Semiautomated Method for Cation-Exchange-Capacity Determination of Reservoir Rocks

The presence of significant amounts of clay in tight-gas sand formations makes the determination of cation exchange capacities (CEC) important for electric-log, self-potential (SP), and gamma ray log interpretation. In the past, CEC measurements have been difficult and time-consuming to obtain. However, an automated method that avoids many difficulties of other techniques while determining the CEC's of many samples at one time has been described by Worthington . Our work is a modification of the work done by Worthington. Easily assembled commercial equipment instead of specially built equipment is used to agitate rock samples contained in dialysis membrane bags during ion exchange with barium acetate solution and during washing of the samples to remove excess barium ions. Barium acetate is used as the source of barium ions instead of barium chloride, which is used in Worthington's procedure. to avoid corrosion of the stainless steel equipment. The amount of barium ions on the rock samples is then determined by conductometric titration with magnesium sulfate. The titration procedure is not automated. In addition, the use of the barium ion method was extended to samples with CEC values an order of magnitude lower than those determined by Worthington. Most measured CEC's for the western tight-gas sands ranged from 0.5 to 10 meg/100 g with a few to 19 meg/100 g. A comparison of barium acetate, adsorbed water, and ammonium acetate methods for determining CEC's is made. Introduction The highly reactive surfaces of clay, which act as ion exchangers. have a large effect on the physical and chemical behavior of reservoir rocks. The measure of the amount of exchangeable ions on the clay is called the CEC. The CEC describes the amount of reversible exchange occurring between ions in a liquid phase and a solid phase that does not significantly change the structure of the solid. It is measured in terms of the amount of positive ion substituted per unit weight of dry rock or, more often, in terms of the amount of positive ion substituted per 100 g of solid material. Quantitative interpretation of electrical resistivity and SP logs used to evaluate porosity and water saturation of permeable formations is affected by the clay content of the formation. Gamma ray measurements may also be affected by the presence of certain clays. Hill and Milbum studied the conductivity properties of shaly sandstones and derived an empirical relationship to account for deviation from Archie's relationship resulting from the "effective clay content" of the shaly sands. Johnson and Linke described the types of clays found in reservoir rock and the clay's effects on the interpretation of various logs. In general, clays such as kaolinite and chlorite have an insignificant effect on resistivity reduction. Grim reports a low CEC for kaolinite (3 to 10 meg/100 g) and a higher CEC range for chlorite (10 to 40 meg/100 g). However, measurements in our laboratory and by Worthington as reported by Johnson and Linke indicate that chlorite behaves in a manner similar to kaolinite. Montmorillonite and illite, on the other hand, are effective resistivity reducers. Their CEC's range from 80 to 100 meg/g and 10 to 40 meg/g, respectively. SPEJ P. 231^