The hydrophobically modified polymer with N-hexadecylacrylamide (C16AM) as hydrophobic group (HMP-C16AM) is the most widely used polymeric emulsifier in oil fields. Given its amphiphilic property, ...aqueous HMP-C16AM solution can form a stable emulsion with the oil phase under the action of shear force. In this study, we investigated the emulsifying property of HMP-C16AM for heavy oil from multiple angles. First, the properties of the heavy oil emulsion using HMP-C16AM were studied under different conditions. Second, a glass-etched visual micromodel was further applied to study the emulsification of heavy oil in the pore throat during the HMP-C16AM flooding. Results showed that the increased stability of the heavy oil emulsion resulted in the small average particle size of oil droplets and increased the viscosity and the viscoelasticity. The combined system of low-salinity water and HMP-C16AM improved the stability of the heavy oil emulsion. As shown by the microscopic oil displacement test, the heavy oil was emulsified in the pore throat, and the heavy oil recovery was enhanced remarkably in the outlet and edge regions of the micromodel after the HMP-C16AM flooding. The study provides theoretical guidance for the next field experiment.
Thermothickening polymers are a novel type of material developed for enhanced oil recovery applications in high-temperature and high-salinity oil reservoirs. However, the existing synthesis process ...of thermoviscosifying polymers is complex and of high cost, both of which limit the wide application of thermoviscosifying polymers. In this study, a thermothickening water-soluble polymer (PHAD) with a low concentration was synthesized by a “graft from” method, with acrylamide and diacetone acrylamide as grafted monomers and hydroxypropyl methyl cellulose as the backbone chain. The basic parameters for PHAD copolymers were systematically studied in comparison with their homopolyacrylamide counterpart. The results show that the PHAD copolymers exhibited excellent thermothickening ability even when the polymer concentration was 0.2 wt % (total salinity is 9350.08 mg·L–1) upon increasing the temperature from 25 to 90 °C, where the apparent viscosity enhancement changes from 4.0 to 13.3 times with increasing the diacetone acrylamide content in PHAD copolymers. The PHAD copolymers also showed good salt tolerance, thermal stability, and viscoelastic properties under harsh reservoir conditions, which are attributed to the synergistic effect of the rigid heterocyclic ring structure and hydrophobic intermolecular association of thermoresponsive monomers within polymer chains. Moreover, the core displacement experiment and etched glass microscopic model show that the PHAD copolymers have good migration in porous media. Due to its high sweep efficiency, the PHAD copolymer has a higher recovery factor (14.0%) than polyacrylamide (4.3%), which makes it more suitable for salt-tolerant and temperature-tolerant tertiary oil recovery chemicals.
Kinetic hydrate inhibitors (KHIs) are polymers used in a chemical method to prevent gas hydrate plugging of oil and gas production flow lines. The main ingredient in a KHI formulation is one or more ...water-soluble amphiphilic polymers. Several classes of KHI polymers contain pendant heterocyclic 5-rings including poly(N-vinylpyrrolidone) (PVP) and poly(2-isopropenyl-2-oxazoline) (PiPOx). Here, we present a KHI performance study on polymers based on the 5-ring vinylic monomer 5-methyl-3-vinyl-2-oxazolidinone (VMOX), which has only recently been manufactured in large quantities. Low molecular weight PVMOX homopolymers were produced in quantitative yield using radical polymerization, with or without a chain transfer agent. For example, PVMOX-2.4k (Mn = 2400 g/mol) had a cloud point at 2500 ppm of 73 °C in deionized water. The polymers were screened for KHI performance using slow constant cooling tests (1.0 °C/h) in high-pressure rocking cells with a synthetic natural gas blend. At 2500 ppm, PVMOX-2.4k gave a better performance than PVP or PiPOx at a similar molecular weight but not as good as poly(N-vinylcaprolactam) (PVCap). Isobutyl glycol ether was shown to enhance the KHI performance of PVMOX. PVMOX gave improved performance with increasing concentration but not as steep of an improvement as some of the best amide-based KHI polymers. A 1:1 copolymer of VMOX with N-vinylcaprolactam gave improved performance compared to the PVMOX homopolymer.
