Abstract

The present paper summarizes the results from a study on improved spontaneous imbibition of water into oil saturated, fractured, low permeable (2-3 mD) chalk material of the Ekofisk type by means of surfactants. The experiments were performed at room temperature, using long (55 cm) and short (5.0 cm) core samples of different wettabilities i. e., water-wet, mixed-wet, and nearly oil-wet. For the two former systems, an anionic surfactant of the type alkyl-propoxythoxy sulfate was used. A cationic surfactant, dodecyltrimethylammonium bromide, was used for the oil-wet condition. The objectives were to determine the imbibition mechanism and to evaluate the potential of using a surfactant solution compared to brine regarding oil recovery and oil production rate. The imbibition mechanism in the water- and mixed-wet systems at high IFT was found to be a countercurrent flow governed by capillary forces. At low IFT, the fluid flow appeared to be countercurrent at the start. Later, the oil expulsion was very slow and governed by gravity forces. Forces related to gradients in the interfacial tension appeared to be active in the countercurrent flow regime (Maragoni effects). Surprisingly, spontaneous oil expulsion from the nearly oil-wet core took place in the presence of the cationic surfactant by a countercurrent flow mechanism. The mechanism is discussed in terms of reversed micelle formation turning the chalk more water-wet just across the water/oil interface.

Introduction

Chalk oil reservoirs, like the Ekofisk field in the North Sea, often are highly fractured. The matrix blocks are characterized by a rather low permeability, in the range of 0.1–10 mD, and fairly high porosity, 30–40 %. The fractures, which play a very important role in the oil recovery process, are transport routs for both the oil displaced from the matrix blocks and the water injected into the reservoir to enhance the oil recovery. For a water-wet reservoir rock, significant improvements in the oil recovery can be obtained by water injection as observed for the Tor formation of the lower Ekofisk field. The Lower and Upper Ekofisk formation of the Ekofisk field are mixed-wet to nearly oil-wet. Laboratory experiments show that the nature of water imbibition is different for the water-wet Tor formation and the Ekofisk formation. The water saturation change, Sw, achieved after spontaneous imbibition is in the order of 50 % for the Tor formation, and about 20-25% for the Ekofisk formation. In general, there also appears to be a decrease in Sw when going from the Lower to the Upper Ekofisk formation. Water is now injected into the Lower Ekofisk formation with success. If, however, the area fraction of water-wet surface is low, the spontaneous imbibition of water ceases, and the oil can only be displaced by drainage of the matrix blocks, which of course is nearly impossible due to the very low permeability of the chalk, 0.1 to 10 mD, and the high permeability of the fractures. Early breakthrough of the injected fluid in the production well will probably occur.

In the present paper, we summarize the experimental results focusing on improving the imbibition of water into Ekofisk-like chalk material by adding surface active material. The mechanism of the imbibition process was studied by changing the IFT, size of the core material, and wettability The four questions posed are:

  1. Will gravity forces be active in oil expulsion at low IFT ?

  2. Is it possible to improve the oil recovery by adding a highly efficient surfactant to the injected water ?

  3. Is the expected decrease in the imbibition rate at low IFT too large for practical applications?

  4. Is it possible to imbibe water into nearly oil-wet chalk using surfactants?

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