Flow characterization of glauconitic sandstones by integrated Dynamic Neutron Radiography and image analysis of backscattered electron micrographs

Abstract

The effects of porosity and pore geometry on the fluid saturation and immiscible displacement of greensand reservoir rocks were investigated on eight samples from the North Sea basin. Dynamic Neutron Radiography (DNR) was used to image the front stability during displacement experiments where oil was infiltrating water-saturated samples. The flow characteristics were related to petrography and pore geometry, which may be determined by image analysis of backscattered electron (BSE) micrographs. The fluid saturation observed in each DNR image was modelled in a corresponding BSE image as a means of evaluating the effect of pore geometry on front stability. Piston-like displacement and channelling were observed and these flow patterns were found to reflect variations in pore geometry. The samples with piston-like displacement have homogeneous pore space, whereas the samples with channelling have heterogeneous pore space, with spatial variations in porosity and pore size. The modelled saturation distribution was interpreted using results from petrographic and petrophysical analyses. The results suggest that the micropores of the glauconite grains and clay minerals contain water, whereas the oil is moving through the intergranular pore space during the displacement experiment.

Petrophysical properties of greensand as predicted from NMR measurements

“…Based on the Kozeny's equation, we estimated Sp (Kozeny) by using permeability determined on the cores and macro-porosity. Sp by image analysis depends on the resolution of the image (Solymar et al 2003). However, Sp from image analysis at the present pixel size…”

“…Nitrogen adsorption methods (BET) yield high specific surface value as nitrogen enters the pores in the sample. By using image analysis to determine the specific surface area, usually a much smaller value is derived, and the value depends upon the resolution (Solymar et al 2003).…”

Permeability in Rotliegend gas sandstones to gas and brine as predicted from NMR, mercury injection and image analysis

“…On average gas permeability is eight times higher than brine permeability; which is significantly larger than the effect of gas slip estimated in Section 4.4. One reason for the difference in permeability to brine and water could be a layer of bound water on the mineral surface that reduces the pore volume that is available to brine flow (Andreassen and Fabricius, 2010;Heid et al, 1950;Luffel et al, 1993;Solymar et al, 2003). Another reason could be the reversible collapse of fibrous illite during drying (De Waal et al, 1988;Luffel et al, 1993).…”

Rock physics model of glauconitic greensand from the North Sea

“…Porosity in this sediment is found at two scales: macroporosity between grains and microporosity within grains (Figure 1). Greensand petroleum reservoirs occur world-wide, e.g., the mid-Cretaceous Safaniya Sandstone Member in Saudi Arabia (Cagatay et al, 1996), the Lower Cretaceous Glauconitic sandstone in Alberta, Canada (Tilley and Longstaffe, 1984), the Upper Cretaceous Shannon sandstone in Wyoming, USA (Ranganathan and Tye, 1986), a Lower Cretaceous Greensand offshore Ireland (Winn, 1994) and a late Paleocene Greensand in central part of the North Sea (Solymar, 2002;Solymar et al, 2003;Hossain et al, 2009;Hossain et al, 2011a, b;Stokkendal et al, 2009). However, evaluation of greensand reservoirs has challenged geologists, engineers and petrophysicsts.…”