Simulation of Geological Storage of Carbon Dioxide in ... - CO2CRC

Simulation of Geological Storage ofCarbon Dioxide in the Onshore Perth Basin(Appendix 7 of report no. RPT06-0162)Ennis-King, J. 1 and Wu, G. 11 CSIRO ClaytonOctober 2005, CO2CRC report no. RPT05-0087

Simulation of Geological Storage of CarbonDioxide in the Onshore Perth BasinJ. Ennis-King and G. WuCRC for Greenhouse Gas Technologies,CSIRO Petroleum,Private Bag 10, Clayton South, VIC 3169CO2CRC Report No.: RPT05–0087October 7, 2005

Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC)GPO Box 463Level 3, 24 Marcus Clarke StreetCANBERRA ACT 2601Phone: +61 2 6200 3366Fax: +61 2 6230 0448Email: pjcook@co2crc.com.auWeb: www.co2crc.com.auReference: Ennis-King, J, Wu, G, 2005. Simulation of Geological Storage ofCarbon Dioxide in the Onshore Perth Basin. CSIRO Petroleum. CO2CRCReport No. RPT05-0087c○CO2CRC 2005Unless otherwise specified, the Cooperative Research Centre for GreenhouseGas Technologies (CO2CRC) retains copyright over this publicationthrough its commercial arm, Innovative Carbon Technologies Pty Ltd. Youmust not reproduce, distribute, publish, copy, transfer or commercially exploitany information contained in this publication that would be an infringementof any copyright, patent, trademark, design or other intellectual propertyright. Requests and inquiries concerning copyright should be addressedto the Communication Manager, CO2CRC, GPO Box 463, CANBERRA,ACT, 2601. Telephone: +61 2 6200 3366.

Contents1 Executive Summary 12 Introduction 23 Simulation capabilities 34 Simulation model 44.1 Absolute permeability . . . . . . . . . . . . . . . . . . . . . . 44.2 Vertical permeability . . . . . . . . . . . . . . . . . . . . . . . 44.3 Relative permeability . . . . . . . . . . . . . . . . . . . . . . . 54.4 Temperature, pressure and salinity . . . . . . . . . . . . . . . 54.5 Model parameters . . . . . . . . . . . . . . . . . . . . . . . . . 55 Results 75.1 Radial grid with effective vertical permeability . . . . . . . . . 75.2 3D grid with explicit flow barriers . . . . . . . . . . . . . . . . 116 Conclusions 24i

List of Figures1 Vertical cross-section of gas saturation in the base case for theradial model after 20 years of injection at 1 million tonnes peryear. The radial distance is 1000m. The top of the modelcorresponds to a depth of 1200m, and the bottom to 2000m.Red corresponds to high gas saturation and blue to zero gassaturation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 Vertical cross-section of gas saturation in the base case for theradial model 180 years after the end of the injection phase.The radial distance is 1000m. The top of the model correspondsto a depth of 1200m, and the bottom to 2000m. Redcorresponds to high gas saturation and blue to zero gas saturation.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 Vertical cross-section of gas saturation in the base case for theradial model 1980 years after the end of the injection phase.The radial distance is 1000m. The top of the model correspondsto a depth of 1200m, and the bottom to 2000m. Redcorresponds to high gas saturation and blue to zero gas saturation.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 Vertical cross-section of gas saturation in the high permeabilitycase for the radial model after 20 years of injection at 1million tonnes per year. The radial distance is 1000m. Thetop of the model corresponds to a depth of 1200m, and thebottom to 2000m. Red corresponds to high gas saturationand blue to zero gas saturation. . . . . . . . . . . . . . . . . . 95 Vertical cross-section of gas saturation in the high permeabilitycase for the radial model 180 years after the end of theinjection phase. The radial distance is 1000m. The top of themodel corresponds to a depth of 1200m, and the bottom to2000m. Red corresponds to high gas saturation and blue tozero gas saturation. . . . . . . . . . . . . . . . . . . . . . . . . 106 Vertical cross-section of gas saturation in the high permeabilitycase for the radial model 1980 years after the end of theinjection phase. The radial distance is 1000m. The top of themodel corresponds to a depth of 1200m, and the bottom to2000m. Red corresponds to high gas saturation and blue tozero gas saturation. . . . . . . . . . . . . . . . . . . . . . . . . 10ii

