As one promising CO_(2)capture,utilization and storage(CCUS)technology,miscible CO_(2)-enhanced oil recovery(CO_(2)-EOR)significantly outperforms immiscible flooding in enhancing oil production and storing CO_(2).Howe...As one promising CO_(2)capture,utilization and storage(CCUS)technology,miscible CO_(2)-enhanced oil recovery(CO_(2)-EOR)significantly outperforms immiscible flooding in enhancing oil production and storing CO_(2).However,achieving CO_(2)miscible flooding is often hindered by the high minimum miscibility pressure of CO_(2)-oil system in many reservoirs.To address this issue,this study focuses on the mechanisms for enhancing CO_(2)-oil miscibility using different types of surfactants and their blends with ethanol.The effects of fatty alcohol polyoxyethylene ethers(EO),fatty alcohol polyoxypropylene ethers(PO),tributyl citrate(TC),and glyceryl triacetate(GT)on the CO_(2)-oil miscibility pressure are quantitatively analyzed,as well as their synergy with ethanol.Results demonstrated that all tested surfactant additives reduce the CO_(2)-oil miscibility pressu re.For ethe r-based surfactant additives,an increase in the degree of polymerization(CO_(2)-philic groups)weakens the effectiveness to reduce miscibility pressure.Oxygen atoms in the functional group contribute more significantly to miscibility enhancement than carbon atoms.Among ester surfactants,GT achieved the best reduction effect of miscibility pressure(11.82%at 3.0 wt%),attributed to its symmetrical short side-chain structure and ester groups.Furthermore,ethanol exhibited a significant improvement for surfactants in enhancing miscibility.Notably,the reduction of CO_(2)-oil miscibility pressure increases to 27.9%by 3.0 wt%GT blended with 5.0 wt%ethanol.These findings demonstrate that blending surfactants with ethanol is a feasible and effective strategy to facilitate miscible CO_(2)flooding.This study provides valuable insights and practical guidance for the field implementation of miscible CO_(2)-EOR.展开更多
Weak water-drive offshore reservoirs with complex pore architecture and strong permeability heterogeneity present major challenges,including rapid depletion of formation energy,low waterflood efficiency,and significan...Weak water-drive offshore reservoirs with complex pore architecture and strong permeability heterogeneity present major challenges,including rapid depletion of formation energy,low waterflood efficiency,and significant lateral and vertical variability in crude oil properties,all of which contribute to limited recovery.To support more effective field development,alternative strategies and a deeper understanding of pore-scale flow behavior are urgently needed.In this work,CT imaging and digital image processing were used to construct a digital rock model representative of the target reservoir.A pore-scale flow model was then developed,and the Volume of Fluid(VOF)method was applied to simulate and optimize waterflooding schemes aimed at boosting oil recovery.Optimization focused on adjusting injection rates,varying the oil–water viscosity ratio,and implementing a water-alternating-gas(WAG)process.Results show that,for equal injection volumes,higher injection rates cause early water breakthrough through high-permeability pathways,yielding slower gains in recovery.Lowering the oil–water viscosity ratio improves mobility control,suppresses viscous fingering,enlarges sweep volume,and enhances recovery.When CH_(4)becomes fully miscible,it dissolves into the crude oil,lowering viscosity and eliminating interfacial tension,thereby providing greater displacement efficiency than partially miscible injection.Following a switch from water to gas injection,residual oil saturation decreases and becomes more uniformly distributed,indicating that the combined action of water and gas significantly improves both sweep efficiency and microscopic displacement.展开更多
The miscibility and crystallization of solution casting biodegradable poly（3-hydroxybuty- rate）/poly（ethylene succinate） （PHB/PES） blends was investigated by differential scanning calorimetry, rheology, and opti...The miscibility and crystallization of solution casting biodegradable poly（3-hydroxybuty- rate）/poly（ethylene succinate） （PHB/PES） blends was investigated by differential scanning calorimetry, rheology, and optical microscopy. The blends showed two glass transition temperatures and a depression of melting temperature of PHB with compositions in phase diagram, which indicated that the blend was partially miscible. The morphology observation supported this result. It was found that the PHB and PES can crystallize simultaneously or upon stepwise depending on the crystallization temperatures and compositions. The spherulite growth rate of PHB increased with increasing of PES content. The influence of compositions on the spherulitic growth rate for the partially miscible polymer blends was discussed.展开更多
