higher fat melting temperatures were recorded in whole-fat camel milk powder and in their anhydrous form as compared to those of bovine milk. PRACTICAL APPLICATION: Camel milk powder is an emerging non-bovine dairy product. Understanding its rehydration ability and evaluating the impact of spray drying on its protein quality are promising approaches to obtain high-quality camel milk powder with high reconstitution ability. Findings of this study indicated that spray drying is a suitable technique to produce highly soluble camel milk powders with low denaturation extent. These results will benefit the research and development department of food industry (especially those producing camel milk powder) as well as the direct consumers.

New methods for qualitative and quantitative determination of wettability from relative permeability curves: Revisiting Craig’s rules of thumb and introducing Lak wettability index

Craig’s rules of thumb have been widely used for qualitative determination of wettability from relative permeability curves. According to the first rule, in water-wet rocks the water relative permeability at residual oil saturation (ROS) is generally less than 30%, whereas in oil-wet systems this is greater than 50%. The second rule considers a system as water-wet, if saturation at the crossover point of relative permeability curves is greater than water saturation of 50%, otherwise oil-wet. The third rule states a relationship between the interstitial water saturation (IWS) and wettability.In this study, Craig’s second rule is modified. A Reference Crossover Saturation (RCS) is defined as RCS=12+Swc-Sor2 (Swc and Sor stands for IWS and ROS, respectively). For water-wet rocks the crossover point of relative permeability curves lies to the right of RCS, whereas this point lies to the left of the RCS for oil-wet systems. Furthermore, a RCS definition is derived for characterization of wettability in gas–liquid systems. In addition, it is explained that Craig’s third rule should be discarded from the analysis. A new index, called Lak wettability index, is presented by combining Craig’s first rule and modified Craig’s second rule. Lak index, which varies from −1 to +1, describes wettability quantitatively from a single pair of relative permeability curves. Twenty water–oil relative permeability experiments were performed and an agreement was observed between the predictions made by Craig’s first rule, modified Craig’s second rule, and Lak index, whereas Craig’s second and third rules failed in characterizing wettability.

Effects of Plasma Treatment on the Bioactivity of Alkali-Treated Ceria-Stabilised Zirconia/Alumina Nanocomposite (NANOZR).

Zirconia ceramics such as ceria-stabilized zirconia/alumina nanocomposites (nano-ZR) are applied as implant materials due to their excellent mechanical properties. However, surface treatment is required to obtain sufficient biocompatibility. In the present study, we explored the material surface functionalization and assessed the initial adhesion of rat bone marrow mesenchymal stem cells, their osteogenic differentiation, and production of hard tissue, on plasma-treated alkali-modified nano-ZR. Superhydrophilicity was observed on the plasma-treated surface of alkali-treated nano-ZR along with hydroxide formation and reduced surface carbon. A decreased contact angle was also observed as nano-ZR attained an appropriate wettability index. Treated samples showed higher in vitro bovine serum albumin (BSA) adsorption, initial adhesion of bone marrow and endothelial vascular cells, high alkaline phosphatase activity, and increased expression of bone differentiation-related factors. Furthermore, the in vivo performance of treated nano-ZR was evaluated by implantation in the femur of male Sprague–Dawley rats. The results showed that the amount of bone formed after the plasma treatment of alkali-modified nano-ZR was higher than that of untreated nano-ZR. Thus, induction of superhydrophilicity in nano-ZR via atmospheric pressure plasma treatment affects bone marrow and vascular cell adhesion and promotes bone formation without altering other surface properties.