7 Vertical cross-section of one realisation of the stochastic distributionof shales for a 3D grid. The lateral width is 3000m.The shale units are 200m by 100m laterally. The top of themodel corresponds to a depth of 1200m, and the bottom to2000m. The shales are in red, the Cattamarra formation is inyellow, the Cadda is in olive, and the Yarragadee is in green. . 118 Horizontal cross-section at a depth of 1710m (bottom of theCadda formation) of one realisation of the stochastic distributionof shales for a 3D grid. The lateral width is 3000m inboth directions. The shale units are 200m by 100m laterally.The shales are in red . . . . . . . . . . . . . . . . . . . . . . . 129 Vertical cross-section of gas saturation for a 3D grid with astochastic distribution of flow barriers after 20 years of injectionat 1 million tonnes per year. The lateral width is 3000m.The shale units are 200m by 100m laterally. The top of themodel corresponds to a depth of 1200m, and the bottom to2000m. Red corresponds to high gas saturation and blue tozero gas saturation. . . . . . . . . . . . . . . . . . . . . . . . . 1310 Horizontal cross-section at a depth of 1710m of gas saturationfor a 3D grid with a stochastic distribution of flow barriersafter 20 years of injection at 1 million tonnes per year. Thelateral width is 3000m. The shale units are 200m by 100mlaterally. Red corresponds to high gas saturation and blue tozero gas saturation. . . . . . . . . . . . . . . . . . . . . . . . . 1411 Vertical cross-section of gas saturation for a 3D grid with astochastic distribution of flow barriers 180 years after the endof injection. The lateral width is 3000m. The shale units are200m by 100m laterally. The top of the model corresponds toa depth of 1200m, and the bottom to 2000m. Red correspondsto high gas saturation and blue to zero gas saturation. . . . . 1512 Horizontal cross-section at a depth of 1510m of gas saturationfor a 3D grid with a stochastic distribution of flow barriers at200 years after 20 years of injection at 1 million tonnes peryear. The lateral width is 3000m. The shale units are 200mby 100m laterally. Red corresponds to high gas saturation andblue to zero gas saturation. . . . . . . . . . . . . . . . . . . . . 16iii

13 Vertical cross-section of gas saturation for a 3D grid with astochastic distribution of flow barriers 920 years after the endof injection. The lateral width is 3000m. The shale units are200m by 100m laterally. The top of the model corresponds toa depth of 1200m, and the bottom to 2000m. Red correspondsto high gas saturation and blue to zero gas saturation. . . . . 1714 Vertical cross-section of gas saturation for a 3D grid with astochastic distribution of flow barriers 180 years after the endof injection. The ratio of vertical to horizontal permeabilityin the sand units is 0.5. The lateral width is 3000m. Theshale units are 200m by 100m laterally. The top of the modelcorresponds to a depth of 1200m, and the bottom to 2000m.Red corresponds to high gas saturation and blue to zero gassaturation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1715 Horizontal cross-section at a depth of 1510m of gas saturationfor a 3D grid with a stochastic distribution of flow barriersafter 20 years of injection at 1 million tonnes per year. Theratio of vertical to horizontal permeability in the sand units is0.5. The lateral width is 3000m. The shale units are 200m by100m laterally. Red corresponds to high gas saturation andblue to zero gas saturation. . . . . . . . . . . . . . . . . . . . . 1816 Vertical cross-section of gas saturation for a 3D grid with astochastic distribution of flow barriers after 20 years of injectionat 1 million tonnes per year. The lateral width is 3000m.The shale units are 500m by 300m laterally. The top of themodel corresponds to a depth of 1200m, and the bottom to2000m. Red corresponds to high gas saturation and blue tozero gas saturation. . . . . . . . . . . . . . . . . . . . . . . . . 1917 Vertical cross-section of gas saturation for a 3D grid with astochastic distribution of flow barriers 40 years after the endof injection. The lateral width is 3000m. The shale units are500m by 300m laterally. The top of the model corresponds toa depth of 1200m, and the bottom to 2000m. Red correspondsto high gas saturation and blue to zero gas saturation. . . . . 2018 Horizontal cross-section at a depth of 1730m of gas saturationfor a 3D grid with a stochastic distribution of flow barriers 40years after the end of injection. The lateral width is 3000m.The shale units are 500m by 300m laterally. Red correspondsto high gas saturation and blue to zero gas saturation. . . . . 21iv

