A laboratory feasibility study of dilute surfactant injection for the Yibal field, Oman

Abstract

The Yibal field, the largest oil field in Oman, comprises 15% of the oil production of the country. The field has had a high ultimate recovery factor and in order to maintain the current recovery trend, different management strategies have been sought. One of the options is the injection of a dilute surfactant in addition to the current waterflooding. The cores from the chalky Shuaiba formation were saturated with brine and oilflooded to restore the initial reservoir condition after cleaning. Nineteen samples were waterflooded followed by dilute surfactant injection. Eight samples were flooded with dilute surfactant. The reason for this scheme is that some parts of the reservoir under study were totally watered out while others are still untouched. In addition to these experiments, nine capillary (static) imbibition experiments were conducted to treat fractured zones where recovery by capillary imbibition during injection is a possibility. Twelve different surfactants (at different concentrations) were tested. Five surfactants were non-ionic, two cationic, four anionic, and one was a mixture of anionic and non-ionic. The selection of the optimum concentrations was based on IFT values at different concentrations. The results were evaluated in terms of the final oil recovery. The average waterflooding recovery was found to be 75.1% of OOIP (out of 19 experiments) whereas surfactant injection yielded an average of 69.9% of OOIP (out of 8 experiments). This indicates that the surfactant injection is not preferable and not recommended over waterflooding for the untouched portion of the reservoir where the rock matrix dominates the flow (unfractured portions). An additional recovery by surfactant solution injection succeeding waterflooding was obtained and found to vary between 0% and 7.4% of OOIP. The surfactant injection is, therefore, recommendable in the pre-waterflooded unfractured zones as long as the proper surfactant type is selected. Half of the surfactant solutions yielded higher and faster capillary imbibition recovery than brine. For the untouched fractured zones of the chalky reservoir, it is more effective to start the injection with surfactant addition rather than waterflooding alone. Surfactant types and concentrations yielding the best performances were identified and listed in this paper.