Primary Miscible / Near-Miscible Flooding, and CO2 Sequestration

Also see the example optimization problem and solution given at Generalized Automatic Global Predictive Optimization, determining the optimal production strategy for SPE5. The optimal producing bhp for gas flooding in the optimal producing strategy (3755.3 psia) is just below the first-contact miscible pressure (3874.8), and demonstrates that the given lab MMP (3000) is meaningless in the reservoir, and that "miscible flood" models assuming a fixed miscibility pressure do not apply in general.  That example also demonstrates the non-existence of any competent AI/ML method in engineering for design, optimization, or forecasting.

The high oil recovery of about 80% obtained in the 20-year SPE5¹ WAG (water-alternating-gas) flood illustrates the very high recovery potential of miscible or near-miscible gas flooding as a primary recovery method.  Recovery by miscible flooding is limited only by sweep efficiency.  Fully miscible flooding of an oil by a solvent is possible at pressures higher than the highest saturation pressure or critical point pressure found along the closed oil-solvent mixture phase diagram, called the first-contact-miscible (FCM) pressure.  The requirement "closed" means that either a dewpoint, bubblepoint, or the critical point can be found (exists) for any given mixture.  Light oils usually have closed oil-solvent phase diagrams for many possible solvents.  Sensor automatically prints out phase diagrams for the initially specified oil and a number of default and optionally specified solvent compositions.  CO2 usually makes the best solvent, rather than any produced surface gas.  Near-miscible levels of recovery may be obtained when the producer operates in the immiscible region somewhat below the FCM pressure, because miscible conditions exist along most of the displacement front.  But multiple-contact miscibility as observed in laboratory slimtube tests is generally not achieved in the reservoir due to multiphase flow, gravity, and heterogeneity, which cause mixing that prevents it.

Depletion of an oil reservoir destroys the possibility of uniform and high sweep efficiency in flooding.  Primary miscible flooding, where applicable, makes "primary" and "secondary" techniques (depletion and waterflooding) unnecessary and very costly.  Spe5 makes a good example:

Spe5 is a 20 year compositional PRIMARY wag flood  of an initially undersaturated oil, with bhp control of 4500 for the injector and 3000 for the producer.  The solvent/oil phase diagram output by Sensor shows that the FCM pressure is 3874.8 psi for the specified injectant (solvent). Recovery after 20 years of flooding is 79.3% oil, 12.8% gas. Below is the phase diagram printed in the Sensor output file for the solvent (injected gas)/oil system:  The FCM pressure is the highest psat in the closed phase diagram (3874.8 psi).  The Sensor data file is spe5.dat.

INJECTANT = ENTERED INJECTANT No.  1 (entered under P-Z keyword)

