Enhanced Gas Recovery: Factors Affecting Gas-Gas Displacement Efficiency

This paper is part of a series of papers on the results of Enhanced Gas Recovery (EGR) research conducted at the Alberta Research Council during 2003 to 2007. In this Joint Industry Project (JIP), the soundness of the concept of gas-gas displacement for enhancing gas recovery was investigated via laboratory investigations, compositional modelling and economic analyses. The results of Phase I gas-gas displacement tests conducted at relative high pressure and temperature (6.2 MPa and 70°C) in 4 cm diameter 30 cm long Berea core were recently reported (1,2) . In the second phase (2004-2005) of the JIP, the main targets were low pressure volumetric (closed) reservoirs in advanced stages of exploitation and also gas bearing strata overlaying oil sand intervals. Pressure maintenance of a depleting gas reservoir by waste gas injection can serve to: 1) arrest the decline in gas production rate, prevent premature well abandonment and increase ultimate recovery; 2) discourage the advance of an aquifer (if present) into the gas zone; and 3) in the case of Gas-Over-Bitumen situations, mitigate declining reservoir pressure during natural gas production to enable exploitation of the underlying oil sands. One example of a field application of this EGR technology was the GRIPE Project operated by Paramount Resources during 2005 and 2006. A series of gas-gas displacement tests were conducted at room temperature and at pressures between 0.7 and 3.5 MPa in the presence of connate water in 5 cm diameter x 2 m long sand-packs. Experimental parameters, such as nature of the injection gas, displacement pressure and displacement rate were systematically varied to study their effect on the displacement efficiency. Numerical simulations of the experimental results were also conducted to gain a better understanding of the interrelationship between the different variables. The laboratory results showed that during low velocity displacement of methane by flue gas in a homogeneous linear sand-pack, molecular diffusion has a dominating effect on the recovery of marketable methane. Reasonable values of molecular diffusion coefficient for different gas-gas displacement conditions were obtained by matching the experimental test results with the numerical simulation. In spite of anticipated adverse effects of mixing between displaced and displacing gas due to molecular diffusion under low pressure and low flow velocity conditions, incremental recoveries of marketable methane under the experimental conditions were encouraging and suggest that EGR by gas-gas displacement can prolong the productive life and increase natural gas recovery from many volumetric gas reservoirs.