19 Vertical cross-section of gas saturation for a 3D grid with astochastic distribution of flow barriers after 20 years of injectionat 4 million tonnes per year. The lateral width is 3000m.The shale units are 200m by 100m laterally. The top of themodel corresponds to a depth of 1200m, and the bottom to2000m. Red corresponds to high gas saturation and blue tozero gas saturation. . . . . . . . . . . . . . . . . . . . . . . . . 2220 Horizontal cross-section at a depth of 1710m of gas saturationfor a 3D grid with a stochastic distribution of flow barriersafter 20 years of injection at 4 million tonnes per year. Thelateral width is 3000m. The shale units are 200m by 100mlaterally. Red corresponds to high gas saturation and blue tozero gas saturation. . . . . . . . . . . . . . . . . . . . . . . . . 23List of Tables1 Porosity, Permeability and depth for each formation in themodel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 Ratio of vertical to horizontal permeability for each formationin the model . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6v

1 Executive SummaryNumerical simulations of the onshore Perth Basin were undertaken to assessthe suitability of the site for the underground storage of carbon dioxide.Given the absence of a regional seal between the injection location and theoverlying fresh water, simulations indicated a risk of the injected carbondioxide coming into contact with the fresh water, although the time for thisoccur might be long. Thus the site examined is unlikely to be suitable forunderground storage.1

2 IntroductionUnderground storage of carbon dioxide as a means of reducing atmosphericemissions of CO 2 has been examined both theoretically and practically overthe last decade (see Ennis-King and Paterson (2002) for a review of the recentliterature). There is also a large amount of relevant research and field experiencein the energy sector e.g. natural gas storage in deep saline formations,carbon dioxide in enhanced oil recovery, and acid gas injection. It is clearfrom this experience that underground storage of CO 2 is eminently feasible,and in fact StatOil is currently sequestering around a million tonnes per yearinto the Utsira formation below the North Sea (Kongsjorden et al., 1997).Other pilot scale field tests are currently occurring or are in preparation(Hovorka and Knox, 2002; Pawar et al., 2001)In order to assess the suitability of a particular site for storage of carbondioxide, it is necessary to have an adequate geological model, and then touse this as the basis for a numerical simulation which can predict the behaviourof the injected carbon dioxide in the subsurface. Unlike a typicaloil-field development, the model needs to be simulated not only during theinjection phase, but also for at least a few thousand years after injection, inorder to check on the storage integrity. In this report, geological informationsupplied by Rick Causebrook and Tess Dance of Geoscience Australiawas used to construct a simple simulation model of part of the onshore PerthBasin, discussed in section 4. The capabilities of the simulator and validationagainst other simulation codes is discussed in section 3. Section 5 presentsan analysis of the results, and conclusions are given in section 6.2

3 Simulation capabilitiesThe simulation code used for this study was TOUGH2 Version 2.0, developedby Karsten Pruess and associates at the Earth Sciences Division ofLawrence Berkley National Laboratories, California, USA (Pruess et al.,1999). TOUGH2 can handle non-isothermal, multicomponent, multiphaseflow. It was chiefly developed for geothermal reservoir engineering, hydrology,and nuclear waste disposal, but has recently been applied by severalgroups to the underground disposal of carbon dioxide. A special module forTOUGH2 has been developed, which contains a high accuracy equation ofstate for carbon dioxide, and improved representations for the solubility ofcarbon dioxide in brine. This module, based on the standard EWASG module(Battistelli et al., 1997) allows for a full treatment of brines, including saltdeposition, and for the solubility of water in carbon dioxide. Thus formationdry-out can occur near the wellbore i.e. the injection of dry carbon dioxidecauses evaporation of residual water (and salt deposition) close to the wellbore,so that eventually the residual water saturation there falls to zero. Themodule also accounts for the increased density of the formation water upondissolution of carbon dioxide, an effect which drives the long-term dissolutionof carbon dioxide in the subsurface (Lindeberg and Wessel-Berg, 1997;Ennis-King and Paterson, 2003ab).The international code comparison project on underground storage ofcarbon dioxide (Pruess et al., 2002ab) provided a useful means of assessingthe performance of various simulation packages for the task. The codesincluded commercial offerings such as Eclipse and CMG-GEM. It was foundthat operator error was responsible for the most serious differences betweensimulation results. When this was corrected, differences then seemed tocentre on the use of different formulations for carbon dioxide properties.TOUGH2, when used properly, was found to compare well to a range ofother codes, and this confirms its suitability for the current project.3