A predictive model for the wettability of chalk

Wettability is usually measured in special core analyses of limited plug samples according to typically costly and time-consuming procedures. For comparative purposes, wettability is considered an index. The two most frequently used wettability indices are the Amott–Harvey wettability index and the U.S. Bureau of Mines (USBM) index. The Amott–Harvey wettability index is linked to imbibition characteristics and the USBM index is associated with the area under capillary pressure curves. To provide a fast analytical method, a mathematical model for predicting the wettability of chalk is presented. The model is calibrated using experimental wettability data and subsequently applied to two wells in Danish chalk oil fields in the North Sea and to outcrop chalk samples. The model supplements traditional labor-intensive laboratory measurements and predicts water wettability variations with depth by modeling both depth and porosity dependencies; in addition, it provides estimates of the effects of the aging time and displacement temperature of chalk wettability measurements in the laboratory.

Foam mat drying of taro (Colocasia esculenta): The effect of ultrasonic pretreatment and drying techniques on the drying behavior, flow, and reconstitution properties of taro flour

AbstractIn this study, the effect of ultrasound pretreatment (UP) and drying methods (convective hot air [CHAD] and microwave oven [MOD]‐assisted foam mat drying [FMD]) on the drying time, behavior and rate, diffusion coefficient, moisture content, color, bulk and tapped densities, flow properties, and reconstitution behavior of taro (Colocasia esculenta) flour were assessed. For this purpose, the taro foams with and without ultrasonic pretreatment (35 kHz, 80°C for 25 min) were dried in a convective oven (70°C, 20% ventilation rate) and microwave oven (460 W). The UP resulted in a higher drying rate, diffusion coefficient, moisture content, Hausner ratio (HR), and wettability time, and a lower drying time compared to the nontreated samples. Combined CHAD and MOD‐assisted FMD improved the disadvantages of the CHAD method for example: long drying time (decreased around 63.33% for CHAD and 59.16% for UP + CHAD), low drying and diffusion rates, high moisture content, low brightness (L*) values, high Carr index (CI), HR, and times of wettability and solubility. Additionally, the CI and HR of the taro flours ranged between 4.29–13.96 and 1.04–1.16 which can also be classified as very good and low levels, respectively.Practical ApplicationsTaro flour is an important alternative flour source in baby food, cake, chips, noodle, and so forth due to its pasting, functional, and nutritional properties. In order to obtain taro flour, the taro corm (root) is washed, peeled, and sliced then generally blanched with distilled water, dried, and ground. The quality of taro flour depends on the preparation method such as the drying method and conditions. Foam mat drying (FMD) is an alternative technique and it has several advantages like low drying time and energy consumption, a high drying rate, increased drying surface area, porous structure, and the desired instant properties of the obtained powders. Ultrasound pretreatment (UP) has also some advantages such as increasing the mass transfer rate, reducing drying time, and energy consumption. In this study, the advantages of these techniques (FMD and UP) were combined and the effects on the drying behavior, time, effective moisture diffusivity, and the physical and powder properties were observed.

Characteristics of Shale Gas Reservoir in Jiyang Depression and its Significance in Drilling and Exploitation

Physical and geochemical characteristics of shale play conclusive role in confirming operation measures during drilling and stimulation. The properties of shale samples from Jiyang depression were investigated through X-ray diffraction, scanning electron microscope, adsorption isothermal, high pressure mercury intrusion, methylene blue trihydrate, pressure pulse decay, tests of specific water wettability and shale stability index. Correlations, geological and engineering significances of them were discussed. Results show that shale reservoir in Jiyang depression has exploitation value corroborated by good characteristic parameters: 2.86% TOC, 69.9% brittle mineral, 26.14% clay mineral, high permeability of 0.011 × 10-3μm2, large Langmuir volume (5.82 cm3/g) and Langmuir specific area (0.91 m2/g), effective porosity (3.77%) and thickness (130.66m). Langmuir specific area is the key control on methane adsorption and storage verified by its moderate positive relativity with Langmuir volumes rather than TOC. High illite content (69.29%) may lead to instability of borehole and velocity sensitivity damage. Microfractures provide channels for filtration, invasion and loss of drilling fluid. Large specific water wettability (4.36 × 10-7g/m2) and smaller shale stability index (19.99 mm) dispalyed that shale formation were unstable once contacting with fresh water. Countermeasures must be adopted during drilling and fracturing to reduce reservoir damage and complex downhole conditions.

