Case02a Gas depletion followed by CO2 injection using COMP3 flow mode

 

In the current case we simulate gas depletion followed by CO2 injection using the COMP3 flow mode from Pflotran-OGS and solve for the thermal field. Note that the user is assumed to be familiar with Case01 so that only key data exclusive to the current case will be discussed.

 

The files for the current case are provided in PFO_001\Case02a\Data. These comprise:

 

•PFO_001_CASE02A.dat input data file for the geomechanical model (ParaGeo)

•PFO_001_CASE02A.in input data file for the flow model (Pflotran-OGS)

•fluid_data: folder containing data files for CO2 equation of state database and saturation functions which are required for the flow simulation in Pflotran-OGS. These are the same files as in the previous case Case01

•grdecl_data: folder containing the grid data for the flow model previously generated in Case01 Grid Generation.

 

Note that the ParaGeo input data file is identical to that in Case01b so will not be discussed here.

 

 

Pflotran input data (PFO_001_CASE02A.in)

 

Here the key data in the PFO_001_CASE02A.in file that is relevant to the use of the COMP3 model to simulate gas depletion followed by CO2 injection is discussed. Note that for specific details on each of the keywords / cards the user is referenced to the official Pflotran-OGS web page.

 

 

Simulation Block

 

Data File

 

SIMULATION

 SIMULATION_TYPE SUBSURFACE

PROCESS_MODELS

  SUBSURFACE_FLOW Flow

     MODE COMP3

    OPTIONS

      RESERVOIR_DEFAULTS

    /

  /

  GEOMECHANICS_EXTERNAL

          DATA_FILE         PFO_001_CASE02A.dat

          TRANSFER_METHOD   memory

  /

/

END

 

•MODE option within SUBSURFACE_FLOW is set to COMP3, a flow model with Gas, Solvent and Water where the solvent is usually CO2 (optional). It is a generalisation of the GAS_WATER model to enable simulation of CO2 injection into depleted gas reservoirs where:

▪Solvent and Gas components share the reservoir vapour phase

▪Solvent component may dissolve in the aqueous phase

 

 

 

Fluid properties

 

Because of the different flow model used to that in Case01b, additional fluid data needs to be defined. In this case methane will be modelled with the Equation Of State for an ideal gas and CO2 will be considered the solvent.

 

Data File

 

FLUID_PROPERTY

 PHASE LIQUID

 DIFFUSION_COEFFICIENT 2.0d-9

/

 

FLUID_PROPERTY

 PHASE GAS

 DIFFUSION_COEFFICIENT 2.0d-5

/

 

EOS WATER

SURFACE_DENSITY 1000.0 kg/m^3

END

 

EOS GAS

 FORMULA_WEIGHT 16.043d0

 SURFACE_DENSITY 1.0995 kg/m^3

 DENSITY IDEAL

 ENTHALPY IDEAL_METHANE

 VISCOSITY CONSTANT 0.01107 cP

END

 

EOS SOLVENT

 SURFACE_DENSITY 1.0 kg/m^3

 CO2_DATABASE fluid_data/co2_dbase.dat

END

 

•Note that in the current case EOS SOLVENT is used to define the CO2 properties (which were specified under EOS GAS in Case01b). However the same CO2 database file as in the previous cases is input.

 

•EOS GAS is used to define methane properties, which will be modelled as an ideal gas.

 

•Appropriate properties for methane are defined in the different option cards.

 

 

 

 

 

Equilibration

 

When using the COMP3 model instead of the GAS_WATER model, additional data is required in EQUILIBRATION. These include the water-gas phase contact depth and the capillary pressure between gas and water at that depth.

 

Data File

 

EQUILIBRATION

 PRESSURE 13.5 MPa

 DATUM_D  1350 m

 WGC_D    1500 m

 PCWG_WGC 0.0    Bar

TEMPERATURE_TABLE

   D_UNITS m

   TEMPERATURE_UNITS C

  RTEMPVD

     0.0     10.0

     1500.0  62.5

  /

END

/

 

•WGC_D defines the Water-Gas contact depth. It is set to 1500 m (below the reservoir bottom surface) so that the modelled reservoir is full of gas.

 

•PCWG_WGC defines the capillary pressure between gas and water at the Water-Gas contact depth.

 

 

 

Wells

 

An additional WELL_DATA definition is required to accommodate the two types of wells (producer and injector). However they are connected to the same cells and they are defined to operate in sequence (i.e. they are not simultaneously active). Consequently they may physically represent the same well in the field.

 

Data File

 

WELL_DATA prod

 CIJK_D   1 1 3 4

 WELL_TYPE PRODUCER

 THETA_FRACTION 0.25

 BHPL 5 MPa

 SHUT

 TIME 1 y

 OPEN

 TARG_GM 0.1 Mt/year

 TIME 21 y

 SHUT

END

 

WELL_DATA injg

 CIJK_D   1 1 3 4

 WELL_TYPE SOLVENT_INJECTOR

 INJECTION_ENTHALPY_P  14.72 MPa

 INJECTION_ENTHALPY_T  15.5 C

 THETA_FRACTION 0.25

 BHPL 25 MPa

 SHUT

 TIME 25 y

 OPEN

 TARG_SM 0.25 Mt/year

 TIME 45 y

 SHUT

END

 

•The WELL_TYPE that operates during gas production is set to PRODUCER

 

•The WELL_TYPE that operates during CO2 injection is set to SOLVENT_INJECTOR

 

•Note that two sets of WELL_DATA are required to accommodate both well types but they represent the same well in the field as:

 

▪They are connected to the same cells (CIJK_D input values are the same)

 

▪They operate in sequence so that they are not active at the same time

 

 

 

 

 

Results

 

The results discussed here consist of the hdf format ParaGeo output results which are provided in PFO_001\Case02a\Results\PFO_001_CASE02A_parageo_results. Note that as the user is assumed to be familiar with Case01 the different result types being output by Pflotran-OGS or the Eclipse format results from ParaGeo will not be discussed.

 

The next two figures below show contour plots for gas saturation, methane gas mass fraction, CO2 solvent mass fraction and pore pressure with principal stress directions and magnitudes indicated by the coloured arrows after 20 years of gas depletion and after 20 years of CO2 injection respectively. Note the change in the directions of the most tensile principal stress (principal stress 1) and the intermediate principal stress (principal stress 2) during those operations. The most compressive principal stress (principal stress 3) remains aligned with the vertical axis throughout the simulation.

 

PFO_001_Case02a_1

Contour plots for gas saturation (a), methane gas mass fraction (b) CO2 solvent mass fraction (c) and pore pressure with principal stress directions and magnitudes indicated by the coloured arrows (d, e, f) after 20 years of gas depletion (t =21 years)

 

 

 

PFO_001_Case02a_2

Contour plots for gas saturation (a), methane gas mass fraction (b) CO2 solvent mass fraction (c) and pore pressure with principal stress directions and magnitudes indicated by the coloured arrows (d, e, f) after 20 years of CO2 injection (t =45 years)

 

 

The figure below shows the evolution of different properties obtained from selecting the target cells that will be interrogated and performing a "Plot Selection Over Time" filter. Note how the vertical displacement in the top surface cell above the well (a) is consistent with the evolution of pore pressure in the bottom hole (c). In figure (d) notice how during depletion the gas is slowly being replaced by water in the bottom hole, and then both components are displaced by CO2 at the same location during injection.

 

PFO_001_Case02a_3

Evolution of vertical displacement at top surface in a cell aligned with well location (a). Evolution of element temperature (b), pore pressure (c) and the three components mass fractions (d) in a well cell