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 dilute surfactant in addition to the current waterflooding scheme.
The cores from the chalky Shuaiba formation were saturated with brine and oilflooded to restore the initial reservoir condition after cleaning process. Nineteen samples were exposed to waterflooding succeeded by dilute surfactant injection. Eight samples were directly exposed to dilute surfactant injection. The reason behind this scheme is the fact that some parts of the reservoir under study were totally watered out, due to field waterflooding, and some parts are still untouched. In addition to these experiments, nine capillary (static) imbibition experiments were also carried out. This is with the intension of covering fractured zones, where recovery by capillary imbibition during injection is a possibility. The investigation considered testing of twelve different surfactants available in the market at different concentrations. Five of these surfactants were nonionic, two cationic, four anionic, and one mixture of anionic and non-ionic. The selection of the optimum concentration values for each surfactant was based on the measured 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 (modified waterflooding) 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, however, was obtained and found to be varying 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, therefore, more effective and recommendable 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.
Introduction
Different schemes of surfactant injection received a great deal of attention in late 1970s and early 1980s mainly due to the financial affordability of the field applications. Considerable amount of work was published during this period. Early studies were performed on sandstone reservoirs1–8. More recently, laboratory and field scale applications were reported for chalks9–15. The economics of the surfactant injection is critical due to the high cost of the chemicals. Therefore, optimum injection scheme has to be designed16–21. Adsorption is an important aspect of the process that plays a role in the optimization and cost analysis of the projects22–30. This literature provided not only the previous experience on surfactant flooding but was useful to gain an insight into the surfactant types used for different rock types and injection schemes applied.
Based on the recent experience on chalky carbonates, the surfactant injection was thought to be an alternative for the Yibal field to maintain the high recovery trend. The Yibal field, tight and fractured carbonate reservoir, containing light oil, has been producing with a high recovery factor. Considering the fact that the field also has a high STOIIP, the possibility of EOR application(s) needs to be evaluated. This is a crucial issue to extend the field life but a challenging application of effective reservoir management.