Performance of environmental friendly water-based calcium carbonate nanofluid as enhanced recovery agent for sandstone oil reservoirs

Recent studies have demonstrated great potential of nanofluids as enhanced oil recovery (EOR) agent. Many mechanisms behind increasing oil recovery inducing alteration of surface wettability toward more water wet have been proposed when displacing oil by nanofluids. In this study, the effect of calcium carbonate nanofluid on wettability alteration of sandstone rock samples was studied. Nanoparticle was synthesized by chemical precipitation method and characterized by X-ray diffraction (XRD), fourier-transform infrared spectroscopy (FTIR), field emission scanning electron microscope (FE-SEM) and dynamic light scattering (DLS). The average particle size of nanoparticles was 55.4 nm. The rock samples were aged in oil to ensure desired oil-wet conditions. The effects of nanofluid concentration and temperature on wettability change were investigated and optimum concentration of calcium carbonate nanofluid was determined. Adsorption of nanoparticles on rock surface was characterized by FE-SEM. The wettability alteration was evaluated by measuring/calculating contact angle (static, advancing and receding), surface energy, liquid area fraction, and wettability index. 2-D oil-wet glass micromodel was used to compare the performance of optimum nanofluid concentration obtained from wettability tests against normal water flooding. Results showed alteration of wettability from oil-wet to water-wet for all nanofluid concentrations. More wettability alteration was observed at higher temperature (80 °C) than ambient conditions (25 °C). Optimum nanofluid concentration (0.025 wt%) was obtained that changed contact angle from 116.41° (non-treated surface) to 46.99° at 25 °C and 28.7° at 80 °C. Flooding oil-wet 2-D glass micromodel with optimum nanofluid concentration resulted in 20% additional ultimate recovery factor compares to normal water flooding.

Mechanism of the Wettability Impact on Surfactant Imbibition in Dodecane-Saturated Tight Sandstone

As a result of low porosity and low permeability in tight reservoirs, spontaneous imbibition plays an important role in oil recovery. While wettability has a crucial impact on the imbibition, the mechanism of the effect of wettability on imbibition is still unclear. In this study, we screened the surfactants with similar interfacial tension (IFT) at the same concentration to study the wettability properties, which will contribute to improve the oil recovery tight reservoir. Two betaine surfactants, lauramide propyl hydroxysulfonated betaine (LHSB) and cocamidopropyl betaine (CAB), were screened according to their IFT. Through spontaneous imbibition experiments, the recovery of LHSB (27.26%) was better than that of CAB (21.69%). The contact angle method and U.S. Bureau of Mines method were used to analyze the core wettability comprehensively. After LHSB treatment, the core wettability index turned to hydrophobic and the wettability index changed from −0.06 to −0.56. However, the wettability index did not change significantly after CAB treatment (−0.15). Further molecular simulations showed that, after the surface of the rock is modified by LHSB, the oil droplets spread gradually and the interface energy with the quartz was enhanced. In combination of the analysis of the ζ potential and adsorption measurement, LHSB is predicted to be adsorbed in a single-layer configuration, while CAB formed a double-layer configuration, which caused the difference in wettability. It can be speculated that, when the surfactant changes the pore surface of the tight sample toward hydrophobic, the oil-wet surface promotes the residual oil droplets to form a continuous phase and, at the same time, reduces flow resistance of the water phase, thereby increasing the spontaneous imbibition rate and the ultimate recovery. This study will help to understand the wettability impact on imbibition by the surfactant and contribute to the improvement of tight oil reservoir recovery.

