CHEMICAL FLOODING OF OIL-RESERVOIRS 8 - SPONTANEOUS OIL EXPULSION FROM OIL-WET AND WATER-WET LOW PERMEABLE CHALK MATERIAL BY IMBIBITION OF AQUEOUS SURFACTANT SOLUTIONS

Spontaneous imbibition experiments in nearly oil-wet, low permeable ch alk material saturated with oil are performed at ambient conditions wi th and without the cationic surfactant dodecyltrimethylammonium bromid e present in the aqueous solution. Without surfactant present in the w ater, the rate of imbibition is, as expected, very small, and only app roximate to 13% of the oil was expelled from the core within 90 days. After that time, a sudden increase in the oil production was observed by exchanging the water with a 1.0 wt% surfactant solution. If the sur factant is present from the beginning, an oil production plateau of ap proximate to 65% recovery was obtained within 90 days. The imbibition mechanism at low interfacial tension is discussed in terms of the phas e behavior of the oil-brine-surfactant system, and the ability of the surfactant to enhance the water wettability. Imbibition experiments ar e also performed in order to study the potential of using surfactants to improve the oil recovery from a low permeable, 1-2 mo, water-wet ch alk reservoir. The experiments were conducted under reservoir conditio ns, i.e. at 130 degrees C and 270 bar using reservoir crude oil and co re material from an actual oil reservoir. Single reservoir core plugs of 4-5 cm in length were combined into a long composite core of 50-55 cm and placed vertically. Mixtures of alkyltrimethylammonium bromides were dissolved in brine and used as surfactant solutions. The experime nts simulated floods conducted under tertiary and secondary flood cond itions. Negligible extra oil was produced by using surfactants after a water flood, tertiary flood condition. Centrifuge experiments at ambi ent conditions documented the difficulties in remobilizing discontinuo us oil after a water flood in a water-wet material. If the surfactants are present from the start in a secondary flood, a crossover from a c ountercurrent flow governed by capillary and surface tension gradient forces to a cocurrent flow based on gravity forces appears to be the f low mechanism. (C) 1998 Elsevier Science B.V.