4 Simulation model4.1 Absolute permeabilityAs is common in sites with few wells, there was not a large amount of permeabilitydata available. The supplied data was analysed in two ways. Firstof all, arithmetic averages were taken of the permeability measurements ineach formation, and a 90 % confidence interval was estimated for this meanvalue. This gives quite a wide range of permeability values. The secondapproach was to attempt to correlate permeability against the correspondingporosity values. Inserting the average porosity in this correlation then givesa measure of the average permeability. This second technique tends to givelower values of the permeability due to the fitting process. In both cases thenet-to-gross values (based on proportion of shale) were taken into account inupscaling the permeability to the whole formation.4.2 Vertical permeabilityThe proposed injection strategy for this location was to inject deep, and usethe overlying stratigraphy to retard and perhaps trap the upward motionof the carbon dioxide due to buoyancy. Previous experience indicates thatthe estimation of vertical permeability is one of the key uncertainties in themodelling process. Two approaches are considered here.The first approach uses an upscaling technique due to Begg and King(Beggand King, 1985). Given a random distribution of thin lateral flow barrierswith a certain volume fraction, and fixed lateral extent, the average verticalpermeability can be computed from the following analytical formula:k vk h=1 − F s(1 + sl/3) 2 (1)where F s is the volume fraction of shale, s is the average vertical frequency ofshales, l is the lateral extent of each shale, and k v and k h are the vertical andhorizontal permeabilities. F s and s can be estimated from log information.Estimation of l depends on an understanding of the depositional environment.For this site, a width/depth ratio of 8-20 was estimated for the shales, givingan estimated lateral extent of 80-500m, with the lateral shape expected tobe elongated. There is an additional factor to be considered of the ratio ofvertical to horizontal permeability (at a fine scale) of sand units, which istaken here to be between 0.2 and 0.5.The outcome of this first approach is an effective vertical permeability foreach interval. In this kind of model, the carbon dioxide rises vertically from4

the injection area (as well as spreading laterally). However the absence ofactual flow barriers probably underestimates the potential for trapping alongthe flow path, since the residual trapping mechanism will be more effectiveon a longer flow path.The second approach is that of object modelling, where one explicitly generatesstochastic realisations of shale distributions which meet the criterionof total shale fraction. The advantage of this technique is that it captures thelengthening of the flow path due to the tortuous vertical migration, allowingfor greater trapping at residual gas saturation.4.3 Relative permeabilityThe choice of relative permeability curves used to model two phase flow ofcarbon dioxide and water is known to have an important influence on thelong-term distribution of carbon dioxide, especially when vertical migrationoccurs. One of the critical parameters is the residual gas saturation on imbibition.Literature data (Suzanne et al., 2003) indicates a wide range ofvalues, between 5 % and 40 % can occur, depending on the local pore structure.No core flood measurements were available at this site, so guided bywork done in other basins, the base case used here is a residual gas saturationof 20 %, with one variant case using 30 %.4.4 Temperature, pressure and salinityUsing a mean surface temperature of 15 C, a fit to the temperature datayielded a gradient of 23.6 C/km. In the absence of extensive pressure data, atypical hydrostatic pressure gradient was assumed. Well completion reportsindicated a salinity of 30,000 ppm below 1340m depth, with a transitionbetween 1340m and 1160m depth to a salinity of 1000 ppm.4.5 Model parametersUsing the techniques discussed above for analysing the geological data, thefollowing model parameters were obtained for the simulation model. For thepurposes of this simulation, the formations are assumed to horizontal andof uniform thickness across the area of the injection site. Table 1 gives thedepth, the average porosity, and the ranges of permeability for each formation,while Table 2 gives the estimates of the ratio of vertical to horizontalpermeability, as this is used in the model with effective vertical permeabilityrather than explicit shale barriers.5