In this paper, melt blends of poly（propylene carbonate） （PPC） with poly（butylene succinate） （PBS） were characterized by dynamic mechanical analysis （DMA）, differential scanning calorimetry （DSC）, tensile t...In this paper, melt blends of poly（propylene carbonate） （PPC） with poly（butylene succinate） （PBS） were characterized by dynamic mechanical analysis （DMA）, differential scanning calorimetry （DSC）, tensile testing, wide-angle X-ray diffraction （WAXD）, polarized optical microscopy and thermogravimetric analysis （TGA）. The results indicated that the glass transition temperature of PPC in the 90/10 PPC/PBS blend was decreased by about 11 K comparing with that of pure PPC. The presence of 10% PBS was partially miscible with PPC. The 90/10 PPC/PBS blend had better impact and tensile strength than those of the other PPC/PBS blends. The glass transition temperature of PPC in the 80/20, 70/30, and 60/40 PPC/PBS blends was improved by about 4.9 K, 4.2 K, and 13 K comparing with that of pure PPC, respectively; which indicated the immiscibility between PPC and PBS. The DSC results indicated that the crystallization of PBS became more difficult when the PPC content increased. The matrix of PPC hindered the crystallization process of PBS. While the content of PBS was above 20%, significant crystallization-induced phase separation was observed by polarized optical microscopy. It was found from the WAXD analysis that the crystal structure of PBS did not change, and the degree of crystallinity increased with increasing PBS content in the PPC/PBS blends.展开更多
An effective parameter in the miscible-CO_2 enhanced oil recovery procedure is the minimum miscibility pressure(MMP)defined as the lowest pressure that the oil in place and the injected gas into reservoir achieve misc...An effective parameter in the miscible-CO_2 enhanced oil recovery procedure is the minimum miscibility pressure(MMP)defined as the lowest pressure that the oil in place and the injected gas into reservoir achieve miscibility at a given temperature. Flue gases released from power plants can provide an available source of CO_2,which would otherwise be emitted to the atmosphere, for injection into a reservoir. However, the costs related to gas extraction from flue gases is potentially high. Hence, greater understanding the role of impurities in miscibility characteristics between CO_2 and reservoir fluids helps to establish which impurities are tolerable and which are not. In this study, we simulate the effects of the impurities nitrogen(N_2), methane(C_1), ethane(C_2) and propane(C_3) on CO_2 MMP. The simulation results reveal that,as an impurity, nitrogen increases CO_2–oil MMP more so than methane. On the other hand, increasing the propane(C_3)content can lead to a significant decrease in CO_2 MMP, whereas varying the concentrations of ethane(C_2) does not have a significant effect on the minimum miscibility pressure of reservoir crude oil and CO_2 gas. The novel relationships established are particularly valuable in circumstances where MMP experimental data are not available.展开更多
Liquid crystalline polymer-polyamide 66 (LCP/PA66) blends were compounded by using a Brabender mixing followed by compression moulding. The LCP employed was a semi-flexible liquid crystalline copolyesteramide based o...Liquid crystalline polymer-polyamide 66 (LCP/PA66) blends were compounded by using a Brabender mixing followed by compression moulding. The LCP employed was a semi-flexible liquid crystalline copolyesteramide based on 30% (molar fraction ) of p-amino benzoic acid (ABA ) and 70% (molar fraction) of poly (ethylene terephthalate) (PET). The LCP/PA66 blends were investigated in terms of the thermal and dynamic mechanical properties. It was found that PA66 and LCP components of the blends are miscible in the molten state, but are partially miscible in the solid state. The inclusion of the semi-flexible LCP into PA66 retards the crystallization rate of PA66. Furthermore, the melting temperature and the degree of crystallinity of PA66 are reduced considerably due to the LCP addition.展开更多
A model has been developed to describe the microstructure evolution in the atomized droplets of Cu-Fe alloy during cooling through the metastable miscibility gap.Calculations have been performed for Cu85Fe15 alloy to ...A model has been developed to describe the microstructure evolution in the atomized droplets of Cu-Fe alloy during cooling through the metastable miscibility gap.Calculations have been performed for Cu85Fe15 alloy to investigate the process of liquid-liquid phase transformation.The numerical results indicate that the minority phase droplets are nucleated in a temperature region around the peak of the supersaturation.The average radius of the Fe-rlch droplets decreases and the number density of the minority phase droplets increases with decreasing the atomized droplet size.The simulated results were compared with the experimental ones.The kinetic process of the liquid-liquid phase transformation was discussed in detail.展开更多
