CO2 Sequestration Through the use of Geomechanics in GEM and WinProp

Tutorial Exercise From SPE Paper 125167-PP

Computer Modelling Group Ltd.

December 2009

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The Objective of this tutorial is to demonstrate how to build and run a CO2 Sequestration model in GEM to simulate the effect of Geomechanics in regards to cap rock leakage. This tutorial is broken into 3 parts: Creating the WinProp fluid model, Building the grid and importation of reservoir properties, and setting up of the Geomechanics module. This tutorial was created from the dataset used in the SPE paper "Geomechanical Risk Mitigation for CO2 Sequestration in Saline Aquifers", SPE paper #125167-PP. This paper and dataset was developed by CMG's David Tran, Vijay Shrivastava, Long Nghiem, and Bruce Kohse. Creating the WinProp Fluid Model

1. Open CMG's Launcher tool and double click to open WinProp PVT and component development tool.

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2. In WinProp double click on the Titles/EOS/Units and enter the comment "2D CO2 Sequestration Component Set". Also Choose the Units to be kPa & deg C. Click OK.

3. Open the Component Selection/Properties Window and under Options in the top of the

window and click Insert Library Component.

4. Select CO2 and C1 and press OK.

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5. In the Component Selection/Properties window go to the Aqueous Phase Tab. In here choose a Henry's Law Constant Correlation of Li-Nghiem's Method for solubility calculations. Press OK to close the window.

6. Open the Composition Window and insert values of 0.001 for CO2 and 0.999 for C1 under the Primary Stream. We enter this large value for C1 as we are using it as a trace component so that the gas phase exists in each grid block and gas phase properties are continuously calculated. Click OK to close the window.

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7. In the top toolbar in WinProp under the Simulator PVT section select CMG GEM EOS Model to add a GEM form to the dataset. Double click on this form to open the CMG GEM EOS model data window and input the values and options as shown below:

8. Once these values are entered press OK to close the window. Save the Dataset as CO2_Sequ_PVT.dat and run it by pressing the Run (Play) button on the top toolbar. This will generate a .gem file which contains the properties that will be used in creation of the dataset.

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Building the Grid and Importation of Parameters 1. In Launcher double click on the Builder icon to open and create a new Builder dataset. 2. In the first window choose the following options and press OK twice to move on:

3. We will start by creating the grid. To do this under the Reservoir Section go to Create Grid and choose Cartesian Grid.

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4. Input the data as shown below and press OK to apply:

5. Click the Specify Properties Button on the main Toolbar and input the following:

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6. Also Specify the Volume Modifier for the Whole Grid to be 1 for both the Matrix and Fracture. After all of this data has been input press OK and Save the dataset.

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7. Since we wish to only apply the fracture properties to the cap rocks for simulation of leakage we want to null the properties over the remainder of the reservoir. To do this, select the NULL Blocks - Fracture from the properties dropdown bar and enter the Edit Reservoir Property Mode by clicking the button on the top toolbar. Click on the top left 1st grid cell and drag and highlight all of the grid cells shown below. When all the proper cell are highlighted release the button. In the popup change the value for these blocks to 0. This Nulls these blocks out from having any fracture properties. Repeat for each of the regions shown.

Nulled Blocks

Not Nulled Blocks

Nulled Blocks

Nulled Blocks

Not Nulled Blocks

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8. We also need to define volume modifiers on some of the boundary blocks to simulate an infinite acting pressure boundary for the reservoir. In the properties dropdown again choose Volume Modifiers. Enter into the Edit Reservoir Property mode if you are not still in it. highlight the blocks 1,1,20 to 1,1,33 on the left side and 411,1,20 to 411,1,33 on the right side. Change the values of these blocks to a very large number, such as 10000000.

9. Lastly in the Reservoir Section we need to setup a sector to define the aquifer in the bottom of the reservoir that we are injecting into. This allows us to see what is going out of the region due to the cap rock leakage more easily. Under Reservoir in the Tree view double click on Sectors to open the Sector window. Create a new Sector called AQUIFER. Select all the layers from Layer 13 to Layer 33 and Press the "Add the blocks in selected regions to the sector" button. Click Apply and OK to close. Normally we would then define an aquifer to have influx into the reservoir, but due to the addition of the volume modified blocks on the edges we can assume that no additional pressure support or influx due to an aquifer is required.

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10. There should only be one warning for the Reservoir section now relating to Rock Compressibility. Ignore this as we will be defining it in detail when setting up the Geomechanics.