Table 1: Porosity, Permeability and depth for each formation in the modelFormation Depth (mSS) Porosity (% ) Perm. (mD) low/av./highYarragadee (Upper) 313-1019 26.8 238/814/1390Yarragadee (Lower) 1019-1725 20.5 7.7/58.5/109Cadda 1725-1914 18.5 119/526/932Cattamarra 1914- 20.7 1.0/8.5/24Table 2: Ratio of vertical to horizontal permeability for each formation inthe modelFormationk v /k h low/average/highYarragadee (Upper) 0.0038/0.018/0.045Yarragadee (Lower) 0.0031/0.016/0.039Cadda 0.0028/0.013/0.034Cattamarra 0.0092/0.026/0.075The injection rate in the base case is taken to be 1 million tonnes of carbondioxide per year for 20 years (about the scale of the Sleipner project), as amedium scale storage operation. The injection strategy is to inject as deepas possible, but with the constraint that adequate permeability is needed togive sufficient injectivity. Since the average permeability of the Cattamarrais too low for sufficient injectivity with a single well, the Cadda was chosenfor injection, at a depth of around 1760-1900m with a vertical well.The first model (with an effective vertical permeability) was carried outon a radial grid with 95 blocks in the radial direction and 110 blocks of 20mvertically (10450 blocks in all), extending down from 310m (the top of theYarragadee) down to 2510m, into the Cattamarra, and out to 3000m radially.The second model (with explicit flow barriers) was a 3D grid, with 31blocks of 100m in each lateral direction, and 100 blocks vertically, for a totalof 96100 blocks. The vertical extent was from 310m down to 2310m, againinto the Cattamarra.6

Figure 1: Vertical cross-section of gas saturation in the base case for theradial model after 20 years of injection at 1 million tonnes per year. Theradial distance is 1000m. The top of the model corresponds to a depth of1200m, and the bottom to 2000m. Red corresponds to high gas saturationand blue to zero gas saturation.5 Results5.1 Radial grid with effective vertical permeabilityThe base case will be the average values of the absolute permeability andthe average values of the ratio of vertical to horizontal permeability in eachformation. For this base case, Figure 1 shows a vertical cross-section of thedistribution of gas saturation in the radial model after 20 years of injection.The pressure gradient due to injection causes the carbon dioxide to spreadout laterally much more than vertically, since the vertical permeability isonly a small fraction of the horizontal permeability. Thus most of the carbondioxide is still contained in the Cadda formation at this stage.After injection ceases, the driving force is now buoyancy, and the injectedcarbon dioxide rises vertically. Figure 2 shows the distribution of gas saturation180 years after the end of the injection phase. There is slow verticalmigration into the lower part of the overlying Yarragadee formation. Residualgas trapping will reduce the amount of mobile carbon dioxide, eventuallystopping the vertical migration. Fig. 3 shows the result 1980 years afterthe end of the injection phase. In this case the vertical motion of the carbondioxide has stopped well below the fresh water interface that is around 1200mdepth.7

Figure 2: Vertical cross-section of gas saturation in the base case for the radialmodel 180 years after the end of the injection phase. The radial distance is1000m. The top of the model corresponds to a depth of 1200m, and thebottom to 2000m. Red corresponds to high gas saturation and blue to zerogas saturation.Figure 3: Vertical cross-section of gas saturation in the base case for theradial model 1980 years after the end of the injection phase. The radialdistance is 1000m. The top of the model corresponds to a depth of 1200m,and the bottom to 2000m. Red corresponds to high gas saturation and blueto zero gas saturation.8

Figure 4: Vertical cross-section of gas saturation in the high permeabilitycase for the radial model after 20 years of injection at 1 million tonnes peryear. The radial distance is 1000m. The top of the model corresponds toa depth of 1200m, and the bottom to 2000m. Red corresponds to high gassaturation and blue to zero gas saturation.However the uncertainty in the permeability estimation both horizontallyand vertically presents a risk that the base case may be far from reality. Acontrasting case is to use the high estimates of horizontal permeability, combinedwith a value of 0.1 for the ratio of vertical to horizontal permeability.Fig. 4 shows the gas saturation in this case at the end of the injection phase.Comparing to Fig. 1, the lateral extent is greater due to the higher absolutepermeability, and the vertical migration is well into the lower part of theYarragadee formation. When 200 years has elapsed since the start of injection,the carbon dioxide has made significant vertical progress, as indicatedby Fig. 5, where gas saturation is seen in the upper part of the Yarragadeeformation. At this stage, only a small proportion of the carbon dioxide isstill mobile, as evidenced by Fig. 6, which shows that after 2000 years, thecarbon dioxide has not moved much further upwards than it had a 200 years.Nevertheless, in this high permeability case (which from the point of viewof containment, is a “worst case”), the carbon dioxide is much closer to thefresh water interface, and this indicates a greater risk for loss of containmentor undesirable effects on the potable water in overlying aquifers.9

