An Experimental Study of Water Imbibition in Chalk from the Ekofisk Field

Abstract

Member SPE-AIME Abstract The Ekofisk Field in the North Sea produces from two lithologically similar, low permeability fractured chalk formations. The Ekofisk Formation of Danian age is separated from the lower Tor Formation of Maastrichtian age by a tight zone of about 15 m thickness. With a projected primary recovery factor of only 18%, the secondary recovery target is substantial. A pilot waterflood has been conducted in the Tor Formation and expansion of waterflooding to a large area of the Tor is planned. For waterflooding to recover substantial oil in a highly fractured rock large saturation changes in the matrix rock must be achieved through water imbibition. Water imbibition in Tor samples is high and variable and the imbibition in Ekofisk samples is low and also variable. The first objective of the laboratory investigation reported here was to establish correlations between imbibition and rock parameters in the highly water wet Tor samples. The second objective was to determine the reasons for differences in Ekofisk and Tor water imbibition. The volume of water imbibed and the initial water saturation in Tor samples correlated very well with porosity for the two wells examined. The water porosity for the two wells examined. The water saturation after imbibition was constant at about 60 percent of pore volume, independent of porosity. percent of pore volume, independent of porosity. Imbibition in the Ekofisk samples from the one well examined varied widely and did not correlate with any of the rock parameters examined. Pore structure studies and extraction experiments with strong solvents indicate that the difference between Tor and Ekofisk samples is due to rock surface chemistry differences. The correlation developed here with the Tor samples provides a sound basis for comparing data from provides a sound basis for comparing data from different wells, for examining the effect of various handling and laboratory procedures, and for data input in models. The indication of rock surface chemistry differences provides direction for further research aimed at explaining in greater detail differences between the Ekofisk and Tor rocks with respect to imbibition. Introduction The Ekofisk Field is located near the center of the North Sea on a relatively simple anticlinal feature (Figure 1). The reservoir rock is chalk, and the productive zones are confined to the Ekofisk and Tor productive zones are confined to the Ekofisk and Tor Formations, which are Danian and Maastrichtian in age, respectively. The field was discovered in 1969 and initial production began in 6 1971 The estimated original oil in place is 0,8 10 stock tank m and the projected primary recovery is about 18 percent. About two thirds of the original oil in place is in the Ekofisk Formation and one third in the Tor Formation. The chalk sediments in the Ekofisk area have rather unique characteristics. The main constituent in this rock is skeletons of coccolithophores, which are unicellular algae. Usually the whole skeleton, the coccosphere is disaggregated into the distinctive coccoliths and further disaggregation produces the plate shaped crystals termed coccolith platelets. plate shaped crystals termed coccolith platelets. Geological studies have shown that the rates of sedimentation are significantly different for the Ekofisk and Tor Formation. The Danian age was dominated by moderately organic productivity and high carbonate dissolution rate, while the Mastrichtian age had high organic productivity and low carbonate dissolution rate. P. 93

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“…Brine-oil-rock system will exhibit neutral wettability, if rock does not show any preference to either brines. Or in other words, neutral wettability is defined as a condition when both fluids equally wet the rock surface (Tiab and Donaldson 2010).…”

“…The interfacial tension (IFT) has the dimensions of force per unit length (newton/metre), which is the modern standard expression of the units. In the earlier literature, however, it is expressed as dynes/centimetre, which is numerically equal to millinewtons per metre [(N 9 10 -3 )/m or mN/m] (Tiab and Donaldson 2010).…”

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