CO2-EGR: Using CO2 to Enhance Gas or Condensate Recovery

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A large number of gas fields in The Netherlands are in decline and within the next decade the production in many fields will cease. End-of-field-life techniques are currently being investigated. One of the options to extend economic field life is enhanced gas recovery (EGR). EGR is a technique to increase the ultimate recovery of natural gas and gas condensate through the injection of carbon dioxide (CO2), or another drive gas like nitrogen, in a gas field. The understanding of the field properties or EGR process parameters that determine the viability of EGR has been limited so far. The objective of this work is to find these success factors of EGR using CO2 as a drive gas and to investigate the potential for this type of utilization of CO2. A parameteric study was done on a realistic model of a gas field in the size range 1–5 bcm, varying a range of field properties (such as depth, GIIP, permeability) and EGR scenario parameters (such as injection rate, timing of the start of injection) to find the parameters that most affect additional recovery or the net present value of the EGR operation. This was done for both dry and wet (condensate) gas fields. For dry gas fields, the key parameters are field size (with larger fields presenting a better case for EGR), permeability (higher additional recovery is obtained in reservoirs of better quality). Typical additional recovery is in the range of 4–12% of the GIIP, depending on the concentration of CO2 that is allowed in the produced gas stream. EGR should not be started before the end of primary recovery; optimum injection rates are typically up to twice the production rate. A higher concentration results in more additional sales gas, at the cost of gas treatment to comply with sales gas specifications. In wet gas fields, best results are obtained when EGR starts as early as possible, to avoid condensate drop out in the reservoir. Although gas treatment requirements after CO2 breakthrough may cause loss of methane production, additional condensate recovery can be as large as 40%.