The first offshore methane hydrate production test was conducted in the Eastern Nankai Trough area of Japan in 2013 subjecting gas hydrates reservoir to large drawdowns by reducing bottomhole pressure (BHP) for in-situ dissociation of gas hydrates. This pioneering test has proven the feasibility of depressurization method through demonstration of gas production from a deep water gas hydrates reservoir. Approximately 119,500 Sm3 of gas was produced during a continuous flow period of 6 days. However, reservoir response to a range of drawdown conditions was not possible due to unintended water production through gas line. This paper evaluates the issues with drawdown control that was affected by the water production.

Gas and water released from the dissociation of gas hydrates were separated using a downhole gas separation system. The separated gas and water were produced to surface via two dedicated gas and water lines. Drawdown was planned by pumping out water into the water line using an electrical submersible pump (ESP). Drawdown control was designed to regulate the liquid level (or hydrostatic pressure) in the gas line by controlling the ESP frequency as well as the water line surface backpressure.

Analysis of production data supported by flow simulations indicated that water production through the gas line was the main reason for the loss of drawdown control, i.e., the rising of liquid level was due to water production caused by the produced gas. Water entrainment by gas stream was not the main reason for the water production due to low gas velocity. The produced gas lightened the water column in the gas line, raised the liquid level and lowered the BHP. This process mimics a gas lifting process in a predominantly liquid production well. Since the BHP was largely affected by the liquid holdup as well as the volume of gas in the gas line, it was difficult to regulate the drawdown as intended.

The analysis concluded that water production through the gas line could be prevented if a backpressure regulator was installed at the surface gas line and / or the ESP had high tolerance to the presence of free gas. Lessons learnt from this first field trial has provided valuable information for improving the methods for controlling large drawdown in methane hydrate production system.