Wettability Quantification in Mixed-Wet Rocks Using a New NMR-Based Method

Summary Nuclear magnetic resonance (NMR) measurements have been attractive options for wettability characterization of reservoir rocks as they are sensitive to the type of fluid in contact with the grain surface. Several NMR-based wettability indices are documented in previous publications. Most of these methods require extensive calibration or involve complex inversion algorithms, which makes them computationally expensive and complicates their applicability in mixed-wet multimodal rocks. In this paper, we introduce a new NMR-based wettability index and verify its reliability in pore-scale and core-scale domains using numerical simulations and experimental measurements, respectively. This new index requires calibration at fully water-saturated water-wet and fully hydrocarbon-saturated hydrocarbon-wet states and can be applied to mixed-wet rocks at any fluid saturation level. This new NMR-based wettability index is a function of the transverse magnetization (T2) of mixed-wet rocks, the bulk relaxivity and saturation of each fluid, and the T2 distributions for fully water-wet and hydrocarbon-wet samples of the same rock type. The reliability of the new index was first tested in the pore-scale domain. For this part, we selected several pore-scale microcomputed tomography (CT) images of carbonate and sandstone rocks. We used a previously developed finite volume simulator to model the T2 responses in these images at fully water-wet and fully hydrocarbon-wet wettability states. Then we generated synthetic partially saturated mixed-wet samples and simulated T2 responses in these synthetic images. We used the simulated T2 results for determining their NMR-based wettability index and verified its applicability in the pore-scale domain. Next, we tested the reliability of the new NMR-based wettability index in the core-scale domain using NMR measurements in four Texas Cream (TC) rock samples, obtained from the Edwards formation. We altered the wettability of the cores to be water-, hydrocarbon-, and mixed-wet by injecting brine, a naphthenic acid in decane solution, or an anionic surfactant solution. We quantified the wettability of these samples using the Amott-Harvey (AH) index and contact angle measurements. Next, we measured the T2 distribution of these samples at different fluid saturation levels. Finally, we quantified the wettability values of these core samples using the new NMR-based index and compared them to those obtained from the AH index and contact angle measurements. We documented successful verification of the proposed method on samples with wettability ranging from −0.90 to 0.98 and from −0.6 to 0.5 (independently quantified using the AH method) in the pore- and core-scale domains, respectively. Results demonstrated that the new NMR-based wettability index reliably estimates the wettability of mixed-wet rocks in a wide range of wettability states. The new wettability index can potentially improve the speed and reliability of NMR-based wettability characterization and is promising for log-scale wettability assessment in mixed-wet rocks.

Simultaneous assessment of wettability and water saturation using 2D NMR measurements

Nuclear Magnetic Resonance (NMR) measurements can be used for estimating porosity, fluid saturation, pore-size distribution, and sometimes wettability. However, quantitative interpretation of rock wettability can be challenging in rocks with complex pore structures in the presence of different fluid types. Conventional NMR-based wettability models are applicable only for rocks with unimodal pore-size distributions. In this paper, we develop a new workflow for simultaneous assessment of fluid saturation and wettability in partially-saturated mixed-wet rock samples using 2D NMR measurements. We verify the reliability of this new workflow in three rock types, including unimodal and multi-modal pore-size distributions.The main inputs to the introduced workflow include the transverse relaxivity (T2) and Diffusivity-T2 (D-T2) or longitudinal-T2 (T1-T2) NMR measurements of the partially-saturated mixed-wet rock samples. Other input parameters include the bulk relaxivities of the saturating fluids and the NMR responses of fully water-saturated water-wet and fully oil-saturated oil-wet samples of the same rock type as the partially-saturated mixed-wet rock. In the verification step, we vary the fluid saturation by injecting brine, decane, or diesel into the rock sample using a core-flood setup. Then, we obtain NMR measurements at different fluid saturations to estimate the water/hydrocarbon saturation and wettability. Results demonstrated that the NMR-based wettability index has an average absolute difference of 0.18 when compared to the Amott Index, and the NMR-based water saturation estimates are within 18% average relative error of the gravimetric-estimates for the brine and diesel saturated rock samples.