Basic Investigations on Enhanced Gas Recovery by Gas-Gas Displacement

Abstract The paper presents basic data on Enhanced Gas Recovery (EGR) by gas-gas displacement for nearly depleted natural gas reservoirs, by injecting waste gases. The soundness of the concept of gas-gas displacement for enhancing gas recovery was investigated via laboratory investigations, compositional modeling and economic analyses. Paramount Resources is field testing the concept in their GRIPE Project in the Athabasca region of Alberta, to enhance production from a gas bearing stratum overlying the oil sand interval. This paper is a part of a series of papers presenting results of EGR research conducted over a four year period (2003- 2006). The main targets were volumetric (closed) reservoirs, in advanced phases of exploitation. Results of basic research on methane displaced from core samples by pure gases (pure CO2 or pure N2), as well as flue gases (mixtures of CO2 and N2) and a CO2/ SO2 mixture, are presented. A series of 13 gas/gas displacement tests in 30cm-long, 4cm diameter, Berea cores were conducted at a temperature of 700C and a pressure of 6,200 kPa. Most of the tests were conducted in the presence of connate water, while others were conducted without connate water (dry cores) to confirm and bench mark the results by other investigators. The tests on consolidated cores showed that for pure nitrogen and pure CO2, used as the displacing medium, the recovery was comparable. In case a mixture of CO2 and nitrogen displaced the natural gas, it was observed that there was a delay in CO2 breakthrough, associated with a period when only a mixture of methane and nitrogen was produced. This is so because solubility of CO2 in connate water is considerably higher than that of nitrogen. This directly leads to a higher gas recovery due to a longer exploitation period, given the fact that up to 20% nitrogen can be tolerated in the produced stream, as opposed to only 1% for CO2 case. For this period of methane and nitrogen production, there are no operational problems associated with the corrosive nature of CO2. EGR by gas-gas displacement is seen as a promising way of prolonging the productive life and economic recovery of many depleting volumetric gas reservoirs. Introduction Alberta currently has 26,000 gas pools, which are in different stages of exploitation. For these gas reservoirs, the pools that should be considered first in the implementation of large-scale EGR-CO2 and CO2 storage have not yet been identified, and no screening criteria have yet been developed. Currently, only the concept of "disused gas reservoirs" has been advanced1 for CO2 storage. This concept implies that only those reservoirs that are in an advanced stage of depletion (with very little marketable gas left), traditionally with extremely low current pressure, or which are water invaded, should be considered. In this context, they have been considered exclusively for CO2 storage and not for enhanced gas recovery (EGR). This paper brings a new concept: that of simultaneous EGR and CO2 storage.