Gas Migration in PMCD Operations: Influence of Mud Type and Properties

Abstract

Pressurized Mud Cap Drilling (PMCD) is commonly used in carbonate formations with massive lost returns. However, gas migration during PMCD is not well-understood and therefore estimates of gas migration rate are very conservative. In this paper, a dynamic well control simulator is used to study wellbore-scale gas migration dynamics during a PMCD process. The simulator is further used to understand the effects of mud type and mud properties on gas migration. It is observed that, the large gas bubbles break into a swarm of smaller bubbles under typical field downhole conditions. The resulting gas swarm migrates at ~0.7 ft/s in water and the migration velocity reduces by ~50% to ~0.4 ft/s in water-based mud (WBM) with oil/surfactant. For synthetic-based mud (SBM), the gas migration rate reduces to ~12-15% of that in water to ~0.1 ft/s as the reservoir gas dissolves in SBM. These results do not include the effect of gas holdup on gas velocity which would further reduce the gas migration velocity during PMCD. This work is pioneering because it provides a deeper understanding of the length-scale dependent multiphase-flow dynamics during gas migration that is being revealed for the first time. This understanding is further used to provide a reliable method to estimate gas migration rates for a given set of fluid properties and operational conditions. The study also shows that a significantly low gas migration rate can be achieved by using a mud with high gas-solubility, reducing surface tension and lowering mud viscosity. These findings can be used to reduce fluid costs and logistics, and to improve planning of PMCD applications.