During pulsating hydraulic fracturing (PHF), reservoir rock can be subjected to constant amplitude, constant mean stress (CACMS) cyclic loading or constant amplitude, increasing mean stress (CAIMS) ...cyclic loading. The influence of increasing mean stress on rock fatigue strength, fatigue lifetime, fatigue damage, and energy evolution of shale is rarely investigated, and which type of cyclic loading is more efficient for PHF has not been determined and demonstrated. In this Article, a series of uniaxial compression tests under these two types of cyclic loading are first conducted. A fatigue lifetime model for CAIMS is established. The shale strength and fatigue lifetime for CACMS and CAIMS are then compared. Their differences are explained by their dissipated energy density evolution and damage evolution. Finally, a nonlinear damage accumulation model to predict damage evolution for CAIMS cyclic loading is proposed. It is suggested that CAIMS is a better cyclic loading type when its amplitude is higher than 30% UCS (uniaxial compressive strength) of rock. This provides a meaningful amplitude threshold for CAIMS parameter optimization in the PHF construction. In this case, compared with CACMS cyclic loading, CAIMS cyclic loading significantly decreases the shale strength by up to 20% UCS and the fatigue lifetime from over 500 to 9. Different from an inverted-S-shaped damage evolution for CACMS cyclic loading, damage evolution for CAIMS cyclic loading exhibits a monotonic increasing trend. The damage variable growth rates of CACMS and CAIMS both show a three-stage trend: (1) their growth rates both decrease; (2) the growth rate for CACMS remains stable, while for CAIMS its growth rate slowly increases; and (3) their growth rates both increase sharply. Correspondingly, the dissipated energy density evolutions for CACMS and CAIMS show a similar three-stage trend. It can be concluded that during the second stage, for CAIMS microfractures and plastic deformation inside specimens are developed and accumulated more and faster than those for CACMS cyclic loading. This explains the phenomenon that the fatigue lifetime and strength of CAIMS cyclic loading are less than those of CACMS. The nonlinear damage accumulation model proposed in this Article can well fit experimental results. This model can be used for accurately describing reservoir rock mechanical property degradation during hydraulic fracture simulation induced by PHF.
Following previous evidence that hydrothermal treatment of bitumen froth does not lead to bitumen upgrading at 250 °C and promotes viscosity increase, the current study explores the free radical and ...cationic reactivity of clay minerals found in bitumen froth in promoting heavier material formation through addition reactions. The current investigation employed α-methylstyrene (AMS) and 1-octene as probe molecules instead of bitumen froth, and their conversion at 250 °C in the presence of clay minerals kaolinite and illite was studied in batch reactors. Thermal conversion of AMS and 1-octene at 250 °C in the absence of minerals was observed to be low. In the presence of clay minerals, not only the conversion of AMS and 1-octene was increased but also reactions such as dimerization of AMS and alkylation of 1-octene and toluene (used as solvents) were mainly promoted, leading to heavier product formation. Double-bond isomerization of 1-octene and cumene formation from AMS was side reactions that were also promoted by the clay minerals. Suppression of mineral-related conversion by pyridine and selectivity to different reaction products that enabled differentiation between free radical and cationic reaction pathways indicated that the mineral-related conversion was predominantly cationic in nature. Using the reactions in the presence of minerals and pyridine as surrogates for alkaline bitumen froth, it was concluded that even under alkaline conditions, minerals could promote heavier material formation through cationic addition.
To study shale adsorption characteristics during the process of CO2 and N2 enhanced shale gas recovery, a high-pressure gas adsorption instrument was used to transfer different adsorbed gases (CH4, ...CO2, and N2) in the adsorption test on shale at different temperature conditions. The Langmuir–Freundlich (L–F) model considering the effect of pressure and temperature was introduced. Further, a modified model for calculating the isosteric enthalpy of adsorption (ΔH) was established. The results showed that the adsorption capacity of CO2 on the shale surface was the strongest and that of N2 was the weakest. The L–F model can better characterize the adsorption behavior of shale under the combination of temperature and pressure. The modified model for calculating ΔH could better characterize the relationship between the ΔH and adsorption amounts. As the adsorbed amount increased, the ΔH at different temperatures showed a trend of decreasing slowly and then decreasing rapidly; the ΔH of CO2 was the largest, and that of N2 was smallest. As the temperature increased, the ΔH revealed a downward trend. With an increase in CO2 pressure and in the mole fraction of CO2 free phase, the adsorption selectivity factor of CO2 over CH4 increased gradually, while the adsorption selectivity factor of N2 over CH4 decreased with the increase in N2 pressure and the decrease in mole fraction of N2 free phase. Temperature will inhibit the preferential adsorption of shale to CO2 and N2.
In this work, for the first time, a new swelling method was applied to study the chemical structure of kerogen. Structural changes during the swelling process were detected via X-ray diffraction ...(XRD). Three types of kerogen samples and three organic solvents were selected for this study. It was found that kerogens have a selective absorption of aromatic hydrocarbons compared with saturated hydrocarbons. Type I kerogen demonstrated the strongest ability to sorb hydrocarbons; it can sorb more than half of its own weight at normal temperature and pressure (25 °C and 1.013 × 105 Pa). However, under the same conditions, type III kerogen can sorb around 40% of its own weight. XRD detection revealed that the microcrystalline structure of kerogen was not affected by liquid organic matter during the swelling process. Three kinds of organic solvent are mainly sorbed in the γ band (amorphous carbon) comprising aliphatic carbon, and the γ band swells under the swelling action, causing the value of d γ to increase. This study explored the ability of kerogen to sorb hydrocarbon compounds and discovered the chemical structural units that occur in kerogen during solvent swelling. It is of great significance for the study of the hydrocarbon generation and expulsion in oil shale under geological conditions and the structure of kerogen.
