No abstract

The application of CO2 foam has caught overwhelming attention for fracturing shales. In applications, high foam deterioration and insufficient viscosity at operating conditions are the major concerns associated with foam fracturing process. In this study, polymer-free CO2 foam possessing high stability has been presented through chemical screening and optimization under HPHT conditions. Initial screening was performed by conducting a series of foam stability experiments considering different commercial anionic surfactants, concentration, and foam stabilizer addition using FoamScan instrument. Foam rheology study was then performed by considering the similar investigated factors under fracturing conditions using HTHP foam rheometer. All the tested solutions were prepared in fixed brine salinity and HPAM polymers with different molecular weights were used in evaluation of the performance of the designed polymer-free foam in term of foam strength. In comparison with other types of surfactant, alpha olefin sulfonate (AOS) exhibited the best foam stability and viscosity at testing conditions. The optimum AOS concentration providing the best performance was found to be 5000 ppm and its combination with 5000 ppm of foam booster (betaine) further increased AOS foam longevity. An improved result on foam stability and viscosity was not obtained by increasing surfactant concentration. Results on foam rheology reveals that CO2 foam generated in the presence of different molecular weight classical HPAM polymers could not provide significant increment in foam viscosity under experimental conditions. It was observed that these types of polymer underwent degradation due to some unfavorable mechanisms which will be expected to negatively affect its performance during fracturing process. On the other hand, polymer-free CO2 foam was found to produce a higher stability and relatively equally high viscosity compared to polymer-stabilied CO2 foam without experiencing degradation at high pressure and temperature conditions. Therefore, based on this study, it is recommended to use polymer-free foam for fracturing shales application. The use of formulated polymer-free CO2 foam which has high stability and viscosity will lead to improved fracture cleanup, minimized formation damage and pore plugging, and efficient proppant placement which will ultimately enhance gas recovery from unconventional shales.

Polymer enhanced foam (PEF) provides an additional strength over conventional CO2 foams for mobilizing oil from the unswept low permeable oil rich zones during an enhanced oil recovery process. The efficiency of the process depends on two major factors i.e. stability and apparent viscosity of PEF. In this study, an experimental investigation of apparent viscosity and stability of polymer enhanced CO2 foam is presented with an objective to assess the polymer performance and to identify the best performing polymer under reservoir conditions of 1500 psi and 80 °C. For this purpose, conventional standard hydrolyzed polymacrylamide (HPAM) polymers and an associative polymer i.e. Superpusher P329 were used in combination with a widely used foamer i.e. alpha olefin sulfonate (AOS) and a foam stabilizer i.e. betaine. Foam stability tests were conducted in the presence of crude oil using FoamScan. Whereas for foam rheological study, a high pressure high temperature Foam Rheometer was utilized and the foam was sheared over the range of 10 to 500 sec−1 inside the recirculating loop. As compared to other HPAMs, an associative polymer i.e. Superpusher P329 significantly amplified foam longevity and provided a more prolonged liquid drainage. A shear thinning behavior was observed for the entire range of shear rate tested and for all the tested foam. HPAMs were found ineffective in improving foam apparent viscosity and the viscosities obtained were found equivalent to that to polymer free foam. Superpusher P329 showed interesting combination with AOS and significant viscosity enhancement has been reported in this paper. This research concluded that Superpusher P329 has the ability to generate strong foam and it is a potiential candidate for mobility control during polymer enhanced CO2 foam flooding process. Keywords: Polymer Enhanced Foam, foam stability, apparent viscosity; CO2 foam.

Foam Assisted Water-Alternating-Gas (FAWAG) flooding is one of the enhanced oil recovery (EOR) technique that had been explored and studied worldwide. The ability of foam to lower the gravity override and create a macroscopic sweeping of oil in the reservoir shows its potential to be a successful technique. However, the rapid degradation of foam at high temperature condition in the presence of light crude oil limits its application. Therefore, introduction of an additive to the surfactant is crucial to maintain foam stability. Nano-scale particle is a well-known material that has attracted a lot of attention due to its unique physiochemical properties. This high surface energy particle has shown to exhibit a catalytic behaviour in various application including EOR chemical flooding. In this study, the effect of four different types of nanoparticles, SiO2 (hydrophilic), SiO2 (hydrophobic), ZnO and TiO2 nanoparticles on foam stability under high temperature condition, and in the presence of light crude oil were investigated. Results from this study has shown that SiO2 nanoparticles of the hydrophilic type at concentration three times lower than the surfactant concentration have significantly improved the foam half-life by 2 times longer than the surfactant alone at temperature of 110°C, in the presence of light crude oil (45° API). No improvement of foam half-life was shown by ZnO nanoparticle used in the surfactant formulation. The presence of SiO2 (hydrophilic) nanoparticles have significantly reduced the detrimental effects of light crude oil and strengthen the foam by increasing the viscosity of surfactant from 4.38 cP to 10.01 cP in the presence of 0.15 wt% SiO2 (philic) nanoparticle. The significant increment in viscosity has maintained the wetness of foam, thus reducing the rate of liquid drainage at the temperature above boiling point of water. The SiO2 (hydrophilic) nanoparticle-surfactant formulation was observed to have produced uniform sized bubbles compared to surfactant formulation alone. This indicates that nanoparticles are able to restrict shrinkage or expansion of bubble by creating a steric layer at the lamella structure which consequently restores the foam stability. The foam stability tests, determined as foam-half-life, were performed using inert nitrogen gas as the gas phase to eliminate other factors that may affect foam stability.

Without regulation pertaining to the use and discharge of surfactant for offshore enhanced oil recovery (EOR) process in Malaysia, we adopted the guidelines from OSPAR (Oslo Paris Convention) that governs the use and discharge of offshore chemicals in the North Sea Region. In OSPAR, the CHARM (Chemical Hazard Assessment and Risk Management) model is being used to assess the risk of offshore chemicals to the marine environment. CHARM prescribes the Predicted Environment Concentration:Predicted No-Effect Concentration (PEC:PNEC) approach which ratio determines the hazard quotient (HQ) in order to rank the chemical by colour banding. Our surfactant formulation achieved a HQ of 2.16 or Silver colour banding with the stipulation that the volume of the discharged produced water is twice the volume of chemical solution (squeeze) injected. Nevertheless, in providing more certainty and confidence for both operators and local regulators to allow for overboard discharge of our flow-back surfactant formulation, we conducted a comprehensive produced water dilution modelling called DREAM (Dose-related Risk and Effect Assessment Model). The model calculates the Environmental Impact Factor (EIF) of each component of the chemical in the discharged produced water. Similar to CHARM, the DREAM uses the PEC:PNEC approach, but its PEC input parameters include environmental influences such as weather profile, current, etc. and incorporates a slick model. Its output is a quantation of the risks to the receiving environment, called the Environmental Impact Factor (EIF); where EIF is more than 1, the impact to the environment is significant. We simulated the chemical fate of individual component of the formulation with the scenario whereby the produced water is not treated prior to discharge. The time-averaged EIFs were more than 1 across all weather windows, indicating the discharge of untreated chemical-containing produced water is likely to have a localized environmental impact. We used both CHARM and DREAM as decision support tools for effective management of operational discharges from offshore projects. Limitations and recommendations from DREAM simulation results in the context of our EOR application are discussed.