Feasibility analysis of fuzzy-ball drilling fluid applied in under-pressured strata of the First and Second Member of Feixianguan Formation， Puguang Gas Field

Abstract:

After more than 20 years of production， the pressure coefficient of the main pay zone （First and Second Member of Feixianguan Formation） of Puguang Gas field has been dropped to 0.60~0.70. However， the overlying Third Member of Feixianguan formation and underlying Changxing Formation are still maintaining virgin formation pressure with the pressure coefficient of 1.20 and 1.10 respectively， which forms a relatively high-low-high pressure system. The second intermediate casing penetrates through the Third Member of Feixianguan formation. If conventional low-density water-based mud with a density of 1.03~1.05 g/cm3 is adopted in the under-pressured intervals of the First and Second Member of Feixianguan formation， severe drilling complications such as lost circulation， differential pressure sticking and formation damage to the main pay zone may still occur. To avoid adverse impacts on production， in this paper， a feasibility analysis was carried out covering lost circulation prevention， plugging， operational depth of 5115 m and sealing while-drilling， it is recommended that the formation should be penetrated by combining fuzzy-ball drilling fluid with sealing while-drilling technology.