Asessment of Solvent Extraction Efficiency In Sandstones Using BET Specific Surface Area Measurements
Authors: French P.R. , Mannhardt K. , De Bree N. , Shaw J.C. 
Published: March 2010

Abstract
Cleaning core is standard procedure prior to routine measurement of parameters such as porosity and permeability and before performing special core analyses. It is shown that the cleaning efficiency of different solvents and the degree of cleanliness of a core can be assessed through specific surface area measurement. Berea and Crossfield Cardium sandstone core plugs containing residual Crossfield Cardium oil from-laboratory core floods were extracted with the azeotropic mixture of chloroform/methanol, toluene, and methylene chloride, and the specific surface areas were measured as a function of cleaning time. Solvent cleaning increased the specific surface areas by a factor of three to four in some instances. Chloroform/methanol was found to be the most efficient of the three tested solvents. It is shown that the presence of residual oil may render a drastically reduced specific surface area for a rock, but the specific surface area can be restored to its original value by extraction with chloroform/methanol. The solvent itself does not seem to alter the clay structure within the rock.

Introduction
It is standard procedure in the petroleum industry to dean core for removal of all reservoir fluids before measuring core properties such as porosity and permeability, or performing laboratory core floods. Cores must also be cleaned before conducting special core analyses such as mercury capillary pressure and surfactant/chemical adsorption measurements. Laboratory resting of cores often requires that the cores first be cleaned of residual fluids and/or contaminants and be restored to the original conditions of wettability that prevail in the reservoir. Despite the importance of core cleaning, there are no generally accepted and convenient methods for choosing the most suitable solvent for any given system, or for assessing the cleanliness of a core.
A number of investigators have addressed the subject of core cleaning. As pointed out by Gant and Anderson (1988). it has been usual lab procedure to consider a core for routine core analysis "clean" when the extract from the care is visually clean (API, 1960). However, although certain solvents are effective in removing some hydrocarbon components from the surfaces of the grains, it is known that asphaltenes can be more difficult to remove from cores. Dubey and Waxman (1991) studied the effect of various solvents on asphaltene desorption to evaluate solvent efficiency in core cleaning operations. Solvents are used, not only to remove residual reservoir fluids, but also mud additives introduced during coring (Basan et al.. 1988).
Previous studies have demonstrated qualitatively the effectiveness of various solvents and combinations of solvents in cleaning cores (Jennings, 1958; Grist et al., 1975: Basan et al., 1988; Gant and Anderson, 1988; Hirasaki et al., 1990). Jennings (1958) reported that toluene used by itself was ineffective in removing all types of contaminants from core. Grist et al (1975) reported the effects of solvent cleaning on wettability. They concluded that different core cleaning methods can result in differing wetting conditions. They suggested cleaning oil-bearing samples with either alternate toluene/methanol cycles or using the azeotropic solvent mixture chloroform/methanol.