The production and properties of two families of anionic surfactants (internal olefin sulfonates and branched C16, 17 alcohol-based alkoxy sulfonates) are described for chemical flooding of oil reservoirs at high temperatures and/or high salinities. Surfactant properties measured include oil/water micro-emulsion phase behaviour obtained using new glassware-based procedures appropriate for higher reservoir temperatures. The results obtained relate to oil/water interfacial tension behaviour and give the "operating window" of the surfactants in terms of their optimal salinity and ability to solubilise oil in the micro-emulsion. The phase tests also give information on the quality of the micro-emulsions obtained where low viscosity and absence of gels is desirable. The surfactants described are promising for EOR and can be produced in commercial quantities. Different IOS products are available with different carbon chain cuts (with range C15 to C28) allowing matching of the IOS to the temperature, salinity and crude oil type of reservoirs. In addition, both IO carbon chain (degree of branching) and the degree of sulfonation influence the surfactant properties of the IOS mixture formed which provides a means for tailoring an IOS surfactant for optimal performance. 1. Introduction In chemically enhanced oil recovery (EOR) the mobilisation of residual oil saturation is achieved through surfactants that generate a sufficiently (ultra) low crude oil/water interfacial tension (IFT) to give a capillary number large enough to overcome capillary forces and allow the oil to flow1. However, reservoirs have different characteristics (crude oil type, temperature and water composition), and the structures of added surfactant(s) have to be tailored to these conditions to achieve an ultra low IFT. In addition, a promising surfactant must satisfy other important criteria including low rock retention, compatibility with the polymer to be used, compatibility with hard water (if present), thermal and hydrolytic stability, acceptable cost/performance balance and commercial availability in sufficient quantities. Because of the well-established relationship between the micro-emulsion phase behaviour and IFT2 it is common in the industry to screen surfactants and their formulations for low IFT through laboratory-based oil / water phase behaviour tests3, 4. This approach works well for tests carried out at room temperature and slightly higher, but at higher temperatures there may be safety issues. Specifically, conventional sealed glass tube test methods may be problematic from a laboratory safety standpoint at higher temperatures due to the vapour pressures from water and crude oil. This paper presents the results of the evaluation of surfactants using improved phase behaviour experimental methods for higher temperatures (up to 150°C). Optimal salinities and solubility parameters have been measured. Two surfactant families have been evaluated, both produced by Shell Chemicals: Internal olefin sulfonates (IOS) which are part of the ENORDET™ O series and proprietary, branched C16, 17 alcohol-based anionic surfactants which are part of the ENORDET™ A series. Both families are suitable for EOR because they have a reduced tendency to form ordered structures/liquid crystals that are undesirable in reservoirs4, IOS products because they are a complex mixture of surfactants of differing chain lengths and the branched C16, 17 alcohol based surfactants because of their randomly branched structures.
The development of structure – property relationships are described for new commercial grade internal olefin sulfonates (marketed as the ENORDET™ O series) and laboratory scale alcohol-alkoxy-sulfate surfactants for use in chemical flooding. Surfactant structure was characterised by an in-house developed liquid chromatography mass spectrometry (LC-MS) technique and properties focused on oil/water microemulsion phase behaviour. Such relationships are important to match the surfactant formulation to particular reservoir conditions (temperature, salinity and crude oil). The relationship between IOS structure (by LC-MS) and optimal salinity (by phase tests) has been modeled by the empirical HLB number and by a semi-empirical molecular model. An IOS 24-28 based surfactant system gave excellent microemulsion performance with several, regionally different crude oils and an initial correlation of performance with the composition crude oils has been made. The IOS surfactants described have been produced on a pilot scale and with consistent quality. This commercially available family, and the commercially available alcohol-alkoxy-sulfate family, cover most of the salinity and temperature reservoir conditions expected, though for high temperature and high salinity reservoirs, alcohol based sulfonates will most likely be required. Finally, the chemistry of production of the IOS surfactants and their handling properties are summarised. Part 2 of this paper (SPE-129769-PP) describes work to formulate an IOS mixture that was subsequently used in a successful ASP field test.
Accurate laboratory screening of surfactants for their ability to give ultra-low interfacial tensions in oil/brine systems is important as a pre-cursor to laboratory core flow tests and surfactant flooding processes in the field. Screening is usually judged by visualisation of middle-phase micro-emulsions in oil/brine systems. Three laboratory methods are described which enable the phase behaviour of oil/water systems containing surfactants to be more safely visualised and measured in glassware at higher temperatures. Higher temperature test conditions result in significant vapour pressures from crude oil and water, and some glass tube test methods currently used in the industry may not be appropriate from a laboratory safety standpoint. The new methods have been verified in our laboratories for higher temperature use and provide useful screening methods for higher temperature reservoirs (up to 150°C).
A load-pulsing technique has been used to determine the velocity of transgranular stress-corrosion cracks in Admiralty Metal tested in a 15N aqueous ammoniacal solution. In this technique, small load pulses are periodically superimposed onto an otherwise constant tensile load during crack propagation, producing markings on the fracture surfaces which delineate the positions of the crack front. The spacing between crack-front markings (Δx) was measured for values of the time interval between pulses (Δt) in the range 2 to 500 s. A one-to-one correspondence was observed between pulses and markings in this range, so that the crack velocity was given by Δx/δt. The velocity was found to be constant over much of the crack length for each value of Δt, indicating that an extensive Stage II region exists in this system. The Stage II velocity was constant: 1.9 × 10−7 m s−1 for values of Δt greater than 100 s; this value of velocity is approximately five times larger than those obtained by conventional methods, and this difference is attributed to the influence of grain boundaries in the latter case. The Stage II velocity increased with decreasing Δt for Δt < 100 s, and this is attributed to fatigue effects. It is concluded that the load-pulsing method can provide a convenient and reliable technique for the determination of Stage II velocities for transgranular stress-corrosion cracking.
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