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Colloidal gas aphrons (CGA) have the unique ability to form a bridge in the pores of reservoirs, which stops fluid invasion. Sizing microbubbles in accordance with the rock pore size distribution is imperative for effective sealing during drilling. The effects of time, temperature and pressure on the stability and size of the microbubbles needs to be better understood in order to design a fluid that will sufficiently block the pores of the formation for extended periods. In this study, the effects of time, pressure and temperature on the size of microbubbles and the stability of microbubble (CGA)-based drilling fluids were investigated. The change in the CGA diameter with time was determined by using a microscopic imaging technique. Effects of base fluid viscosity and surfactant concentration on the size and stability of the microbubbles were also investigated. Introduction CGA-based drilling fluids have been successfully used in high-angle and horizontal well drilling in highly depleted reservoirs(1). Microbubbles in CGA-based drilling fluids form a bridge in front of the pores of the rock. This bridge is believed to stabilize the rock while sustaining minimal damage to the formation. Stability of the microbubbles and how bubble size changes as a function of downhole conditions (i.e. temperature and pressure) are some of the major concerns associated with the application of CGA-based drilling fluids. A stable CGA structure requires maintaining an ideal film wall thickness of 4 to 10 microns(2). Another factor affecting CGA stability is the rate of transfer of the surfactant molecules between the viscous water shell and the bulk phase due to gravity drainage or temperature gradients. This leads to a surface tension gradient at the surface of the shell. As a result, the Marangoni Effect will counteract this deformation(3–4). Increasing the viscosity of the shell can help to minimize the transfer of surfactant molecules. Usually a biopolymer is added to adjust the shell viscosity(3). The third property that the CGA structure must have is low diffusivity, which is the ability of the air that is in the core to transfer to the aqueous shell. CGA bubble size and stability have been the subject of earlier studies(5–12). Longe(6) analyzed the bubble size distribution of CGAs for soil and groundwater decontamination applications. Longe's analyses included effects of surfactant concentration, surfactant type and electrolytes on the stability of the CGAs over the time. Jauregi et al.(7) also investigated the stability of CGAs as a function of surfactant concentration. Results from both studies indicated that the stability of CGAs increase with increasing surfactant concentration. Chaphalkar et al.(8) measured the size distribution of CGAs using a particle size analyzer. The CGAs were virtually non-existent after 20 minutes for three different types of surfactant. Roy et al.(9) reported similar results. Amiri and Woodburn(10) studied the rate of drainage, as well as the CGA bubble size, by recording the images of the CGAs over time. They reported that after 10 minutes, the bubble shape had changed from spheres to polyhedral structures.

Colloidal gas aphron-based drilling fluids are designed to minimize formation damage by blocking the pores of the rock with microbubbles, which can later be removed easily when the well is open for production. Sizing colloidal gas aphron (CGA) bubbles in accordance with the rock pore size distribution is essential for effective sealing of the pores during drilling. The physical properties (i.e. viscosity, density, fluid loss, etc.) of the CGA-based drilling fluids also need to be understood in order to use these fluids more effectively. In this study, the physical properties of colloidal gas aphron-based drilling fluids are investigated. The results of rheology, API filtration loss and density measurement tests using various CGA-based drilling fluid formulations are presented. The effects of polymer and surfactant concentration, surfactant type, shear rate, mixing time and water quality on the CGA bubble size have been studied. Results of CGA bubble size characterization experiments are also reported. Introduction Colloidal gas aphron-based drilling fluids have recently been used for drilling at-balance in an attempt to eliminate the problems associated with overbalanced and underbalanced drilling. In order to achieve an at-balance drilling situation, the fluid pressure must be maintained at a level greater than the formation pressure, but the difference should be kept at a minimum level to avoid invasion of the fluid into the formation(1). Colloidal gas aphron drilling fluid simulates such a situation by building a bridge in front of the pores of the rock. It is believed that this bridge stabilizes the rock while allowing minimal damage to the formation. This system has been successfully implemented in high-angle and horizontal well drilling in highly depleted reservoirs(2), as well as with vertical wells. Simply put, aphrons are bubbles, approximately 10 to 100 microns in diameter. The term colloidal gas aphrons was first used by Sebba(3). Like regular foams, aphrons are typically composed of a gaseous (colloidal gas aphrons) or liquid (polyaphron) core. Unlike foams, however, aphrons have a thin aqueous protective shell. Aphron stability is determined by the rate of mass transfer between the viscous water shell and the bulk phase. This transfer is known as the Marangoni effect(3–5). If the mass transfer rate is high, aphrons will be unstable. Therefore, the shell fluid is designed to have certain viscosity to minimize the Marangoni effect. The shell is composed of an inner layer and an outer layer. Figure 1 illustrates a typical aphron. The inner layer consists of surfactant molecules which supports and separates the air core from the viscous layer. The outer layer, which also supports the viscous layer, is hydrophobic outwards and hydrophilic inwards. Since this bubble is in contact with the bulk water, it is believed that there is another layer in which the surfactant molecules are hydrophobic inwards and hydrophilic outwards. This indicates that there is a region in between the aphron outer shell and the bulk phase layer where a hydrophobic globule will be comfortable and, therefore, oil can adhere to the gas aphron(3).

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One of the major unsolved complex problems confronting the petroleum and chemical industries at present is the untimely deposition of heavy organic and other solids dissolved or suspended in the fluid flow systems. The production, transportation, and processing of petroleum can be significantly affected by flocculation and deposition of such compounds in the course of industrial processing systems, including transfer conduits, reactors, and refineries and upgrading equipment, with devastating economic consequences. Heavy organics such as paraffin, wax, resin, asphaltene, diamondoid, mercaptdans, and organometallic compounds can precipitate out of the crude oil solution due to various forces causing blockage in the oil reservoir, well, pipeline, and in the oil produc-