Circulating bubble columns or loop reactors form one of the important classes of modified bubble columns. The present paper analyses the performance of external loop air lift reactors (EL‐ALR). The EL‐ALR has many advantageous features especially at large scale. These arise from its feature of having controlled liquid circulation which is the key parameter for the design and operation of EL‐ALR contactors. Therefore, a reliable and generalised circulation model has been presented. The reliability of the model depends on the accuracy of the predictive methods for the gas hold‐up and the two‐phase fractional pressure drop. Detailed analysis of these fundamental parameters of gas‐liquid flows has been presented. The effects of design (area ratio of riser to downcomer, height to diameter ratio and volume of reactor) and operating (gas flow rates and sparging locations) parameters on the performance of the EL‐ALR have been analysed in detail. A rational basis has been developed for the estimation of pressure drop, mixing time and mass transfer coefficient. An optimum combination of design and operating parameters has been suggested and a criterion has been developed for the optimum location of spargers. An attempt has been made to provide critical analysis of the published information and to construct a coherent picture of EL‐ALR.

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Incompatibilities between drilling fluid filtrates and formation connate fluids are believed to cause changes in rock wettability and hence impair production for a given reservoir rock. This paper summarizes test results and analysis of an experimental study to systematically assess the effect of incompatibilities on rock wettability and in turn on its permeability. The rocks considered are dolomite, limestone and sandstone. The parameters that were studied are the ph, monovalent to divalent cation ratio, total cation concentration (salinity) and the drilling fluid surfactants. The Dynamic Wilhelmy Plate Technique was used in the measurement of wettability changes. The analysis of test data was conducted using a defined deviation number that allowed quantitative evaluation of the various effects that were considered in the study. Results have shown that the most significant in rock wettability changes are the surfactants in the drilling fluids. Dolomite rock had the maximum reduction in water wetness when surfactants are absent, while the reduction is about the sameforall rocks when surfactants are present. Also, among all five water-base muds studied, the calcium treated gypsum mud had the least water wetness.