Positron annihilation spectroscopy(PAS)was utilized to investigate the relationship between the free-volume hole properties and miscibility of dynamically vulcanized EPDM/PP blend.The results showed that the noncrysta...Positron annihilation spectroscopy(PAS)was utilized to investigate the relationship between the free-volume hole properties and miscibility of dynamically vulcanized EPDM/PP blend.The results showed that the noncrystalline region of PP and EPDM in the blend was partially miscible and the miscibility of the blend became worse when the weight percent of EPDM was<50%.This was also demonstrated by DMTA and mechanical properties of the blends with various compositions.展开更多
Poly(hydroxybutyrate)/poly(vinyl alcohol)(PHB/PVA)blends plasticized with glycerol were prepared by melt blending of PHB and glycerol plasticized PVA.The PHB/PVA-glycerol compositions were 90:10,75:25 and 50:50 w/w,be...Poly(hydroxybutyrate)/poly(vinyl alcohol)(PHB/PVA)blends plasticized with glycerol were prepared by melt blending of PHB and glycerol plasticized PVA.The PHB/PVA-glycerol compositions were 90:10,75:25 and 50:50 w/w,being the concentration of glycerol in the PVA mixture of 10 wt%.The blends were characterized by infrared spectroscopy,dynamic-mechanical thermal analysis,and scanning electron microscope of the fragile fractured surface.The results showed one single phase blend,indicating miscibility corroborated by the presence of a single glass transition temperature.The blending method proved to be an efficient way to tune PHB properties keeping its biodegradable nature since both PVA and glycerol are fully biodegradable materials.展开更多
The influence of solvent on the miscibility of polystyrene（PS） and poly（styrene-co-acrylonitrile）（PSAN） blends has been investigated viscometrically. The miscibility of different PS/PSAN blend（30/70, 50/50 and ...The influence of solvent on the miscibility of polystyrene（PS） and poly（styrene-co-acrylonitrile）（PSAN） blends has been investigated viscometrically. The miscibility of different PS/PSAN blend（30/70, 50/50 and 70/30） compositions in acetone and benzene at 20, 30, and 40 °C was investigated on the basis of the sign of Chee（ΔB and ）, and Sun’s（α） criteria. The values of these parameters were evaluated from the analyses of reduced viscosity data of binary（solvent/polymer） and ternary（solvent/polymer1/polymer2） polymer systems. These investigations indicated partial miscibility for both the blend systems. However, PS/PSAN/acetone blend system showed somewhat higher partial miscibility than the PS/PSAN/Benzene blend system highlighting the impact of solvent over the polymer-polymer interactions and hence their miscibility. The results obtained through viscometry were also corroborated by the refractive index and density results for the blends under study. The effect of temperature on miscibility in both the cases was almost negligible.展开更多
The miscibility of flue gas and different types of light oils is investigated through slender-tube miscible displacement experiment at high temperature and high pressure.Under the conditions of high temperature and hi...The miscibility of flue gas and different types of light oils is investigated through slender-tube miscible displacement experiment at high temperature and high pressure.Under the conditions of high temperature and high pressure,the miscible displacement of flue gas and light oil is possible.At the same temperature,there is a linear relationship between oil displacement efficiency and pressure.At the same pressure,the oil displacement efficiency increases gently and then rapidly to more than 90% to achieve miscible displacement with the increase of temperature.The rapid increase of oil displacement efficiency is closely related to the process that the light components of oil transit in phase state due to distillation with the rise of temperature.Moreover,at the same pressure,the lighter the oil,the lower the minimum miscibility temperature between flue gas and oil,which allows easier miscibility and ultimately better performance of thermal miscible flooding by air injection.The miscibility between flue gas and light oil at high temperature and high pressure is more typically characterized by phase transition at high temperature in supercritical state,and it is different from the contact extraction miscibility of CO_(2) under conventional high pressure conditions.展开更多