Figure 5: Vertical cross-section of gas saturation in the high permeabilitycase for the radial model 180 years after the end of the injection phase. Theradial distance is 1000m. The top of the model corresponds to a depth of1200m, and the bottom to 2000m. Red corresponds to high gas saturationand blue to zero gas saturation.Figure 6: Vertical cross-section of gas saturation in the high permeabilitycase for the radial model 1980 years after the end of the injection phase. Theradial distance is 1000m. The top of the model corresponds to a depth of1200m, and the bottom to 2000m. Red corresponds to high gas saturationand blue to zero gas saturation.10

Figure 7: Vertical cross-section of one realisation of the stochastic distributionof shales for a 3D grid. The lateral width is 3000m. The shale unitsare 200m by 100m laterally. The top of the model corresponds to a depthof 1200m, and the bottom to 2000m. The shales are in red, the Cattamarraformation is in yellow, the Cadda is in olive, and the Yarragadee is in green.5.2 3D grid with explicit flow barriersThe use of the effective vertical permeability in the radial case has somedrawbacks in this particular setting. The derivation of the vertical permeabilityis based on an average vertical flow through a distribution of barriers.However in this case, the interest is more in the breakthrough time i.e. thetime elapsed before carbon dioxide reaches the vicinity of the fresh wateraquifers.The first case considered uses a shale distribution based on a net-to-grosstarget of 0.4, with shale widths being 200m in one direction and 100m in theother, and thicknesses being 20m (due to the grid coarseness, this correspondsto 2 by 1 grid laterally and 1 grid block vertically). Mathematically the shalesare taken to be ellipsoids, so they protrude into the grid blocks immediatelyabove and below the layer in which the centre of the shale is located. Fig.7 shows a snapshot of a vertical cross-section through one realisation of thedistribution. It can seen that the connectivity of the shales means that flowpaths must go out of the plane of the cross-section to connect all the sandunits. A horizontal cross-section is shown in Fig. 8 at a depth of 1710m.The accompanying file small_shales.mpg contains an animation of scanningthrough the successive layers of the simulation grid, showing the shales inred.11

Figure 8: Horizontal cross-section at a depth of 1710m (bottom of the Caddaformation) of one realisation of the stochastic distribution of shales for a 3Dgrid. The lateral width is 3000m in both directions. The shale units are200m by 100m laterally. The shales are in red12

Figure 9: Vertical cross-section of gas saturation for a 3D grid with a stochasticdistribution of flow barriers after 20 years of injection at 1 million tonnesper year. The lateral width is 3000m. The shale units are 200m by 100mlaterally. The top of the model corresponds to a depth of 1200m, and thebottom to 2000m. Red corresponds to high gas saturation and blue to zerogas saturation.It is assumed that for the sand units the ratio of vertical to horizontalpermeability is 0.2. This takes into account smaller flow barriers that mayexist within the sand units.Fig. 9 shows a vertical cross-section of the gas saturation after 20 yearsof injection at 1 million tonnes per year. The effect of the shale barriers onthe migration path is clear, with the carbon dioxide being forced laterally,as well as making its way into the Yarragadee formation. A lateral crosssectionat a depth of 1710 m (just above the injection interval) is shown inFig. 10. Again the effect of the shale distribution on the migration pathcan be seen, and the bias in the lateral orientation of shales has an obviouseffect on the lateral spread of the carbon dioxide. The accompanying filesp3_smallshales_vertical_0to20yr.mpg andp3_smallshales_lateral1710m_injection.mpg contain animations of thevertical and lateral spread of the carbon dioxide respectively.As before, in the post-injection phase the carbon dioxide migrates upwards,leaving some behind trapped at the residual gas saturation. Fig. 11shows that the carbon dioxide has migrated a considerable vertical distanceinto the Yarragadee formation. A horizontal cross-section of the gas saturationat a depth of 1510 m is shown in Fig. 12. It can be seen that the carbondioxide has found a few migration paths near the centre of the grid. As it well13

