Field Scale Tests & Modeling to Forecast Gas Migration Rates for Pressurized Mud Cap Drilling (PMCD) Applications

Abstract

Abstract Understanding gas migration rates is a critical part of well control discipline; for PMCD significant discrepancies exist between field observations of gas migration rates and common industry estimation methods (e.g. SPE212546). The objective of this work was to study realistic field scale scenarios in a test well and develop a predictive model using the data. The model helps in simulating field scenarios and efficient planning of fluid logistics with significant economic impact. We present an industry first well scale study that demonstrates the impact of flow regime and solubility using Argon & Nitrogen on gas migration. A 5200-ft-deep vertical test-well with 9-5/8″x2-7/8″ annulus equipped with 4 downhole pressure/temperature gauges and fiber optic DTS/DAS is utilized for the field-scale experiments. Tests were conducted by injecting gas at the bottom and observing its migration in the static mud column, followed by flushing to simulate PMCD shut-in and flushing cycles. The impact of gas solubility, bottomhole pressure (BHP) and gas injection rates were studied. Nitrogen/Argon gases were used to introduce solubility differences. To estimate gas migration rates, a mathematical model was developed and calibrated with the test data. The flow regime and gas solubility in the mud were found to be the most important factors in determining gas migration rates. Flow regime transition from Cap to Taylor bubbles was observed when BHP was rapidly dropped from 3600 to 2290 psi, with a threefold increase in gas migration rates. Argon gas migration rates were ~half that of nitrogen under the same test conditions which is in line with the gas solubility differences between the two gases. The gas migration rate increased non-linearly with injection rates between 0.1 to 6 bpm. The simulation model utilized well known Cap bubble correlations along with Henry's law and was found to adequately model the test results. Reported field case histories were also modeled and the results compared well with reported gas migration rates (~0.01-0.4 ft/sec) for both water and oil-based muds. The minimum mud bullheading rate to flush gas was found to be 100 gpm (0.6 ft/sec) and was ~15 % higher than the gas migration rate; this helps in determining the minimum field required flushing rate. The gas migration model accounts for solubility in the mud, pressure in the wellbore influencing gas flow regime, and gas influx rates and predicts results in-line with observed field case histories.