In this paper the miscibility of poly (ε-caprolactone) (PCL) and aliphatic polycarbonate (APC) is studied by using DSC. The results show that PCL and APC are miscible in all ranges of composition. The interaction par...In this paper the miscibility of poly (ε-caprolactone) (PCL) and aliphatic polycarbonate (APC) is studied by using DSC. The results show that PCL and APC are miscible in all ranges of composition. The interaction parameter between the polymers is calculated from the melting point depression data. Using optical microscope, the shapes of the PCL spherulites in the blends are observed.展开更多
基金supported by the National Natural Science Foundation of China(52374063,52204065 and 52304098)the Shandong Provincial Natural Science Foundation(ZR2023ME049)+2 种基金the Fundamental Research Funds for the Central Universities(25CX07004A)State Key Laboratory of Intelligent Construction and Healthy Operation and Maintenance of Deep Underground Engineering(SDGZK2429)Shenzhen Science and Technology Program(JCYJ20220818095605012)。
文摘As one promising CO_(2)capture,utilization and storage(CCUS)technology,miscible CO_(2)-enhanced oil recovery(CO_(2)-EOR)significantly outperforms immiscible flooding in enhancing oil production and storing CO_(2).However,achieving CO_(2)miscible flooding is often hindered by the high minimum miscibility pressure of CO_(2)-oil system in many reservoirs.To address this issue,this study focuses on the mechanisms for enhancing CO_(2)-oil miscibility using different types of surfactants and their blends with ethanol.The effects of fatty alcohol polyoxyethylene ethers(EO),fatty alcohol polyoxypropylene ethers(PO),tributyl citrate(TC),and glyceryl triacetate(GT)on the CO_(2)-oil miscibility pressure are quantitatively analyzed,as well as their synergy with ethanol.Results demonstrated that all tested surfactant additives reduce the CO_(2)-oil miscibility pressu re.For ethe r-based surfactant additives,an increase in the degree of polymerization(CO_(2)-philic groups)weakens the effectiveness to reduce miscibility pressure.Oxygen atoms in the functional group contribute more significantly to miscibility enhancement than carbon atoms.Among ester surfactants,GT achieved the best reduction effect of miscibility pressure(11.82%at 3.0 wt%),attributed to its symmetrical short side-chain structure and ester groups.Furthermore,ethanol exhibited a significant improvement for surfactants in enhancing miscibility.Notably,the reduction of CO_(2)-oil miscibility pressure increases to 27.9%by 3.0 wt%GT blended with 5.0 wt%ethanol.These findings demonstrate that blending surfactants with ethanol is a feasible and effective strategy to facilitate miscible CO_(2)flooding.This study provides valuable insights and practical guidance for the field implementation of miscible CO_(2)-EOR.
基金funded by the Research Project of CNOOC(China)Co.,Ltd.Shanghai Branch,grant number 202417716474Research Project of CNOOC Limited,grant number KJZX-2024-0102.
文摘Weak water-drive offshore reservoirs with complex pore architecture and strong permeability heterogeneity present major challenges,including rapid depletion of formation energy,low waterflood efficiency,and significant lateral and vertical variability in crude oil properties,all of which contribute to limited recovery.To support more effective field development,alternative strategies and a deeper understanding of pore-scale flow behavior are urgently needed.In this work,CT imaging and digital image processing were used to construct a digital rock model representative of the target reservoir.A pore-scale flow model was then developed,and the Volume of Fluid(VOF)method was applied to simulate and optimize waterflooding schemes aimed at boosting oil recovery.Optimization focused on adjusting injection rates,varying the oil–water viscosity ratio,and implementing a water-alternating-gas(WAG)process.Results show that,for equal injection volumes,higher injection rates cause early water breakthrough through high-permeability pathways,yielding slower gains in recovery.Lowering the oil–water viscosity ratio improves mobility control,suppresses viscous fingering,enlarges sweep volume,and enhances recovery.When CH_(4)becomes fully miscible,it dissolves into the crude oil,lowering viscosity and eliminating interfacial tension,thereby providing greater displacement efficiency than partially miscible injection.Following a switch from water to gas injection,residual oil saturation decreases and becomes more uniformly distributed,indicating that the combined action of water and gas significantly improves both sweep efficiency and microscopic displacement.
基金ACKNOWLEDGMENT This work was supported by the Key Science Foundation of Education Ministry of China and the Anhui Science Foundation.
