Gas injection methods such as Continuous Gas Injection, Water Alternating Gas and conventional Gas-Assisted Gravity Drainage (GAGD) have been widely used to improve/enhance oil recovery for conventional oil reservoirs. However, applications to naturally fractured basement reservoirs are still limited. This paper will introduce a case study of a new and effective GAGD method conducted in a Huff ‘n’ Puff fashion to improve oil recovery for a fractured basement reservoir in Cuu Long Basin, offshore Vietnam.

A GAGD pilot, which consists of 4 cycles, was conducted in which dry gas was periodically injected into an existing production well in an isolated area. It was expected that as the injected gas rose to the top to form a gas zone, it would push the Gas Oil Contact (GOC) downwards and might also push the Water Oil Contact (WOC) to the lower part of the producer or even away from the bottom of its wellbore. Before commencing the 4 cycles, gas injection, asphalting and reservoir simulation studies were conducted. In addition, a thorough forward plan was carefully devised before each cycle to determine possible effects of important operating parameters to the final outcome of that cycle.

From the results of the 4 cycles, it could be concluded that the gas injection volume is well correlated with cumulative water-free oil production, a parameter which indicates the effectiveness of the method in terms of the gravity-drainage mechanism. It could also be found that the final incremental oil gain of each cycle depended upon, not only, the gas injection volume, but also other important factors such as gas injection, shut-in time… suggesting that a non-linear optimization exercise is necessary to make the whole pilot economically successful.

The success of the GAGD pilot proves that it could be a simple and effective Improved Oil Recovery (IOR) method for fractured basement reservoirs. That can be a foundation for further application of the method to other reservoirs in the Cuu Long Basin.

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