Sagdox geometry

Abstract

There is provided a process to recover bitumen from a subterranean hydrocarbon reservoir. The process includes injecting steam and oxygen separately into the bitumen reservoir. When mixed in the reservoir, the mix is in the range of 5 to 50% O.sub.2. The process also includes producing hot bitumen and water using a horizontal production well, and producing/removing non-condensable combustion gases to control reservoir pressure.

Claims

1. A process to produce bitumen from an at least partially depleted steam-swept bitumen-comprising reservoir: wherein a reservoir in a natural state containing 100% of a native bitumen has been previously subjected to an initial extraction to produce the at least partially depleted steam-swept reservoir by: installing a steam assisted gravity drainage (SAGD) system within the reservoir, the SAGD system comprising: a production well having a horizontal distal portion and a vertical proximal portion in communication with an extraction pump; and a steam injection well having a horizontal distal portion above the horizontal distal portion of the production well and a vertical proximal portion in communication with a steam source; operating the SAGD system, by injecting steam through the steam injection well to the horizontal distal portion thereof into the reservoir with the effect that steam heat and steam pressure are applied to the bitumen thereby reducing viscosity of the bitumen and mobilizing the bitumen to flow downward under gravity drainage; and extracting bitumen and water from the bitumen-comprising subterranean reservoir into the horizontal distal portion of the production well; the process comprising: subjecting the at least partially depleted steam-swept reservoir to a secondary extraction comprising: installing an oxygenatious gas injection well with a gas outlet in the at least partially depleted steam-swept reservoir above the horizontal distal portion of the production well, the gas injection well being separate from the SAGD system and horizontally spaced apart from the SAGD system; operating the oxygenatious gas injection well by injecting oxygenatious gas through the gas outlet and igniting the bitumen in a combustion zone in the at least partially depleted steam-swept reservoir with the effect that one of: combustion heat energy; oxygenatious gas pressure; steam heat and steam pressure generated from vaporized water within the at least partially depleted steam-swept reservoir; and combustion gas pressure is applied to the bitumen, thereby reducing viscosity of the bitumen and mobilizing the bitumen to flow downward under gravity drainage into the horizontal distal portion of the production well, wherein a volume to volume ratio of oxygenatious gas in the secondary extraction relative to water used to produce steam in the initial extraction is in the range of 5% to 50%.

2. The process according to claim 1 wherein the initial extraction produces an at least partially depleted steam-swept reservoir having between 10-25% residual bitumen of the native bitumen, and wherein the secondary extraction comprises: heating the residual bitumen with combustion gases in the combustion zone; stripping light fractions from the residual bitumen; pyrolyzing the residual bitumen to produce coke; oxidizing the coke; and producing a combustion swept zone having substantially no recoverable bitumen.

3. The process according to claim 1 wherein the ratio is in the range of 10% to 40%.

4. The process according to claim 1, comprising: installing a produced gas (PG) extraction well with an inlet within the at least partially depleted steam-swept reservoir, the PG extraction well being separate from the SAGD system and horizontally spaced apart from the SAGD system; and operating the PG extraction well to extract non-condensable gas.

5. The process according to claim 4, comprising: controlling the formation of the combustion zone by controlling one of: the injection of oxygenatious gas; and the extraction of produced gas.

6. The process according to claim 5, wherein one of: a plurality of oxygenatious gas injection well outlets; and a plurality of PG extraction well inlets, are spaced apart horizontally to control the formation of the combustion zone.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a sketch of the preferred well configuration for a SAGDOX Geometry process added on to a SAGD process.

(2) FIG. 2 illustrates various alternative options for configuration of SAGDOX wells.

(3) FIG. 3 illustrates a horizontal slice in mid play for a SAGDOX process based on a University of Calgary Simulation study.

(4) FIG. 3A is productivity chart for a SAGD process where steam alone is injected into the well.

(5) FIG. 4 is a schematic sketch of an integrated cogeneration process for steam and electricity in a SAGDOX operation with an air separation unit

(6) FIG. 5 is in addition to FIG. 4 illustrates the addition of a conventional steam boiler thereto.

TECHNICAL DESCRIPTION OF INVENTION

(7) Introduction

(8) SAGDOX is a bitumen EOR process that can be added on to SAGD and uses mixtures of steam and oxygen. Steam provides heat directly, oxygen adds heat by combusting residual bitumen in a steam-swept zone.

(9) While it is possible to start a SAGD project using steam only and then implement SAGDOX by adding oxygen to the steam, this is not preferable because of high corrosion rates in a saturated steam and oxygen system, particularly using carbon steel pipes. The preferred strategy is to separately isolate steam and oxygen injection and allow mixing to occur in the reservoir. The separation can be accomplished by packers (swellable and mechanical downhole packers) or by using separate injector wells.

