Oilfield Glossary
Oilfield Glossary
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base exchange

1. n. [Formation Evaluation]
The quantity of positively charged ions (cations) that a clay mineral or similar material can accommodate on its negatively charged surface, expressed as milli-ion equivalent per 100 g, or more commonly as milliequivalent (meq) per 100 g. Clays are aluminosilicates in which some of the aluminum and silicon ions have been replaced by elements with different valence, or charge. For example, aluminum (Al+++) may be replaced by iron (Fe++) or magnesium (Mg++), leading to a net negative charge. This charge attracts cations when the clay is immersed in an electrolyte such as salty water and causes an electrical double layer. The cation-exchange capacity (CEC) is often expressed in terms of its contribution per unit pore volume, Qv. In formation evaluation, it is the contribution of cation-exchange sites to the formation electrical properties that is important. Various techniques are used to measure CEC in the laboratory, such as wet chemistry, multiple salinity and membrane potential. Wet chemistry methods, such as conductometric titration, usually involve destruction or alteration of the rock. Although quicker and simpler to perform, they are less representative of electrical properties in situ. The multiple salinity and membrane potential methods are more direct measurements of the effect of CEC on formation resistivity and spontaneous potential.
2. n. [Drilling Fluids]
Quantity of positively charged ions (cations) that a clay mineral (or similar material) can accommodate on its negative charged surface, expressed as milliequivalents per 100 grams. CEC of solids in drilling muds is measured on a whole mud sample by a methylene blue capacity (MBC) test, which is typically performed to specifications established by API. CEC for a mud sample is reported as MBC, methylene blue test (MBT) or bentonite equivalent, lbm/bbl or kg/m3.