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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TGT field is located offshore Vietnam, at a water depth of 45m and approximately 100km Southeast of Vung Tau. The field started production in 2011 and is currently producing about 13k bopd. The produced crude is 38-40 deg. API with viscosity of 0.45 cP at reservoir conditions and classified as light oil. The field consists of three wellhead platforms with nearly 40 production wells, all being gas lifted. Emulsion, which is rarely encountered in downhole environment and not reported in the surrounding oil fields, was diagnosed to be present in production tubing of TGT wells by two indications: measured bottom-hole flowing pressure (BHFP) remarkably higher than calculated value, and emulsion observed on surface well fluid samples. Upon completion of the laboratory testing for chemical selection, in early 2020 a field trial was carried out by injecting demulsifier into the production tubing of selected wells via the chemical injection line or together with lift gas. A successful field trial resulted in a clear reduction of BHFP of the wells along with a production increase by approximately 13% from 8 tested wells. Due to the low-cost of the application and major economic gain compared to other IOR methods, long-term downhole demulsifier injection has been applied in additional wells for increasing the field production. Unlike for heavy oil developments, there is a lack of publications on downhole emulsion and demulsification for light oil fields. This paper describes a case study for application in a light oil field, covering identification of the wells having an emulsion issue in the production tubing, laboratory testing for selecting demulsifier, challenges in chemical deployment and the field trial results.