Figure 10: Horizontal cross-section at a depth of 1710m of gas saturationfor a 3D grid with a stochastic distribution of flow barriers after 20 years ofinjection at 1 million tonnes per year. The lateral width is 3000m. The shaleunits are 200m by 100m laterally. Red corresponds to high gas saturationand blue to zero gas saturation.14

Figure 11: Vertical cross-section of gas saturation for a 3D grid with astochastic distribution of flow barriers 180 years after the end of injection.The lateral width is 3000m. The shale units are 200m by 100m laterally. Thetop of the model corresponds to a depth of 1200m, and the bottom to 2000m.Red corresponds to high gas saturation and blue to zero gas saturation.known in secondary oil migration, the fact that the carbon dioxide followsrelatively narrow migration paths reduces the effect of residual gas trapping.The accompanying files p3_smallshales_vertical_20to200yr.mpgand p3_smallshales_lateral1510m_20to200yr.mpg contain animations ofthe vertical and lateral spread of the carbon dioxide in the 180 years afterinjection ceases.A final snapshot of the distribution of gas saturation 920 years afterinjection ceases (Fig. 13) indicates that vertical migration has continuedslowly, with carbon dioxide draining upwards along the established migrationpaths. At this stage the carbon dioxide has not reached the level of the freshwater.In the case just examined it was assumed that the ratio of vertical tohorizontal permeability in the sand units was 0.2. What happens if this ratiohas a higher value of 0.5? Fig. 14 shows the vertical cross-section after200 years (compare Fig. 11) and Fig. 15 the lateral cross-section at 1510mdepth (compare Fig. 12). The effect of the greater vertical permeability,while retaining the same distribution of flow barriers, is that the carbondioxide follows the same paths but does so more rapidly. After 200 years,in the vertical cross-section the carbon dioxide is well into the Yarragadeeformation.The aspect ratio of the shale units can only be estimated roughly from15

Figure 12: Horizontal cross-section at a depth of 1510m of gas saturation fora 3D grid with a stochastic distribution of flow barriers at 200 years after 20years of injection at 1 million tonnes per year. The lateral width is 3000m.The shale units are 200m by 100m laterally. Red corresponds to high gassaturation and blue to zero gas saturation.16

Figure 13: Vertical cross-section of gas saturation for a 3D grid with astochastic distribution of flow barriers 920 years after the end of injection.The lateral width is 3000m. The shale units are 200m by 100m laterally. Thetop of the model corresponds to a depth of 1200m, and the bottom to 2000m.Red corresponds to high gas saturation and blue to zero gas saturation.Figure 14: Vertical cross-section of gas saturation for a 3D grid with astochastic distribution of flow barriers 180 years after the end of injection.The ratio of vertical to horizontal permeability in the sand units is 0.5. Thelateral width is 3000m. The shale units are 200m by 100m laterally. The topof the model corresponds to a depth of 1200m, and the bottom to 2000m.Red corresponds to high gas saturation and blue to zero gas saturation.17

Figure 15: Horizontal cross-section at a depth of 1510m of gas saturationfor a 3D grid with a stochastic distribution of flow barriers after 20 yearsof injection at 1 million tonnes per year. The ratio of vertical to horizontalpermeability in the sand units is 0.5. The lateral width is 3000m. The shaleunits are 200m by 100m laterally. Red corresponds to high gas saturationand blue to zero gas saturation.18