11. The next section to define is the Components section. In the first part of this tutorial a WinProp model called CO2_Sequ_PVT.gem was created. This can be directly imported into Builder by going into the Component section and selecting "Import WinProp-Generated Model...". Select the file and Open it. You will see an error regarding Solubility Data (SOLUBILITY) and an error in the Reservoir Section related to Global Component definitions.

12. Double Click on the Solubility Data error in the Tree view and change the Henry's Constant for the C1 component to 0. The Methane is just a trace component and thus we do not consider to be soluble. Leaving it in can lead to errors with improper hydrocarbon fractions since they will become completely soluble in the reservoir aquifer. You will notice the error may still be there. This is due to a known bug that will be fixed in a later release. If the dataset is run it can be noted that there will be no errors.

13. The error in the Reservoir Section will be fixed when we define the Initial Conditions.

14. To define the Rock-Fluid Interactions go to the Rock-Fluid section and create a New Rocktype. Deselect the Capillary Pressure columns and then Copy/Paste the Relative Permeabilities from the Excel file Relative_Perms.xls for the Water-Oil Table and Liquid-Gas Table. Make sure to click the Gas Saturation Liquid-Gas Kr Table dependency prior to pasting into the Liquid-Gas Table.

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15. Go to the Hysteresis Modelling Tab and select Model Hysteresis effect in gas relative permeability, krg, values: and insert a value of 0.4. Click Apply and OK to close this. If an error appears it should be ignored for the time being.

16. Open the Initial Conditions section and set the Block Saturation at each grid block average over the depth interval spanned by the grid block (VERTICAL DEPTH_AVE). Choose Water, Gas (WATER_GAS) as the Gravity-Capillary equilibrium and use the default of Ignoring ALL capillary pressure curves.

17. Click on the Init. Region Parameters tab and enter the following information:

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18. After you click Apply and OK you will notice that the Reservoir Section now only has a warning. The last Initial Conditions parameter we need to define is the Default Separator. Double click on this to open the window and change the Separator Format to Short Format. Next enter Atmospheric pressure (101.325 kPa) and Temperature (15.56 oC) and click Apply and OK.

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19. Under the Numerical Controls input the following values in each respective section:

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20. After this has been setup click Apply and OK to close the window. Skip the Geomechanics portion as we are going to come back to this later. Under the Wells and Recurrent Section Add a New Well. This will be Named Injector with Type INJECTOR.

21. Define an OPERATE Constraint of MAX STG surface gas rate as 200000 m3/day on CONT. Define a second OPERATE constraint of MAX BHP bottom hole pressure of 51710 kPa on CONT.

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22. Also, change the Injected Fluid to SOLVENT with a CO2 Mole Fraction of 1 before pressing OK.

23. Double click on the Well to open the Well Events window. Add a new date at 2005-01-01 and Shut-in the well at this date under the Options Tab.

24. Double click on PERF under the well name in the tree view to open the Perforations window. Add perforations into the following blocks. This can be done by clicking the Mouse Button in the Perforations Tab or by manually entering the block addresses. This second method is recommended due to the refinements and ease of simply typing, rather than trying to zoom and click in the proper box. Also change the Well Index Type to GEO calculated from geometry, isotropic under the General Tab and insert a new Radius of 0.3048 meters.

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25. Lastly, we need to add a range of dates. Open the dates screen and add a range from 2005-01-01 to 2010-01-01 in 1 month increments. Place a STOP at 2010-01-01. The model is now complete except for the addition of the Geomechanics.

Implementing Geomechanics 1. To implement the Geomechanics module go to the Geomechanics section in Builder and

open the Calculation Options window. Change the Finite Element Method Dimension 2D or 3D to 3D by checking the box. Click Apply and OK to close. A warning will appear which should be ignored.

2. Double click on Geomechanical Rock Types in the Geomechanics section to open the window. Add a New Rock Type with the following information:

3. Add another Rock Type and Copy the values from Rock Type 1 to it. Repeat this again for a Third Rock Type but change the Rock Type Model to Mohr-Coulomb for it.

4. Add a Fourth Rock Type with the following values then click Apply and OK:

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5. Go to the Reservoir Section and open the window for Compaction/Dilation Regions. Create a new region with the following values:

6. Create a second region with the same values. Create a 3rd and 4th region with the same values except change the exponents from e-006 to e-005 for both the CCPOR MATRIX and CCPOR FRACTURE.

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7. We need to now define which grid blocks are related to which regions. Go to the Specify Property window and enter the following information for Deformation Rock Type of the Matrix and Fracture and the Rock Compaction Set Number for both the Matrix and Fracture:

Press OK and Save the dataset.

