High Temperature Sealing Performance of Novel Biodegradable Colloidal Gas Aphron (CGA) Drilling Fluid System

Drilling in depleted oil and gas reservoirs, highly developed pores/fractures and other low-pressure areas is often accompanied by serious drilling fluid invasion. Huge fluid intrusion not only causes borehole instability, but also creates severe formation damage and reduces well productivity. Therefore, reducing fluid intrusion is crucial in these reservoirs1). Granular, fibrous, and lamellar solid materials are commonly added in water-based drilling fluids as plugging agents. By designing solid particles size compatible with the pore throat, bridging occurs in the reservoir near the borehole wall to achieve filtration control2). As the long-time circulation in the high temperature and high-pressure wellbore, the solid phase material tends to take place complicated problems such as coalescence, settlement, acid dissolution, gelatinization, viscoelastic damage, etc. Therefore, maintaining the size distribution to ensure good sealing against specific rocks is difficult. Moreover, the solid particles remaining in the reservoir have a negative impact on production. Recently, a colloidal gas aphron (CGA) drilling fluid technology has been successfully applied internationally to solve the huge leakage problems, as listed in Table 1. The fluid system has been proved to have the advantages of reducing fluid invasion and reservoir damage significantly, no need for additional pressurization equipment, high cutting carrying efficiency, shortening drilling cycle and reducing drilling cost3)~5). A large number of aphrons generated by surfactants and polymers exist in the CGA fluid. Aphrons are microbubbles composed of a gas core and a thick multilayered aqueous surfactant shell. These robust and elastic microbubbles are ideal bridging materials with a wide size distribution and can be deformed to adapt to various pores and fractures6)~8). Pasdar et al. observed the changes in the particle size of the aphrons under a microscope with sudden or stepwise pressure, and found that the aphrons can survive under 2000 psig and ambient temperature. Growcock et al. further proposed that aphrons can survive at 4000 psig and ambient temperature. More importantly, aphrons show little affinity for each other and for mineral surfaces. Consequently, aphrons can be cleaned up to the ground [Regular Paper]