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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.

Poly(dimethyl siloxane) (PDMS) was bulk-modified to develop a new intra-cochlear electrode that can closely hug the inner wall of scala tympani (ST). The hydrophilicity of bulk and surface of PDMS was changed using a sequential method for preparation of interpenetrating polymer networks (IPNs). A series of IPNs, based on PDMS and poly(acrylic acid) (PAAc), was synthesized and characterized by means of attenuated total reflectance Fourier transform infrared spectroscopy, water contact-angle measurement, dynamic mechanical thermal analysis and peel strength tests. The performances of actual-sized fabricated electrodes were assessed inside a transparent model of ST, which was filled with saline. The cell behavior of L929 fibroblasts on materials was studied in vitro.

Foaming of nitrogen stabilized by C14-16 alpha olefin sulfonate in natural sandstone porous media, previously subject to water flooding, was studied experimentally. Foam was generated in-situ by co-injecting gas and surfactant solution at fixed foam quality. Effect of surfactant concentration on the foam strength and foam propagation was examined. X-ray CT scans were obtained to visualize the foam displacement process and to determine fluid saturations at different times. The experiments revealed that stable foam could be obtained in the presence of water-flood residual oil. CT scan images, fluid saturation profiles and mobility reduction factors demonstrated that foam exhibited a good mobility control in the presence of water-flood residual oil. This was further confirmed by a delay in the gas breakthrough. The experiments also proved that immiscible foam displaced additional oil from water-flooded sandstone cores, supporting the idea that foam is potentially an effective EOR method. Foam flooding provided an incremental oil recovery ranging from 13±0.5% of the oil initially in place for 0.1 wt% foam to 29±2% for 1.0 wt% foam. Incremental oil due to foam flow was obtained first by a formation of an oil bank and then by a long tail production due to transport of dispersed oil within the flowing foam. The oil bank size increased with surfactant concentration, but the dispersed oil regime was less sensitive to the surfactant concentration.

The flow of nitrogen foam in Bentheimer sandstone cores previously saturated with a surfactant solution has been investigated experimentally. The displacement process was visualized with the aid of a computed tomography (CT) scanner. CT data were analyzed to obtain water saturation profiles at different times. Pressure drops measured over core segments were recorded to determine foam mobility. It was found that foam undergoes a sharp transition from a weak to a strong state at a critical gas saturation of S gc = 0.75 ± 0.02. This effect was interpreted successfully by the rise of foam yield stress as gas saturation exceeds the S gc. It is suggested that confined jamming is the most likely mechanism responsible for the mobility transition.

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