Diederik Michiel Boersma

Visualization of ASP Coreflood Experiments Using X-ray CT Imaging

Description: This paper describes a series of experiments that used X-ray tomography to visualize the mobilization of remaining oil by Alkaline Surfactant Polymer (ASP) flooding after conventional waterflooding. The experiments were conducted in cores drilled from Bentheim sandstone outcrop material with diameters of approximately 7.55 cm and lengths of 14.9, 27.5 and 100 cm. The crude used in the experiments has an in-situ viscosity of about 100 cP and contains petroleum acids that are converted to soaps in the presence of alkali. Application: In addition to pressure and effluent data collected during conventional coreflood experiments phase and saturation distributions in space and time are needed to more completely interpret the results of core floods. This additional information reveals underlying mechanisms, and assists the development of models that capture the physics of ASP that can ultimately be used to provide field scale predictions for ASP performance. Results, Observations, and Conclusions: • A significant oil bank was created, propagated, and produced in each of the experiments. • The size of the oil bank in terms of length and maximum oil saturation is largely dependent on the post-waterflood saturation distribution along the cores. • Large variations in both the absolute value and the spatial distribution of the post-waterflood saturation distribution were observed in the different experiments. • A characteristic self-similar cross-sectional averaged oil saturation profile develops for floods conducted in the longer length cores. • Detailed analyses of individual frames revealed separate “oil” and “ASP fingers” in the area downstream of the oil bank. Significance: The displacement mechanisms involved in ASP flooding relatively viscous crudes are revealed by X-ray CT visualization. The resulting improved understanding of the process increases the confidence in the potential full-field application of ASP flooding.

Ammonia as Alkali for ASP Floods – Comparison to Sodium Carbonate

Ammonia is logistically preferred over sodium carbonate for alkaline-surfactant-polymer enhanced oil recovery projects (ASP) due to its low molar mass and the possibility for it to be delivered as a liquid. On an offshore platform space and weight savings can be the determining factor in deciding whether an ASP project is feasible. Logistics may also be critical in determining the economic feasibility of projects in remote locations. Ammonia as alkali together with a surfactant blend of alkylpropoxy sulfate – internal olefin sulfonate (APS/IOS) functions as an effective alkali. Surfactant adsorption is low and oil recovery in core floods is high. Static adsorption tests show that low surfactant adsorption is attained at pH > 9, a condition that ammonia satisfies at low solution concentration. It is expected that ammonia has a performance deficiency relative to sodium carbonate in that it does not precipitate calcium from solution. Calcium accumulation in the ammonia ASP solution will occur due to ion exchange from clays. The high oil recovery for ammonia and the calcium accumulation in ASP and SP core floods with APS-IOS blends shows that this surfactant system is effective and calcium-tolerant. Also, phase behavior and IFT measurements suggest that APS/IOS blends remain effective in the presence of calcium. EO/PO sulfates (such as the employed APS) are known commercially available, calcium-tolerant surfactants. However, due to hydrolysis sulfate-type surfactants are suitable for use only in lower temperature reservoirs. Very different behavior was noticed for phase behavior measurements with calcium intolerant surfactants such as alkyl benzene sulfonates (ABS) and internal olefin sulfonates (IOS). In this case calcium addition results in a very high IFT and complete separation of oil and brine. Presumably this will result in low oil recovery. A preferred approach for ASP offshore with divalent ion intolerant surfactants may be the use of a hybrid alkali system combining the attributes of sodium carbonate and ammonia. The concept is to supply the bulk of the alkalinity for an ASP flood by ammonia with all the inherent logistical advantages. A minor quantity of sodium carbonate is added to the formulation to specifically precipitate calcium ions.