文摘The miscibility and crystallization of solution casting biodegradable poly（3-hydroxybuty- rate）/poly（ethylene succinate） （PHB/PES） blends was investigated by differential scanning calorimetry, rheology, and optical microscopy. The blends showed two glass transition temperatures and a depression of melting temperature of PHB with compositions in phase diagram, which indicated that the blend was partially miscible. The morphology observation supported this result. It was found that the PHB and PES can crystallize simultaneously or upon stepwise depending on the crystallization temperatures and compositions. The spherulite growth rate of PHB increased with increasing of PES content. The influence of compositions on the spherulitic growth rate for the partially miscible polymer blends was discussed.
基金This work was supported by the National Natural Science Foundation of China(Nos.270274049 and 220374051).
文摘In this paper, melt blends of poly（propylene carbonate） （PPC） with poly（butylene succinate） （PBS） were characterized by dynamic mechanical analysis （DMA）, differential scanning calorimetry （DSC）, tensile testing, wide-angle X-ray diffraction （WAXD）, polarized optical microscopy and thermogravimetric analysis （TGA）. The results indicated that the glass transition temperature of PPC in the 90/10 PPC/PBS blend was decreased by about 11 K comparing with that of pure PPC. The presence of 10% PBS was partially miscible with PPC. The 90/10 PPC/PBS blend had better impact and tensile strength than those of the other PPC/PBS blends. The glass transition temperature of PPC in the 80/20, 70/30, and 60/40 PPC/PBS blends was improved by about 4.9 K, 4.2 K, and 13 K comparing with that of pure PPC, respectively; which indicated the immiscibility between PPC and PBS. The DSC results indicated that the crystallization of PBS became more difficult when the PPC content increased. The matrix of PPC hindered the crystallization process of PBS. While the content of PBS was above 20%, significant crystallization-induced phase separation was observed by polarized optical microscopy. It was found from the WAXD analysis that the crystal structure of PBS did not change, and the degree of crystallinity increased with increasing PBS content in the PPC/PBS blends.
文摘An effective parameter in the miscible-CO_2 enhanced oil recovery procedure is the minimum miscibility pressure(MMP)defined as the lowest pressure that the oil in place and the injected gas into reservoir achieve miscibility at a given temperature. Flue gases released from power plants can provide an available source of CO_2,which would otherwise be emitted to the atmosphere, for injection into a reservoir. However, the costs related to gas extraction from flue gases is potentially high. Hence, greater understanding the role of impurities in miscibility characteristics between CO_2 and reservoir fluids helps to establish which impurities are tolerable and which are not. In this study, we simulate the effects of the impurities nitrogen(N_2), methane(C_1), ethane(C_2) and propane(C_3) on CO_2 MMP. The simulation results reveal that,as an impurity, nitrogen increases CO_2–oil MMP more so than methane. On the other hand, increasing the propane(C_3)content can lead to a significant decrease in CO_2 MMP, whereas varying the concentrations of ethane(C_2) does not have a significant effect on the minimum miscibility pressure of reservoir crude oil and CO_2 gas. The novel relationships established are particularly valuable in circumstances where MMP experimental data are not available.
基金Supported by the National Natural Science Foundation of China.
文摘Liquid crystalline polymer-polyamide 66 (LCP/PA66) blends were compounded by using a Brabender mixing followed by compression moulding. The LCP employed was a semi-flexible liquid crystalline copolyesteramide based on 30% (molar fraction ) of p-amino benzoic acid (ABA ) and 70% (molar fraction) of poly (ethylene terephthalate) (PET). The LCP/PA66 blends were investigated in terms of the thermal and dynamic mechanical properties. It was found that PA66 and LCP components of the blends are miscible in the molten state, but are partially miscible in the solid state. The inclusion of the semi-flexible LCP into PA66 retards the crystallization rate of PA66. Furthermore, the melting temperature and the degree of crystallinity of PA66 are reduced considerably due to the LCP addition.
基金supports from the National Natural Science Foundation of China(Grant Nos.50271076,50371092,50395104)the Sino-Germany Science Foundation(GZ032/1)the Natural Science Foundation of Liaoning Province of China.
