Colloidal gas aphron (CGA) based drilling fluids, because of their non-coalescing nature, excellent capability in minimizing deep invasion, and also behaving like a flexible bridging material, are indicated for drilling permeable and fractured formations. Their unique feature is to form a solid free, tough, and elastic internal bridge in pore networks or fractures to minimize deep invasion by means of air microbubbles, which can be removed easily during the initial stage of production. CGA based fluids combine certain surfactants and polymers to create the system of microbubbles. Surfactant is used to produce the surface tension to contain the aphron as it is formed, build the multilayer bubble wall, and create interfacial tension to form a non-bonding network capable of bridging openings in permeable and fractured formations. Polymer is used as viscosifier and aphron stabilizer. The surface activity and aggregation behavior of the surfactant affects the stability and also other physico-chemical properties of generated microbubbles. Therefore, selection of a suitable surfactant is important for the generation of microbubbles with the desired rheological and filtration properties. The goal of this paper is to investigate the potential use of a new plant-derived surfactant as an aphronizer surfactant in preparation of CGA based drilling fluids for accomplishing desirable rheological and filtration properties. For this purpose, natural surfactant obtained from leaves of special tree namely Zizyphus Spina-Christi and used for preparation of aphron-based fluids. To evaluate the potential use of new plant-derived surfactant as an aphronizer, various physico-chemical properties of aphron-laden muds were investigated. To achieve the research objectives, laboratory tests of suspension generation, microscopic visualization, initial yield, filtration loss, and rheological behavior with varying concentrations of surfactant and polymer were performed. Effect of base fluid viscosity and surfactant concentration on size/size distribution of microbubbles, rheology, and yield of CGA based drilling fluids will be presented. Three rheological models, namely, Bingham Plastic, Power Law and Casson models were used for characterizing rheological properties of the muds studied.

Colloidal gas aphron (CGA) fluids, consist of gas bubbles with diameters ranging from 10 to 100 μm, surrounded by a thin aqueous surfactant film. This fluid combines certain surfactants and polymers to create the systems of microbubbles. The function of surfactant in CGA is to produce the surface tension to contain the aphrons. Also a biopolymer needs to be considered in the formulation as a viscosifier as well as stabilizer. The aphron-laden fluid appears to be particularly well suited for drilling through depleted zones. The unique feature of aphron based fluids is to form a solid free, tough, and elastic internal bridge in pore networks or fractures to minimize deep invasion by means of air microbubbles. This microenvironment seal readily cleans up with reservoir flow back as production is initiated, thereby reducing cost associated with stimulation processes. This paper presents a comprehensive, comparative study of rheological behavior and filtration properties of CGA based drilling fluids with various concentrations of polymer and surfactant. Laboratory evaluations showed that the CGA based fluid is one of the ideal engineering materials which can control fluid loss or loss circulation during drilling operation, save cost and increase productivity which rheological characteristics and filtration properties of them are greatly influenced by the level of polymer and surfactant concentration.