                   RESERVOIR

                     FLUID       INJECTANT

          ---------------------------------

          1  C1   0.500000      0.770000   

          2  C3   0.300000E-01  0.200000   

          3  C6   0.700000E-01  0.300000E-01

          4  C10  0.200000      0.00000   

          5  C15  0.150000      0.00000   

          6  C20  0.500000E-01  0.00000   

             Z     PSAT      TYPE  TENSION       DXY         DENO        DENG

          --------------------------------------------------------------------

          0.0000  2302.9    BUB PT  2.3187     0.47470      33.775      6.9619   

          0.0200  2342.3    BUB PT  2.2203     0.46726      33.677      7.1253   

          0.0400  2382.1    BUB PT  2.1231     0.45978      33.576      7.2925   

          0.0600  2422.3    BUB PT  2.0269     0.45226      33.472      7.4636   

          0.0800  2462.9    BUB PT  1.9321     0.44470      33.366      7.6386   

          0.1000  2503.9    BUB PT  1.8385     0.43708      33.256      7.8178   

          0.1200  2545.4    BUB PT  1.7464     0.42941      33.143      8.0013   

          0.1400  2587.2    BUB PT  1.6557     0.42169      33.027      8.1892   

          0.1600  2629.5    BUB PT  1.5665     0.41390      32.907      8.3818   

          0.1800  2672.1    BUB PT  1.4790     0.40605      32.783      8.5792   

          0.2000  2715.2    BUB PT  1.3932     0.39814      32.655      8.7816   

          0.2200  2758.6    BUB PT  1.3091     0.39015      32.523      8.9892   

          0.2400  2802.4    BUB PT  1.2269     0.38208      32.387      9.2023   

          0.2600  2846.5    BUB PT  1.1467     0.37394      32.246      9.4211   

          0.2800  2890.9    BUB PT  1.0685     0.36570      32.100      9.6459   

          0.3000  2935.7    BUB PT 0.99248     0.35737      31.949      9.8768   

          0.3200  2980.7    BUB PT 0.91864     0.34894      31.792      10.114   

          0.3400  3026.0    BUB PT 0.84711     0.34041      31.630      10.359   

          0.3600  3071.5    BUB PT 0.77797     0.33176      31.461      10.610   

          0.3800  3117.2    BUB PT 0.71132     0.32299      31.286      10.869   

          0.4000  3163.0    BUB PT 0.64724     0.31408      31.104      11.136   

          0.4200  3208.9    BUB PT 0.58584     0.30504      30.914      11.411   

          0.4400  3254.7    BUB PT 0.52719     0.29584      30.717      11.695   

          0.4600  3300.5    BUB PT 0.47139     0.28648      30.511      11.988   

          0.4800  3346.2    BUB PT 0.41851     0.27695      30.296      12.291   

          0.5000  3391.5    BUB PT 0.36864     0.26722      30.071      12.604   

          0.5200  3436.5    BUB PT 0.32185     0.25729      29.836      12.929   

          0.5400  3480.9    BUB PT 0.27822     0.24713      29.589      13.265   

          0.5600  3524.7    BUB PT 0.23780     0.23673      29.331      13.613   

          0.5800  3567.5    BUB PT 0.20065     0.22606      29.058      13.975   

          0.6000  3609.3    BUB PT 0.16681     0.21510      28.772      14.350   

          0.6200  3649.7    BUB PT 0.13631     0.20381      28.470      14.741   

          0.6400  3688.4    BUB PT 0.10915     0.19217      28.150      15.148   

          0.6600  3725.1    BUB PT 0.85316E-01 0.18014      27.812      15.573   

          0.6800  3759.3    BUB PT 0.64773E-01 0.16766      27.453      16.017   

          0.7000  3790.5    BUB PT 0.47448E-01 0.15468      27.070      16.481   

          0.7200  3818.2    BUB PT 0.33229E-01 0.14115      26.662      16.967   

          0.7400  3841.4    BUB PT 0.21963E-01 0.12699      26.225      17.478   

          0.7600  3859.4    BUB PT 0.13438E-01 0.11209      25.754      18.015   

          0.7800  3871.0    BUB PT 0.73849E-02 0.96353E-01  25.247      18.582   

          0.8000  3874.8    BUB PT 0.34619E-02 0.79629E-01  24.696      19.182   

          0.8200  3869.0    BUB PT 0.12545E-02 0.61733E-01  24.095      19.819   

          0.8400  3851.3    BUB PT 0.28009E-03 0.42423E-01  23.434      20.498   

          0.8600  3818.7    BUB PT 0.18015E-04 0.21372E-01  22.702      21.226   

          0.8800  3766.9    DEW PT 0.10763E-08 0.18812E-02  22.010      21.881   

          0.9000  3690.3    DEW PT 0.52509E-04 0.28018E-01  22.863      20.946   

          0.9200  3579.7    DEW PT 0.95858E-03 0.58116E-01  23.804      19.860   

          0.9400  3420.4    DEW PT 0.64384E-02 0.94063E-01  24.860      18.557   

          0.9600  3183.8    DEW PT 0.30324E-01 0.13974      26.090      16.901   

          0.9800  2794.8    DEW PT 0.13540     0.20605      27.638      14.514   

          1.0000  ** NO PSAT FOUND **

If we substitute CO2 for the specified injection gas in the WAG flood, oil recovery is about the same (78% oil, -8% gas), but 90% of the injected CO2 is sequestered (remains in reservoir at end of run).  CO2 is more miscible with the oil than the originally specified injection gas.  The datafile is spe5c.dat.  The CO2-oil phase diagram is given in spe5c.out as:

 INJECTANT = CO2

      RESERVOIR
       FLUID      INJECTANT
---------------------------------
1 CO2 0.00000      1.000000
2 C1  0.500000     0.00000
3 C3  0.300000E-01 0.00000
4 C6  0.700000E-01 0.00000
5 C10 0.200000     0.00000
6 C15 0.150000     0.00000
7 C20 0.500000E-01 0.00000

