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Abstract As early as 1954 a study into the efficiency of nonionic detergents to displace crude oil from sandstone reservoirs was published by Dunning. In this study different exethylated alkylphenols were investigated and it was concluded that this calss of surfactants was among the most promising, provided the solubility in the injected waters at reservoir temperatures is sufficient. Since then hardly any investigation into these chemicals were carried out. Practically all work on chemical flooding, both in Practically all work on chemical flooding, both in the laboratory and in the field has been focussed on petroleum sulfonates. Why this investigation into modified nonionics? The main reason is the excellent stability against electrolytes, a feature petroleum sulfonates do not have. petroleum sulfonates do not have. By combining the favourable properties of both the nonionics and anionics the so called modified nonionics were introduced for chemical flooding. The products show an excellent stability against electrolytes, especially divalent ions, and no temperature dependence of their solubility. Illustrated by a practical example: sandstone reservoir, average salinity and medium temperature, the development of a flooding recipe based on modified nonionics will be discussed. Attention will be focussed on the laboratory investigation and the optimization of the flooding recipe. It has been shown that a 50% pv slug containing about 0.5% surfactant will give about 35% incremental oil. Introduction As early as 1954 a study into the efficiency of nonionic surfactants to displace crude oil from sandstone reservoirs was published by Dunning. In this study different oxyethylated alkylphenols were investigated and it was concluded that this class of surfactants was among the most promosing, providing the solubility in the injected water under providing the solubility in the injected water under surface and reservoir conditions is sufficient. For a maximum displacement efficiency the mole ratio of ethylene oxide varied between 4 and 12, dependent on crude oil, reservoir water and reservoir conditions. This result was confirmed by Wade et al, who applying the EACN concept found minimum interfacial tension between crude oil and solutions of oxyethylated alkylphenols and alcohols in reservoir water at a mole ratio between 3.5 and 12. Gogarty gives an extensive overview of the research in chemical flooding over the last years. Several types of chemicals are discussed but attention is focussed on petroleum sulfonates. Petroleum sulfonates have been tested since about Petroleum sulfonates have been tested since about 1960 in field trials, demonstration projects etc. showing their potential to increase the recovery factor. Petroleum sulfonates are limited in their application. With increasing salinity the reservoir water, especially the divalent (Ca++) ions the solubility decreases. When a critical value in salinity and divalent ions is reached, this value is roughly 1% salinity, 100 ppm Ca++, petroleum sulfonates precipitate leading to possible plugging (Table precipitate leading to possible plugging (Table 1). Several methods have been proposed and tried out to overcome this problem, f.i. preflushing of a highly saline reservoir with sweet water, addition of other chemicals to the petroleum sulfonate solution, tailoring the petroleum sulfonate with respect to its equivalent weight. The salinity gradient concept is an illustration of this. Nonionic surfactants are reaction products of an alkylene oxide f.i. ethylene oxide, and molecules containing an acidic H-atom like alcohols or alkylphenols. Nonionic surfactants contain hydrophobic and hydrophilic parts, the ratio is expressed as the Hydrophilic-Lilophilic Balance. When a nonionic surfactant dissolved in water is heated phase separation, observed as a haze, will occur phase separation, observed as a haze, will occur at a certain temperature. This temperature is called the cloudpoint. Below the cloudpoint the ethylene oxide groups of the molecule are hydrated and hence the molecule is soluble in water. Above the cloudpoint the water molecules are split off and phase separation occurs. Addition of electrolytes to the non-ionic surfactant solution decreases the cloudpoint. P. 87 |
