Micro-bubbles size, rheological and filtration characteristics of Colloidal Gas Aphron (CGA) drilling fluids for high temperature well: Role of attapulgite

Colloidal Gas Aphron drilling fluids have been successfully applied in petroleum industry to drill low pressure formations and depleted reservoirs. As the increase of drilling depth and reservoir temperature, characteristics of CGA drilling fluids at high temperature are more concerned, yet less investigated. In this paper, attapulgite with better performance at high temperatures was selected to prepare CGA drilling fluids and the bubbles size distribution, rheological and fluid loss performance at 25 °C/90 °C/120 °C/150 °C/180 °C were studied. CGA fluid with Xanthan gum and 3% bentonite were prepared and tested as control group. Result indicates that: 1) The addition of attapulgite significantly reduces the average diameter of aphrons and improves the bubble size distribution of CGA fluids, which is conducive to the stability of CGA fluids; 2) Attapulgite effectively increases low shear rate viscosity (LSRV) of CGA fluid at room temperature, which will help to carry cuttings and seal formation. Power law model is the optimal model to describe the rheological behaviors of attapulgite-based CGA drilling fluids. Rheological parameters of Power law model show that the tested fluids have a shear thinning behavior at all temperatures and the flow behavior index can be controlled to an appropriate range by adjusting the amount of attapulgite; 3) Attapulgite greatly reduces the fluid loss of CGA drilling fluid, especially at high temperature. The attapulgite-based CGA drilling fluids can maintain an acceptable fluid loss (within 15 mL) for drilling operations at 120 °C. At 150/180 °C, 3% attapulgite exhibited the lowest fluid loss volume, which is reduced by 46.7%/25% as compared to 3% bentonite. In addition, the mechanism of attapulgite-based CGA fluid loss control was studied and can be concluded as the “embedded” mud cake structure composed by aphrons and attapulgite.