A systematic CT-scan-aided laboratory study of N2 foam in Bentheimer sandstone cores is reported. The aim of the study was to investigate whether foam can improve oil recovery from clastic reservoirs subject to immiscible gas flooding. Foam was generated in situ in water-flooded sandstone cores by coinjecting gas and surfactant solution at fixed foam quality. It was stabilized using two surfactants, namely, C14–16 α-olefin sulfonate (AOS) and mixtures of AOS and a polymeric fluorocarbon (FC) ester. The effects of surfactant concentration, injection direction, surfactant preflush, and core length on foam behavior were examined in detail. Stable foams were obtained in the presence of waterflood residual oil. It was found that foam strength (mobility reduction factor) increases with surfactant concentration. Foam development and, correspondingly, oil recovery without surfactant preflush were delayed compared to the case with preflush. Gravity-stable foam injection caused a rapid increase in foam strength and an incremental oil recovery almost twice that for unstable flow conditions. Core floods revealed that the incremental oil recovery by foam was as much as (23 ± 2)% of the oil initially in place after injection of 4.0 pore volumes (PV) of foam (equal to the injection of 0.36 PV of surfactant solution). Incremental oil recovery was only (5.0 ± 0.5)% for gas flooding under the same injection conditions. It appears that oil production by foam flooding occurs by the following main mechanisms: (1) residual oil saturation to foam flooding is lower than that to water flooding; (2) formation of an oil bank in the first few injected pore volumes, coinciding with a large increase of capillary number; and (3) a long tail production due to the transport of tiny oil droplets within the flowing foam at a fairly constant capillary number. The observations of this study support the concept that foam is potentially an efficient enhanced oil recovery (EOR) method.

Summary A detailed laboratory study of nitrogen-foam propagation in natural sandstones in the absence of oil is reported. The goal of this study was to elucidate further the mechanisms of foam mobility control. The C14–16 alpha-olefin sulfonate (AOS) surfactant was selected to stabilize foam. X-ray computed-tomography (CT) images were taken during foam propagation to map liquid saturation over time. Effects of surfactant concentration and of total injection velocity were examined in detail because these are key parameters for controlling foam strength and foam propagation under field conditions. The experiments revealed that foam mobility decreases in two steps: During initial forward foam propagation, foam mobility decreases by an order of magnitude compared with water mobility; during a secondary backward liquid desaturation, it decreases further by one to two orders of magnitude for sufficiently high surfactant concentrations. The steady-state mobility-reduction factor (MRF) increases considerably with both surfactant concentration and total injection velocity. A hysteresis was observed for a cycle of increasing/decreasing surfactant concentration or total injection velocity. The observed effects could be interpreted mechanistically in terms of surfactant adsorption and foam rheology. Implications for field application of foam for immiscible and miscible gas enhanced oil recovery (EOR) are discussed.

Studies on Mxene/alginate composite adsorption have opened up a new avenue for designing adsorbents possessing high adsorption capacity and high efficiency.

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This paper describes the design of a multi-gigabit fiber-optic receiver with integrated large-area photo detectors for plastic optical fiber applications. An integrated 250 m diameter non-SML NW/P-sub photo detector is adopted to allow efficient light coupling. The theory of applying a fully-differential pre-amplifier with a single-ended photo current is also examined and a super-Gm transimpedance amplifier has been proposed to drive a of 14 pF to multi-gigahertz frequency. Both differential and common-mode operations of the proposed super-Gm transimpedance amplifier have been analyzed and a differential noise analysis is performed. A digitally-controlled linear equalizer is proposed to produce a slow-rising-slope frequency response to compensate for the photo detector up to 3 GHz. The proposed POF receiver consists of an illuminated signal photo detector, a shielded dummy photo detector, a super-Gm transimpedance amplifier, a variable-gain amplifier, a linear equalizer, a post amplifier, and an output driver. A test chip is fabricated in TSMC's 65 nm low-power CMOS process, and it consumes 50 mW of DC power (excluding the output driver) from a single 1.2 V supply. A bit-error rate of less than 10 has been measured at a data rate of 3.125 Gbps with a 670 nm VCSEL-based electro-optical transmitter.

Organic nonvolatile memory with ultralow power consumption is a critical research demand for next-generation memory applications. However, obtaining a large-area, highly oriented ferroelectric ultrathin film with low leakage current and stable ferroelectric switching remains a challenge for achieving low operation voltage in ferroelectric memory transistors. Here, an ideal ferroelectric neat PVDF ultrathin film with a high degree of orientation is fabricated by a melt-draw technique without post-thermal treatment and assisted stabilization process. The PVDF ultrathin film is self-polarized with predominantly vertical orientation of dipole moments, exhibiting a d 33 of 25 pm V–1 and the ultralow coercive voltage of approximately 3 V characterized by piezoresponse force microscopy. A remnant polarization of 6.3 μC cm–2 is identified based on a PVDF capacitor with an active layer formed by six layers of melt-drawn thin films. By employing a single-layer melt-drawn PVDF ultrathin film as an insulation layer, a bottom-gate-top-contact ferroelectric field-effect transistor is fabricated with a very low operation voltage of 5 V. It exhibits a memory window with an on/off current ratio of 103 at zero gate bias and threshold voltage shift of around 2 V.

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