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Nonionic surfactants are commonly used during well stimulation for several reasons. They reduce interfacial tension between the acid and oil phases, thus improving acid/rock interaction. They are also used to form a stable foam which improves the sweep efficiency during acidizing. However, these surfactants should be employed at temperatures below their cloud point (defined as the temperature at which the surfactant solution becomes cloudy). This temperature signifies the onset of the surfactant salting out, which will reduce the efficiency of the stimulation and may damage the formation. An experimental study was conducted to assess the effect of various acids and stimulation additives on the cloud point of nonionic surfactants. The influences of acids (inorganic and organic), mutual solvents, friction reducers, hydrogen sulfide scavengers, sequestering agents, short chain alcohols, simple salts, scale inhibitors, anionic surfactants on the cloud point of several nonionic surfactants were examined over a wide range of parameters. The results indicated that the cloud point monotonically increased with the acid concentration. However, the rate of increase depended on the acid type and the number of ethylene oxide groups of the surfactant. Salts depressed the cloud point of nonionic surfactants at all hydrochloric acid concentrations examined. Alcohols, methanol and isopropanol enhanced the cloud point of nonionic surfactants. The effect of mutual solvents was found to be a function of the number of ethylene oxide groups of the surfactant, acid and mutual solvent concentrations. Anionic surfactants depressed the cloud point of nonionic surfactants at all sodium chloride concentrations examined. Clay stabilizers (cationic polymers) and hydrogen sulfide scavengers depressed the cloud point whereas scale inhibitor and phosphonic acid did not affect the cloud point significantly. It is extremely important to measure the cloud point of nonionic surfactants before performing a stimulation job. It is also recommended to use the acid formulation and mixing waters that will be used in the field. Introduction Well stimulation is a process aimed at the removal of near-wellbore impairment due to deposition of particulate solids during drilling, workover or production operations.1,2 In this process an acid or mixture of acids is injected into the well to dissolve and remove this damage. Apart from removing deposited material, the acid can react with the rock matrix and enlarge pore sizes.3 This will allow insoluble fines to be flushed out when oil production is resumed (producing well) or during backflow (injection well). Enlarging pore sizes will improve the permeability in the wellbore area, hence the productivity or injectivity of the well will improve.

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