The application of GIS to analysing 2-D models of porous systems in the context of gas mobility

Abstract

The mobility of gas bubbles within the pore fluid of sediments, represented by changes in the spatial distribution of the gas phase, has been studied using a physical analogue model constructed in glass and analysed using a Geographical Information System (GIS). Such glass models have the appearance of petrographical thin sections in which the gas phase can be identified, but the mobility is difficult to quantify. To improve the quantification, images showing the position and size of individual gas bubbles, and images of the glass micro-model (2-D model) have been digitized for analysis in the GIS. The digitization of the 2-D models produces separate overlays outlining the grains and pores. Each individual grain or pore is given a separate label. The area of each pore space, grain and gas bubble can then be automatically calculated in two dimensions and from these measurements the corresponding cumulative frequency distribution curves can be obtained. The results show that the size of gas bubbles within a porous system of specified grain and pore size cannot easily be predicted using standard petrophysical parameters. The shapes of the gas bubble distribution curves appear to be unrelated to the shapes of either the pore or the grain size distribution curves. The results further show that the mobility of a gas phase in a porous medium seems to depend strongly on whether it has been subjected to pressure changes before, rather than on the magnitude of the pressure changes.