Numerical History-Matching of Modeling and Actual Gas Production Behavior and Causes of the Discrepancy of the Nankai Trough Gas-Hydrate Production Test Cases

Comprehensive data sets were obtained during the gas production tests from a gas-hydrate reservoir in Eastern Nankai Trough (2013 and 2017), including the gas and water production rates, downhole pressure, temperature-monitoring data, log- and core-derived reservoir property information, and seismic volume. Numerical simulations using a compositional and thermal-coupling numerical model (MH21-HYDRES) had been conducted earlier and after each production test for production behavior prediction purposes and history-matching to reevaluate the reservoir property information. The actual production behavior demonstrated some deviations from the prediction by the numerical model. For example, the model predicted a gradual increase in the gas production rates under the constant pressure drawdown but real gas rates were almost constant or slightly decreasing, some inconsistencies in the temperature data and derived vertical and temporal production profiles were observed, and differences in the production behavior exist among the wells that were drilled within a 60 m radius area. To reduce the gap between the model and actual results, understand the nature of the gas-hydrate reservoir, and understand the physical processes during the gas-hydrate dissociation, comparison of the modeling results and obtained data was repeated, along with reexamination of the geological and geophysical data. The study results revealed two features, namely, vertical and horizontal heterogeneities of the formation and complexly disturbed situation in the near-wellbore region due to the unconsolidated nature of the sediments, which were determined as the main causes for the discrepancy in the model and actual production behavior.