Studies of waterflooding in low-permeable chalk by use of X-ray CT scanning

Abstract

Low-permeability chalk samples from oil reservoirs located in the Danish sector of the North Sea have been analyzed by use of X-ray computed tomography (CT) scanning. The method has been applied for front tracking during two-phase waterfloods (imbibition) with and without connate water at initial conditions. In general, good agreement was found between the water saturation measured by gravimetry and the value obtained from the CT scans. In the absence of connate water, waterflood oil recovery exceeded 90%. The saturation profiles clearly show that the low oil saturation was obtained before the waterfront had reached the outlet. Therefore, the high displacement efficiency cannot be solely attributed to the capillary end effect. It is argued that it must be due to a rate-dependent suppression of snap-off. Results confirm that the presence of connate water lowers the waterflood sweep efficiency quite substantially. Connate water acts not only as an obstruction to the receding oil phase, but it also promotes snap-off. Stable collars of wetting phase may form far ahead of the connected front because wetting phase can be readily supplied from the smaller pores. In addition to the waterfloods, a nitrogen injection experiment was performed after a secondary waterflood. Vertical injection from the top ensured a favorable mobility ratio and resulted in a very high oil recovery.