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).
