A Relationship Between Pore Structure and Residual Oil Saturation in Tertiary Surfactant Floods

Photomicrography and mercury porosimetry have been used jointly to determine the pore-size distributions of various sandstone samples. The two curves differed drastically from each other for all samples. Two unconsolidated packs consisting of uniform 250u glass beads and mixed 44-250u beads, respectively, as well as the sinters prepared from them, were also investigated. An index, D, measuring the difficulty of recovering waterflood residuals in tertiary surfactant floods has been constructed from the two different porosimetry curves. Reasonably good correlation porosimetry curves. Reasonably good correlation bas been obtained between D and residual oil saturations found in tertiary surfactant floods. Introduction This paper presents our first results in a continuing study of pore structure and oil recovery. The problem of how pore structure might influence oil recovery has been discussed by several authors. There is, however, no known method whereby one could rank various porous media (e.g., different sandstones) on the basis of pore structure in the order of decreasing amounts of expected residual oil saturations assuming identical conditions of flooding (identical oil, water, wetting and pressure gradient for the various sands. In this pressure gradient for the various sands. In this work we have taken an initial step toward this ideal objective. The prime target of the treatment has been the problem of correlating the extent of recovery of waterflood residuals by tertiary surfactant floods with the pore structure. The degree of difficulty presented by the pore structure in the way of recovering the isolated oil masses left behind by a waterflood has been expressed in the form of an index that is calculated from a mercury porosimetry and a photomicrographic pore-size distribution curve obtained on the sample. pore-size distribution curve obtained on the sample. The degree of correlation obtained amounts to a promising start in the case of tertiary surfactant promising start in the case of tertiary surfactant floods, and there also appears to be some correlation between the residual oil saturations found in the waterfloods and the pore structure. In this paper we are considering only the case of water-wet formations and moderate viscosity ratios. THEORY The term "pore structure" ordinarily means the distribution of pore volume by some linear pore dimension (pore-size distribution) and the topographical sequence of pores. Pore-size distributions have been determined by various methods. However, for reservoir rocks the most popular method has been mercury porosimetry. In a typical reservoir rock pore necks alternate with bulges. As the meniscus of penetrating mercury advances past a pore neck, it continues to advance in a nonequilibrium manner, until it comes to an even narrower neck. Since the capillary pressure of penetration of mercury into the pore pressure of penetration of mercury into the pore space between the two necks is determined by the size of the first neck, the pore diameters corresponding to the space between the two necks remain undetected by this method. Let us consider an arbitrary pore segment in the sample and approach it from the outside surface of the sample. Somewhere between the pore segment and the outside, there is a controlling cross-section in the pore space that is defined as follows: once the meniscus of the invading mercury has passed that cross-section, there is no narrower neck in its path all the way to the segment considered. The path all the way to the segment considered. The pore neck is the segment considered the controlling pore neck is the segment considered the controlling cross-section as defined above, even if the pore neck is far removed from the controlling crosssection. Denoting the radius of the pore segment by re and that of the controlling pore neck by r'e we have re greater than r'e. JPT P. 289