Development of an ultra-low-density foam workover fluid based on chemical cross-linking

Foam workover fluids are an effective solution for use in the safe drilling of low-pressure formations. However, existing foam drilling fluid systems are often unable to balance an ultra-low density (< 0.7 g/cm³) with a long-term stability (half-life > 120 h), resulting in foam instability and accidents during drilling. In this study, a high-rigidity liquid film was constructed through chemical cross-linking interactions between the isocyanate (NCO) groups at the ends of a water-soluble polyurethane and the molecular chains of xanthan gum, establishing an ultra-low density and highly stable foam drilling fluid system. The final formulation consisted of water, 2.7% water-soluble polyurethane, 0.5% xanthan gum, 0.05% Na 2 CO 3 , 2% shale inhibitor, and 1.5% lubricant. The colloidal foam exhibited a low density (0.5 g/cm³), a long half-life (213 h), and excellent rheological properties. The system has low fluid loss, good resistance to NaCl and CaCl 2 and demonstrated thermal resistance between 70 and 90 °C. Additionally, the system exhibited a good oil resistance, poor soil contamination resistance, good sand suspension ability and a certain plugging capacity. This ultra-low-density foam drilling fluid significantly enhanced the liquid film strength and foam stability, thereby representing an innovative solution for improving the stability of foam drilling fluid.