   Z   PSAT    TYPE TENSION   DXY   DENO   DENG
--------------------------------------------------------------------
0.0000 2302.9 BUB PT 2.3187 0.47470 33.775 6.9619
0.0200 2296.9 BUB PT 2.2727 0.46371 33.852 7.2289
0.0400 2290.9 BUB PT 2.2251 0.45262 33.931 7.5006
0.0600 2285.0 BUB PT 2.1762 0.44143 34.011 7.7773
0.0800 2279.1 BUB PT 2.1258 0.43014 34.093 8.0592
0.1000 2273.3 BUB PT 2.0739 0.41876 34.176 8.3465
0.1200 2267.5 BUB PT 2.0207 0.40730 34.260 8.6394
0.1400 2261.8 BUB PT 1.9660 0.39575 34.346 8.9382
0.1600 2256.0 BUB PT 1.9099 0.38412 34.433 9.2432
0.1800 2250.4 BUB PT 1.8525 0.37242 34.522 9.5547
0.2000 2244.7 BUB PT 1.7937 0.36066 34.612 9.8729
0.2200 2239.1 BUB PT 1.7336 0.34884 34.703 10.198
0.2400 2233.5 BUB PT 1.6722 0.33697 34.796 10.531
0.2600 2228.0 BUB PT 1.6097 0.32505 34.890 10.872
0.2800 2222.5 BUB PT 1.5459 0.31310 34.985 11.220
0.3000 2217.0 BUB PT 1.4811 0.30111 35.081 11.578
0.3200 2211.5 BUB PT 1.4153 0.28911 35.178 11.945
0.3400 2206.0 BUB PT 1.3485 0.27710 35.276 12.321
0.3600 2200.5 BUB PT 1.2810 0.26508 35.375 12.707
0.3800 2195.0 BUB PT 1.2127 0.25308 35.474 13.105
0.4000 2189.5 BUB PT 1.1439 0.24109 35.573 13.513
0.4200 2184.0 BUB PT 1.0747 0.22914 35.673 13.934
0.4400 2178.5 BUB PT 1.0053 0.21723 35.772 14.368
0.4600 2172.9 BUB PT 0.93575 0.20538 35.871 14.816
0.4800 2167.3 BUB PT 0.86641 0.19360 35.968 15.278
0.5000 2161.7 BUB PT 0.79747 0.18191 36.064 15.756
0.5200 2155.9 BUB PT 0.72917 0.17032 36.157 16.251
0.5400 2150.1 BUB PT 0.66179 0.15885 36.247 16.764
0.5600 2144.2 BUB PT 0.59564 0.14751 36.333 17.296
0.5800 2138.2 BUB PT 0.53105 0.13633 36.414 17.849
0.6000 2132.1 BUB PT 0.46837 0.12533 36.488 18.425
0.6200 2125.8 BUB PT 0.40798 0.11452 36.553 19.026
0.6400 2119.3 BUB PT 0.35028 0.10394 36.608 19.654
0.6600 2112.6 BUB PT 0.29569 0.93598E-01 36.651 20.311
0.6800 2105.7 BUB PT 0.24464 0.83528E-01 36.677 21.002
0.7000 2098.5 BUB PT 0.19756 0.73758E-01 36.683 21.728
0.7200 2091.0 BUB PT 0.15489 0.64318E-01 36.665 22.494
0.7400 2083.1 BUB PT 0.11703 0.55242E-01 36.617 23.306
0.7600 2074.8 BUB PT 0.84336E-01 0.46567E-01 36.531 24.170
0.7800 2066.1 BUB PT 0.57099E-01 0.42918E-01 36.398 25.092
0.8000 2056.7 BUB PT 0.35479E-01 0.39515E-01 36.205 26.082
0.8200 2046.7 BUB PT 0.19461E-01 0.35258E-01 35.937 27.153
0.8400 2035.7 BUB PT 0.87769E-02 0.29965E-01 35.571 28.319
0.8600 2023.6 BUB PT 0.27959E-02 0.23370E-01 35.080 29.599
0.8800 2009.8 BUB PT 0.41152E-03 0.15060E-01 34.421 31.017
0.9000 1993.5 BUB PT 0.23791E-05 0.43385E-02 33.536 32.600
0.9200 1973.0 DEW PT 0.56154E-04 0.10070E-01 34.366 32.333
0.9400 1944.5 DEW PT 0.37147E-02 0.30693E-01 36.272 30.661
0.9600 1898.5 DEW PT 0.44451E-01 0.62689E-01 38.083 28.229
0.9800 1799.9 DEW PT 0.31364 0.11810 39.228 24.243
1.0000 ** NO PSAT FOUND **

The FCM pressure for the CO2/oil mixture is only 2302 psia.  Since the producing pressure 3000 is above the FCM pressure, spe5c is a first-contact miscible wag flood.

If instead we first deplete the reservoir for 20 years with bhp=1000 (recovery = 27.1% oil, 47.7% gas), then waterflood (inject at 4500 psia, produce at 1000 psia) for another 20 years (recovery = 60.2% oil, 76.1% gas), and then do the same original WAG flood for another 20 years, the final recoveries are 74% oil, 39.1% gas.  The data file is spe5dwm.dat

So, when miscible flooding applies and suitable gas is available, it is usually the best possible primary recovery process.  Depletion and waterflooding destroy the possibility for very high miscible recovery because they eliminate the possibility for uniform and high sweep efficiency.

Primary miscible or near-miscible primary floods with horizontal wells could be the most efficient possible production method for originally undersaturated oil reservoirs.  Gravity-stable vertical floods using horizontal wells might maximize production efficiency. Primary miscible recovery eliminates the usual depletion and waterflooding phases of recovery for an approximate order of magnitude improvement in production efficiency.  A prohibiting factor is often availability of injection gas.  Locating gas and power plants close to major oilfields maximizes efficiency of the process.  Beyond gas availability, primary miscible recovery methods may not be applied to qualifying reservoirs / fluids because of any combination of convention (of primary depletion followed by (secondary) waterflooding followed by (tertiary) EOR methods), corporate and investor policy, or regulation.

The above also applies to liquid-rich and gas condensate unconventional systems.

Given the very high potential value of the use of CO2 for (primary or EOR) miscible flooding, it makes no sense to waste CO2 by storing it.  Evidence of that is the inability of anyone to answer our questions in the SPE CCUS Technical Section.