Enhanced Gas Recovery Potential from CO2 Injection into Depleted Dry Gas Reservoirs

Injection of CO2 into depleted dry gas reservoirs is one of several means of sequestrating CO2 for environmental reasons currently under consideration. Large depleted UKCS dry gas fields have the attraction of a large pore volume available for sequestration, without the need to exceed reservoir pressures maintained over geological time. They also offer the possibility of enhanced gas recovery (EGR) by supporting reservoir pressure late in field life when it may limit recovery. However, such injection schemes may not be directly economic without an environmental value placed on CO2 sequestration, since most dry gas reservoirs achieve very high recovery factors through depletion, so the target for EGR is small. Graham Paterson ( graham.paterson@ecltechnology.com ) of ECL Technology Ltd presents the results from a DTI SHARP programme investigation into EGR from CO2 injection using a simple generic simulation model with representative properties for a southern North Sea (SNS) dry gas field.

Model

The model represents a reservoir (or reservoir compartment) of approximately 250 billion scf. It is a homogeneous model with a permeability of 50mD and areal gridblock dimensions of around 200ft and thicknesses of approximately 40ft. The dip is 3 degrees.

To allow for the possibility of CO2 being in super critical liquid form in the reservoir, compositional simulation was used. A typical SNS gas composition was entered and default EoS parameters were applied to produce an EoS model that could be utilised in the study. The model was initialised with a pressure of 4000 psia and four production wells, two located towards the up-dip region of the model and two located in the centre. These wells were controlled on a group production rate of 60 MMscf/d. Each well has a maximum rate of 40 MMscf/d imposed and a secondary bottom hole pressure limit of 200 psia.

The base case is natural depletion using the well constraints defined above with a simulation period of 20 years. A series of runs were set up in which the two downdip (centrally located) wells were converted to injectors, and CO2 was injected over the whole interval at a rate of 40 MMscf/d. It is worth noting that a very high recovery factor is predicted in the natural depletion case, more than 95% GIIP. Thus the target for EGR was only 4.5% GIIP.

Results

A number of cases were considered in which gas injection started at different times during the simulation period, corresponding to injection start-up at different average field pressures. The results, summarised in Figure 1, indicate that although a small EGR may be possible, if CO2 breakthrough occurs during the plateau period, the hydrocarbon gas flow rate declines and recovery is compromised.

Figure 1: Gas Production Profiles

To focus on more attractive economics, a series of cases was run corresponding to late injection start times at average field pressures of 500 psi, 400 psi and 300 psi. By this stage most of the gas reserves have been produced. The results are summarised in the table below.

In most cases there is potential for a small but positive EGR. Examples of cumulative EGRs (defined as production from a particular case minus production from the natural depletion case) are shown in Figure 2. Note that the EGR curves are negative soon after injection. This is because the number of production wells in the model is reduced from four to two. In general, these results show that a greater EGR is obtained in cases with lower average reservoir pressures and lower CO2 injection rates. This confirms that a CO2 injection scheme is more likely to be favourable if initiated late in field life.

Figure 2: Enhanced Gas Recovery with Injection at an Average Field Pressure of 300psia

Figure 2 also includes the results of a case in which a dedicated injector is included downdip of the original producers (and the four original producers left on line). It can be seen that a comparatively high EGR is predicted (up to about 4.6% GIIP). This corresponds to a case where there would have to be some investment in an additional well (unless an existing shut-in well could be utilised).

The main conclusion of this study is that it may be possible to achieve a positive EGR by CO2 injection into a depleted gas reservoir. It is worth noting, however, that a large throughput of CO2 is required to maximise the EGR. This is sensitive to the detailed reservoir description and additional studies are underway to investigate this.

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