In this study, a temporary plugging agent composed of two different types of guest molecules for thermally induced phase change fracturing was constructed for different shale formation temperature ...ranges. This agent, which was constructed through the screening of the host, guest, and solvent, is in a uniform solution state at room temperature. The solution became cloudy with increasing temperature, and a stable gel was formed when the respective gelling temperature was reached. However, the gel turned into a solution again upon further heating. The temporary plugging agent system developed in this paper could realize the phase transition of “solution–gel–solution” only by changing the temperature. Characterization and evaluation of the basic performance of the temporary plugging agent show that the components of this agent form a long fibrous gel through supramolecular interactions, the constructed thermally induced phase change fracturing system has good thermal stability, and the filter loss is small. These findings indicate that the temporary plugging agent system for thermally induced phase change fracturing could form an excellent temporary plugging agent suitable for formations at 90–120 °C through the unique temperature responsiveness of supramolecular gels. In addition, the gel formation process was described by the host–guest structure combined with SEM characterization. The results of physical simulation experiments reveal that the temporary plugging agent has low viscosity, good fluidity, and good injectability at room temperature. After being injected into the formation, the temporary plugging agent could form a gel with sufficient strength at the target formation temperature to plug the fractures. After the formation was further heated, the formed gel gradually broke and caused slight damage to the core. The whole temporary plugging process does not require the addition of an additional crosslinking agent and a gel breaker. This new type of temporary plugging agent has a potential application value in shale gas refracturing.
Kinetic hydrate inhibitors (KHIs), such as poly(N-vinylcaprolactam) (PVCap) and related copolymers, are a well-known method to help combat gas hydrate formation in oil and gas field production flow ...lines. The caprolactam groups in this polymer class have been shown previously to have a particularly strong interaction with hydrate surfaces, inhibiting crystal growth, but probably also gas hydrate nucleation. In an earlier study, we reported on the first alternate KHI polymer class with pendant caprolactam groups based on the 2-methacrylamido-caprolactam (2-MACap) monomer. This report builds on that study, by optimizing the best copolymers from that study and copolymerizing 2-MACap with other comonomers. KHI experiments were carried out in high-pressure steel rocking cells using a structure II-forming natural gas mixture. The KHI performance of some of these copolymers exceeded that of PVCap of similar molecular weight. In addition, the importance of the methyl group in 2-MACap for enhanced KHI performance was confirmed by making and testing polymers with 2-acrylamido-caprolactam, which has no methylated backbone. Further confirmation from 2-MACap copolymers with 1-acryloylpyrrolidine and N-methacryloylpyrrolidine, for which the latter copolymer performed best. Finally, it was shown that a series of well-known synergists for PVCap were able to give excellent KHI performance enhancement of the selected 2-MACap copolymers, although some molecules showed antagonistic effects. This could be due to unhelpful polymer–synergistic interactions or both molecules competing in the same KHI mechanistic processes.
Identifying the variation of fracture structure and coal permeability with effective stress is important for protecting coalbed methane (CBM) reservoirs and formulating a reasonable drainage system. ...This study characterizes the coal fracture and permeability variations with effective stress using three-dimensional (3-D) micro-CT and digital core techniques. Dual-resolution scanning is adopted to characterize the variation of three types of fractures with effective stress and to clarify the stress sensitivity of the porosity of each type of fracture. Type-A fractures (>109 μm3) are most sensitive to effective stress and tend to close as effective stress increases. Moreover, the porosity of this fracture has a negative exponential relationship with effective stress. Type-B fractures (107–109 μm3) are moderately sensitive to effective stress and have the highest contribution of total porosity at high effective stress. Type-C fractures (<107 μm3) are least sensitive to effective stress, and the porosity of this fracture changes slightly. The seepage simulation in the two directions with digital core techniques confirms that the permeability of coal samples presents an exponential decrease with effective stress and shows an anisotropic behavior with space. Combined with the quantitative analysis of variation of pore–fracture models, the compression model of coal under effective stress can be established, which can reveal the reason that the permeability loss rate in the y-axis is more than that in the z-axis. The sensitivity of porosity and permeability is stronger under low stress and weaker under high stress. Therefore, digital core technology can quantitatively analyze the stress sensitivity of coal fracture structure and permeability based on 3-D models. This method is more intuitive to study the stress sensitivity of coal compared with traditional methods.
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