Modelling Trapped Gas Expansion in Water-Drive Gas Reservoirs
Engineering Honours 2015
Australian School of Petroleum
University of Adelaide
This paper presents a simulation study using Schlumberger’s reservoir simulation software ECLIPSE to model trapped gas expansion and remobilisation during its production by blowdown as secondary drainage flow. The study is intended to serve as a preliminary guideline in modelling secondary drainage flow in ECLIPSE since a clear methodology has so far not been identified. Therefore the study undertaken for this report provides an initial insight into the mechanisms involved in the process. Its application is recommended for simulation of water-drive gas reservoirs with a clearly observable presence of residual or trapped gas. This will assist in the evaluation of gas recovery, leading to more accurate reserves estimates
A literature review aiming to identify the main parameters and processes deemed potentially influential on the remobilisation of trapped gas and their subsequent impact on gas production has been conducted and used as a basis for reservoir simulation work. Previous studies in literature show the secondary drainage curve during blowdown is below the primary imbibition curve. This is based on field cases and core experimental studies, which establish the existence of a gas remobilisation threshold above residual saturation to reconnect the gas phase. The flow rate of the remobilised gas depends on how fast the gas relative permeability increases during secondary drainage. Hence changing the gas remobilisation threshold and slope of the gas relative permeability curve will affect the recovery of gas. By assuming no mobility threshold above residual gas saturation, the total recovery of residual gas will be overestimated.
Commonly used hysteresis formulations by Killough and Carlson are unable to model secondary drainage flow below the imbibition curve, thus another method must be used to achieve this. Water Alternating Gas (WAG) hysteresis has been identified as the most appropriate method to model trapped gas expansion as secondary drainage flow below the imbibition curve in ECLIPSE. This is based on an inbuilt WAG Hysteresis model in ECLIPSE, as well as the ODD3P three-phase relative permeability and capillary pressure hysteresis model developed by Hustad. A reservoir model was constructed to test scenarios which model trapped gas expansion as secondary drainage flow using both the WAG Hysteresis models. A sensitivity analysis was performed to quantify the influence of secondary drainage parameters including the gas remobilisation threshold and slope of the secondary drainage curve. All results and recommendations have been discussed and presented in detail.
The conclusion of this study is that the standard formalisms used to model hysteresis (Killough, Carlson) should not be used to model trapped gas expansion due to blowdown as they do not incorporate a gas remobilisation threshold and a secondary drainage curve underlying the imbibition curve. Instead, by adopting a WAG hysteresis model in ECLIPSE, water production will be higher and gas production lower from the correct use of secondary-drainage gas relative permeabilities in a gas reservoir invaded by water. This will lead to a significant improvement in the evaluation of trapped gas recovery and an increase in the quality of results from ECLIPSE during reservoir simulation.