Distribution of alkali and alkaline earth ions in the near well-bore region after invasion of mud filtrate

A modified equation for the Gapon selectivity coefficient was found to fit ion-exchange isotherms of kaolinite and montmorillonite for divalent/monovalent cation exchange. This equation was used to predict the distribution of divalent/monovalent cations in the region near the well-bore after infiltration of mud. A field test was carried out to study sorption/desorption reactions on sedimentary rocks after the infiltration process. This test used a tracer technique in which mud was tagged with tritiated water during drilling an oil well. The fraction of mud filtrate water was determined from the measured fraction of tritiated water. The ionic composition of the pore water sampled by using a kind of formation tester equipment (FTE), is considered as a survey of the ionic distribution of the invaded ionic substituents. Data from the field test confirm the predicted values of divalent cations sorbed/desorbed on the sedimentary rocks after the infiltration process. Distribution of monovalent/divalent cations in the near well-bore region is important for the open-hole log interpretation used for reservoir description.