Experimental results on the combined effect of reservoir heterogeneity, dispersion and gravity segregation between the injected and produced hydrocarbon gases on gas recovery efficiency are presented in this paper.
Laboratory tests were conducted in both one-dimensional and two-dimensional systems at ambient temperature and pressures in the range of 700 kPa to 3500 kPa, conditions typical of Alberta shallow gas reservoirs. The effect of heterogeneity on Enhanced Gas Recovery (EGR) in a system containing two non-communicating intervals was investigated with two parallel 2 m sand-packs of 5.4 and 2 Darcies (D). Methane was recovered by injection of either CO2, or flue gas from this heterogeneous system. The effect of gravity segregation (between injected and produced gas) on EGR was also investigated by means of a 2 dimensional physical model containing upper and lower compartments with permeabilities of 2 and 5 D, respectively. The layers were separated with a thin barrier with moderate permeability allowing cross flow and molecular diffusion between the two compartments. Results show that cross flow and transverse dispersion help to mitigate the adverse effects of heterogeneity during EGR. Density difference between injected and produced gas can either improve or reduce displacement efficiency depending on reservoir geology. Effectiveness of various techniques to mitigate the adverse impact of heterogeneity on EGR was also investigated.
In recent years, enhanced oil and gas recovery by waste gas or CO2 injection has attracted much interest 1–7 because it is considered as a feasible mean to not only improve hydrocarbon recovery but also, to permanently store greenhouse gases underground. Alberta Research Council has been operating a joint Industry Participation (JIP) project on enhanced gas recovery (EGR) for several years. This JIP project consisted of several tasks including laboratory experimentation, numerical simulation, economic evaluation, surface facility designs and field pilot. The main resource targets of this EGR JIP program are Alberta's 4200 gas pools which are in different stages of exploitation and many of them are approaching the end of their production life. The laboratory program aims at achieving better understanding of the gas-gas displacement process within porous media so that contamination of hydrocarbon gas by the injected waste gas can be mitigated. Results from laboratory tests conducted with 2 meter long homogeneous sand-pack to investigate factors such as pressure, displacing gas properties, flow rate and gas solubility in water, on gas-gas displacement efficiency were presented previously in the 2008 CIPC conference8. In the present paper, we present results from displacement tests conducted in heterogeneous porous media.
A schematic diagram of the experimental setup for the displacement tests 1–4 is presented in Figure 1. Two sandpacks with different permeabilities of 5.4 and 2.0 D were arranged in parallel to simulate two non-communicating layers of a gas reservoir where the higher permeable layer was located below the lower permeable layer. Properties of the restored sand-packs are presented in Table 1. The equipment and procedure were very similar to those described previously8.