8. The last thing that needs to be done in Builder is to set up the Outputs. Go to the Input/Output Section and open the Simulation Results Output window. Define the first section, When to Write (WSRF), as shown:

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9. In the Lower area, What to write (OUTSRF) define the following Outputs:

10. Click the Select button for the first (Grid) variable. In the popup window deselect all

then check the boxes for the following in order to implement the output of them (shown in order, SG is first and VDISPL is last to on list): SG SW PRES DENG DENW POROS PERM VELORC Z (for CO2) Y (for CO2) W (for CO2) STRESI STRESJ STRESK STRESSH STRESNORM PRMXDIR PRMNDIR STRNMXP STRNMNP VDISPL

11. Click on the other Select button for the Well Special Variables to bring up the value window. Select the values: PAVG GHGGAS GHGLIQ GHGSOL GHGSCRIT GHGTHY

12. Save the Dataset and Close Builder.

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13. This cap rock leakage model is unique in that it utilizes the Barton-Bandis method for calculating the stresses associated with the bending of rock, leading to the opening of fractures that allow leakage to overlying formations. Barton-Bandis is not currently supported by Builder and thus needs to be defined in the .dat file through a text editing software.

14. Open the dataset in Text Editor and search for the Geomechanic Section.

15. Find the model GEOROCK 3 and replace the line "DRUCKER" with the following: ** B-B model E0 Kni FRS Khf Kccf Krcf GPERMBB 1.981e-05 6.786e7 0 233. 233. 33.

This implements the Barton-Bandis model for the 3rd Rock Type. This Rock Type was set to the small Shale Layers between the different Reservoir zones.

16. The Initial Stresses of the reservoir need to be defined. To do this insert the following

data into the Geomechanics section just before the "RUN" Keyword, which denotes the beginning of the Wells and Recurrent Section:

** Initial effective stress in the reservoir ** H stress : horizontal stress on a plane ** V stress : Vertical stress on a plane ** N stress : Normal stress to a plane ** Sxx Syy Szz Sxy Syz Sxz STRESS3D 3447.4 3447.4 6894.8 0 0 0 STRESSGRAD3D -10.4688 -10.4688 -20.9376 0.0 0.0 0.0 ** Use default boundary conditions, i.e, constrained at bottom and sides

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17. After this has all been done your Geomechanics section should look as follows:

18. Go to your Numerical Section in the Text file and add the following line: NCHECK-CEQ 3 This specifies that the simulator will have a maximum of 3 allowable Newtonian iterations for constraint equation residual checking. Setting this number, rather than taking the default of 0, means the simulator will check up to 3 times to ensure a well's constraint is not violated.

19. Save the dataset and Run it in GEM using 1 CPU. Geomechanics is not parallelized and thus will not work on multiple CPU runs. This capability is being further developed.

20. The run will take approximately 13.25 hours. After it is complete open the .irf file in Results 3D for post-analysis. Results are provided with the course materials.

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RESULTS

1. When Results 3D opens choose Gas Mole Fraction (CO2) as the property to be displayed and play the change over time. It can be clearly seen how the gas moves from the injecting well vertically until it hits the cap rock.

In the time it is moving the pressure increases in the reservoir, more so around the region directly above the well, and the reservoir beings to bend. When the stress reaches the edge of its envelope then the rock fractures, causing the gas and fluids to migrate into the upper formations. This process repeats for the second cap rock as well.

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It can be seen that even though injection stops and there is no further increase in pressure the fractures remain open. This is due to the inelastic deformation of the rock not allowing a full closure (which is modelled with the Barton-Bandis method). Therefore, your reservoir will always have the capabilities of leaking to the other formations if the cap rock was breached at all, even though injection may have stopped a long time prior.

2. Other Properties to examine are the Pressure, Porosity-Effective Current, Displacement Along Z, Effective Stresses in the I, J, and K, and Maximum/Minimum Strains.

3. Graphing the Well Bottom-hole Pressure for the Injector reveals at the specific dates that breakthrough occurred. By looking for pressure drops it can be determined when the cap rock was breached, causing a drop in the Reservoir Pressures and respective drop in the Bottom-hole Pressure. This seems to have occurred in 2001-09-25 for the first cap rock and 2003-02-22 for the second cap rock. The other two spikes in the chart correlate to the fracture beginning to close and thus allowing less fluid/gas to migrate. This in-turn causes a rise in the pressure again, until the fracture is reactivated and opens further again, causing more fluid/gas to migrate and once again dropping the pressure. Keep in mind that the fracture will never completely close.