Gas Migration in PMCD Operations: Instrumented Well Study Provides Fundamental Insights

Significant discrepancy exists between the gas migration rates observed during the field applications of Pressurized Mud Cap Drilling (PMCD) and the widely used Taylor bubble velocity correlation. This impacts the fluid logistics planning and design of fluid properties for PMCD applications. Pilot-scale experiments and simulations have shown the importance of wellbore length-scale for estimating gas migration velocity (Samdani et al., 2021, 2022). Therefore, an industry-first well-scale study of gas migration in synthetic-based mud (SBM) was performed using a 5200-ft-deep vertical test-well (9-5/8″ × 2-7/8″ casing/tubing) located at Louisiana State University (LSU) well testing facilities. This test well is instrumented with 4 downhole pressure gauges and distributed temperature/acoustics sensing (DTS/DAS) fiber optic cables which were used to track the migrating gas and to determine its velocity. In a typical test, bottomhole pressure (BHP) was maintained, while gas migrated in a shut-in well. Tests were conducted by varying gas injection rate (10-250 gpm), total gas influx size (10-20 bbl), and BHP (2200-4500 psi). Gas migration rates indicated presence of Taylor bubbles at lower pressures (<2000 psi) and relatively smaller cap-bubbles at higher pressures (>2700 psi). The observation of pressure-dependent flow regime transition in a wellbore is one of the significant outcomes of this study. Changes in gas influx rate also influenced the gas migration velocity as it impacts the gas holdup and the rate at which gas can dissolve in comparison with the injection rate, under the prevailing flow regime. As a result, increase in influx rate led to higher gas migration velocity. A numerical model was also developed incorporating the experimentally observed relationship between pressure and transition of flow regime, to translate the test results into useful information and predictions for field PMCD. For example, the impact of reservoir gas solubility on gas migration rates was determined using this model while using the test-results based on nitrogen gas migration. The model results for reservoir gas migration rates in SBM showed a reasonable match with field-PMCD data under similar conditions.