"Honey, I Shrunk the Pores!"

Abstract

Introduction When I was young, my parents instilled in me a curiosity about the physical nature of the world. I remember Dad, an avid reader of "The Country Guide," explaining the enormous power or a prairie thunderstorm by telling me "an inch of rainfall on all acre of land is over one hundred tons of water." To bring this analogy closer to our own business, it still amazes me that a reservoir one foot thick with ten per cent porosity will also hold almost one hundred tons of oil per acre of land. Sometimes we get so bogged down by the details of our business, we don't have time to think about some of nature's simple wonders. With thoughts like these in mind, it's neat to take a closer look at a parameter which is taken very much for granted: porosity. To ost of us, the only thing which matters is the absolute value of porosity, and obviously the higher the better! Some interestinghenomena pop to light, however, when one really visualizes things on a pore-scale. With that in mind, let's take closer look at some porous systems from the inside... Conventional Pore Systems Let's quickly review some basic geometric concepts, focussing at the pore-scale. Table 1 shows how Archie(1) and Lucia(2) classified carbonate pore sizes: a typical "medium"-sized pore is about 50 microns (0.002 inches). Pore throats, the inter-pore connections which determine permeability, are much smaller, often in the 1-10 micron range. Obviously, it takes an enormous number of these pores to form a reservoir. We also know that the size of the individual pores may not have much impact on the total porosity. Figure 1 shows the porosity resulting from a cubic arrangement of spherical grains is 48%, regardless of the size of the grains.owever, for a given porosity, the pore system surface area increases as the average pore size decreases (Table 2). The surface areas in Table 2 assume pores are spherical: however, in reality their shapes are much more complex. As a result, urface areas in reservoir rocks may be 100 times higher than shown in Table 2 due to tile dramatic effects of surface roughness.good example is Donaldson's work(3), which showed the TABLE 1: Pore size classifications. Table Available In Full Paper. TABLE 2: Ways to make 11% porosity in 1 m3 of rock. Table Available In Full Paper. surface area of many common reservoir rocks to be about 1 m2/gm. These large surface areas are rather startling at first glance. Imagine one cubic metre or oil-saturated rock with 11% porosity. Table 2 shows the resulting 0.11 m3 of oil would be spread over 10,000m2; accounting for pore surface roughness the area would actually be about 1,000,000 m2. To put it in more real terms, this would be like spreading a couple of cups of oil across the surface of a foothall field. Is it any wonder that oilfield recoveries average only about 25%?