Figure 16: Vertical cross-section of gas saturation for a 3D grid with astochastic distribution of flow barriers after 20 years of injection at 1 milliontonnes per year. The lateral width is 3000m. The shale units are 500m by300m laterally. The top of the model corresponds to a depth of 1200m, andthe bottom to 2000m. Red corresponds to high gas saturation and blue tozero gas saturation.a presumed knowledge of the depositional environment. In the base case,lateral dimensions of 200m x 100m were assumed. What happens if theshales are more laterally extensive, while maintaining the same overall shalevolume fraction? In this case, the shales are assumed to be 500m long and300m wide (i.e. 5 grid blocks by 3 grid blocks), but still one grid block thick(20m).The accompanying file large_shales.mpg contains an animation ofscanning through successive layers of the shale distribution in the grid (theshale units are in red).Fig. 16 shows a vertical profile of the distribution of gas saturation at theend of the injection phase. The larger lateral extent of the flow barriers forcesthe carbon dioxide to migrate further horizontally before it finds a verticalpathway. Eq. (1) indicates that effective vertical permeability can be verysensitive to the lateral dimension l of the flow barriers. The accompanyingfiles p5_largeshales_injection.mpg andp5_largeshales_injection_lateral.mpg contain animations of the evolutionof gas saturation in vertical and horizontal cross-sections respectivelyduring the injection phase (the horizontal section is at a depth of 1730m).Fig. 17 shows a vertical cross-section of the distribution of gas saturation40 years after the end of injection. Comparing to Fig. 16, it can be seen thatthe carbon dioxide has largely moved along existing migration paths, and19

Figure 17: Vertical cross-section of gas saturation for a 3D grid with astochastic distribution of flow barriers 40 years after the end of injection.The lateral width is 3000m. The shale units are 500m by 300m laterally.The top of the model corresponds to a depth of 1200m, and the bottom to2000m. Red corresponds to high gas saturation and blue to zero gas saturation.has not made a great deal of vertical progress. The horizontal cross-sectionat 1730m in Fig. 18 indicates how the “chimneys” of upward moving carbondioxide are larger and more continuous than was the case for small shaleunits. This tends to increase the effect of residual gas trapping, compared tothe thin tortuous pathways in the latter case.The effect of increasing the injection rate from 1 million tonnes per yearto 4 million tonnes per year can be seen in Fig. 19, which gives a verticalcross-section of the distribution of gas saturation at the end of the injectionphase. This should be compared with Fig. 9. The higher injection rateresults in a much greater rate of vertical migration even during the injectionphase. In addition to the increased rate, the carbon dioxide will also migratea greater vertical distance before it becomes trapped as residual gas. In thiscase it is very likely to reach the fresh water aquifers in a short time. Ahorizontal cross-section at a depth of 1710m at the same time (the end ofthe injection phase) shown in Fig. 20 demonstrates how the greater injectionrate causes a greater lateral spread of the plume (indeed to the edges of thegrid).20

Figure 18: Horizontal cross-section at a depth of 1730m of gas saturation fora 3D grid with a stochastic distribution of flow barriers 40 years after theend of injection. The lateral width is 3000m. The shale units are 500m by300m laterally. Red corresponds to high gas saturation and blue to zero gassaturation.21

Figure 19: Vertical cross-section of gas saturation for a 3D grid with astochastic distribution of flow barriers after 20 years of injection at 4 milliontonnes per year. The lateral width is 3000m. The shale units are 200m by100m laterally. The top of the model corresponds to a depth of 1200m, andthe bottom to 2000m. Red corresponds to high gas saturation and blue tozero gas saturation.22

Figure 20: Horizontal cross-section at a depth of 1710m of gas saturationfor a 3D grid with a stochastic distribution of flow barriers after 20 years ofinjection at 4 million tonnes per year. The lateral width is 3000m. The shaleunits are 200m by 100m laterally. Red corresponds to high gas saturationand blue to zero gas saturation.23

6 ConclusionsNumerical simulations have been conducted to examine the suitability of apossible site for underground storage of carbon dioxide in the onshore PerthBasin. A simple geological model was constructed based on data supplied byGeoscience Australia. One of the key features is the absence of an extensiveregional sealing unit above the proposed injection location. Two differentapproaches were used to estimate vertical permeability, one using an upscaledeffective vertical permeability and the other using object modelling of flowbarriers.For deep injection (around 1900m) at a rate of 1 million tonnes of carbondioxide per year, the key question is whether the carbon dioxide will reachthe formations containing fresh water around 1200m. For low values of theabsolute permeability and vertical permeability, the carbon dioxide is trappedas residual gas before it can migrate this far. However for high values of theabsolute permeability and the vertical permeability, there is a risk that thecarbon dioxide could migrate this far. Likewise, a higher injection rate (4million tonnes per year) increases the risk of reaching the fresh water aquifers.24

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