文摘A model has been developed to describe the microstructure evolution in the atomized droplets of Cu-Fe alloy during cooling through the metastable miscibility gap.Calculations have been performed for Cu85Fe15 alloy to investigate the process of liquid-liquid phase transformation.The numerical results indicate that the minority phase droplets are nucleated in a temperature region around the peak of the supersaturation.The average radius of the Fe-rlch droplets decreases and the number density of the minority phase droplets increases with decreasing the atomized droplet size.The simulated results were compared with the experimental ones.The kinetic process of the liquid-liquid phase transformation was discussed in detail.
基金This work was financially supported by 863 Programme of China No.863-715-012-0160
文摘Positron annihilation spectroscopy(PAS)was utilized to investigate the relationship between the free-volume hole properties and miscibility of dynamically vulcanized EPDM/PP blend.The results showed that the noncrystalline region of PP and EPDM in the blend was partially miscible and the miscibility of the blend became worse when the weight percent of EPDM was<50%.This was also demonstrated by DMTA and mechanical properties of the blends with various compositions.
文摘Poly(hydroxybutyrate)/poly(vinyl alcohol)(PHB/PVA)blends plasticized with glycerol were prepared by melt blending of PHB and glycerol plasticized PVA.The PHB/PVA-glycerol compositions were 90:10,75:25 and 50:50 w/w,being the concentration of glycerol in the PVA mixture of 10 wt%.The blends were characterized by infrared spectroscopy,dynamic-mechanical thermal analysis,and scanning electron microscope of the fragile fractured surface.The results showed one single phase blend,indicating miscibility corroborated by the presence of a single glass transition temperature.The blending method proved to be an efficient way to tune PHB properties keeping its biodegradable nature since both PVA and glycerol are fully biodegradable materials.
基金supported by the Higher Education Commission,Islamabad,Pakistan in the form of an Indigenous PhD Fellowship Phase-IV to Zafarullah Khan Marwat.Zafarullah Khan Marwat is also grateful to the Higher Education Department(Colleges)Peshawar Khyber Pakhtunkhwa for grant of study leave for PhD Studies
文摘The influence of solvent on the miscibility of polystyrene（PS） and poly（styrene-co-acrylonitrile）（PSAN） blends has been investigated viscometrically. The miscibility of different PS/PSAN blend（30/70, 50/50 and 70/30） compositions in acetone and benzene at 20, 30, and 40 °C was investigated on the basis of the sign of Chee（ΔB and ）, and Sun’s（α） criteria. The values of these parameters were evaluated from the analyses of reduced viscosity data of binary（solvent/polymer） and ternary（solvent/polymer1/polymer2） polymer systems. These investigations indicated partial miscibility for both the blend systems. However, PS/PSAN/acetone blend system showed somewhat higher partial miscibility than the PS/PSAN/Benzene blend system highlighting the impact of solvent over the polymer-polymer interactions and hence their miscibility. The results obtained through viscometry were also corroborated by the refractive index and density results for the blends under study. The effect of temperature on miscibility in both the cases was almost negligible.
基金Supported by the PetroChina Science and Technology Project(2023ZG18).
文摘The miscibility of flue gas and different types of light oils is investigated through slender-tube miscible displacement experiment at high temperature and high pressure.Under the conditions of high temperature and high pressure,the miscible displacement of flue gas and light oil is possible.At the same temperature,there is a linear relationship between oil displacement efficiency and pressure.At the same pressure,the oil displacement efficiency increases gently and then rapidly to more than 90% to achieve miscible displacement with the increase of temperature.The rapid increase of oil displacement efficiency is closely related to the process that the light components of oil transit in phase state due to distillation with the rise of temperature.Moreover,at the same pressure,the lighter the oil,the lower the minimum miscibility temperature between flue gas and oil,which allows easier miscibility and ultimately better performance of thermal miscible flooding by air injection.The miscibility between flue gas and light oil at high temperature and high pressure is more typically characterized by phase transition at high temperature in supercritical state,and it is different from the contact extraction miscibility of CO_(2) under conventional high pressure conditions.
基金The subject supported by National Natural Science Foundation of China
文摘In this paper the miscibility of poly (ε-caprolactone) (PCL) and aliphatic polycarbonate (APC) is studied by using DSC. The results show that PCL and APC are miscible in all ranges of composition. The interaction parameter between the polymers is calculated from the melting point depression data. Using optical microscope, the shapes of the PCL spherulites in the blends are observed.