(10) The preferred SAGDOX mixture is 35% (v/v) oxygen and 65% steam.

(11) Injector Volumes

(12) Lets define SAGDOX (Z) where Z=% (v/v) oxygen in the steam oxygen mixture.

(13) Table 1 presents properties of SAGDOX injection gases. Some of the features of the gas mixtures are as follows:

(14) As the percent of oxygen in the mix increases, the total volume to inject a fixed amount of energy drops by up to a factor of 10.

(15) For our preferred mix (SAGDOX (35)), to inject the same amount of energy as steam, our volume rates are cut by 76%. We can expect smaller pipe sizes than a SAGD project.

(16) Compared to SAGD steam for SAGDOX(35) our oxygen injection rate is 8.5% of the volume rate. Our O.sub.2 injector (and produced gas) well can be very small.

(17) Preferred Well Configuration

(18) FIG. 1 shows the preferred well configuration for SAGDOX added-on to SAGD. The following features are notable:

(19) The SAGD well pair is conventional—parallel horizontal wells with length of 400-1000 m and separation of 4-6 m. The lower horizontal well is about 2-8 m above the bottom of the reservoir. The upper well is a steam injector. The lower horizontal is the bitumen (+water) producer.

(20) The SAGDOX oxygen injector is above the toe area of the steam injector (1-4 m). The well is not at the end of the pattern (about 5-20 m in from the end).

(21) Two produced gas removal wells are on the pattern boundaries (i.e. only 1 net well) toward the heel area of the SAGD well pair. The wells are completed near the top of the reservoir (1-10 m) below the ceiling.

(22) This configuration enables the following: Separate control of O.sub.2/steam injection Oxygen injection into the steam-swept area Removal of (cool) non condensable gases 2(net) new wells (small vertical wells) compared with SAGD

(23) If the reservoir is “leaky”, with enough capacity to sequester non-condensable gases produced by combustion, we may not need produced gas removal wells or we can reduce the number of produced gas removal wells.

(24) Other Configurations

(25) Of course, our preferred SAGDOX well configuration is not the only way to implement SAGDOX. FIG. 2 shows some other possibilities, including the following:

(26) Using a packer(s) we can isolate a portion of our injector well and simultaneously inject steam and oxygen (FIG. 2(1)). (swellable and mechanical downhole packers) If we can use the toe of the steam injector for oxygen injection we can segregate O.sub.2 and steam to minimize corrosion. Even with some corrosion, we are willing to sacrifice the toe of the injector. Because steam demands for SAGDOX are much less than SAGD (Table 1), there is plenty of “room” to segregate O.sub.2 and steam in the SAGD producer.

(27) Using a packer(s) we can similarly isolate part of the injector well to remove produced gases (FIG. 2(4)).

(28) We can install multiple oxygen injectors, to improve conformance and allow more control (FIG. 2(3)).

(29) Similarly, we can install multiple produced gas removal wells, to improve conformance and control (FIG. 2(6).

(30) Extended Reach Wells

(31) FIG. 2(7) shows how SAGDOX can improve SAGD. Because liquid volumes in the production well are reduced for SAGDOX compared to SAGD we are no longer limited to a horizontal well pair length of about 1000 m. Table 2 shows that we can expect, for the same bitumen production, the produced volume rates for SAGDOX (35) in the lower horizontal well will be about 28% of the volume rate for SAGD. So with reduced hydraulic limits on well length we can extend SAGD wells beyond the 1000 m limit.

(32) This may have to be drilled initially (not as a SAGD add-on). The extended-reach version of SAGDOX can: (c/w SAGD) Increase productivity Increase recovery Decrease number of wells needed to exploit resource
What Aspects of Invention can be Altered and Still Accomplish Goals? Well positions, within limits stated 1 well-multiple wells (better control) O.sub.2 concentration in SAGDOX mix (5 to 50% (v/v) range) Pressure of reservoir

(33) TABLE-US-00002 TABLE 1 Properties of SAGDOX Injection Gases SAGDOX SAGDOX SAGDOX SAGDOX SAGDOX SAGDOX (0) (9) (35) (50) (75) (100) % (v/v) 0 9 35 50 75 100 oxygen % heat 0 50.0 84.5 91.0 96.8 100.0 from O.sub.2 BTU/SCF 47.4 86.3 198.8 263.7 371.9 480.0 mix MSCF/MMBTU 21.1 11.6 5.0 3.8 2.7 2.1 MSCF 0.0 1.0 1.8 1.9 2.0 2.1 O.sub.2/MMBTU MSCF 21.1 10.6 3.3 1.9 0.7 0.0 steam/MMBTU Where: Steam heating value = 1000 BTU/lb O.sub.2 heating value (combustion) = 480 BTU/SCF SAGDOX (0) = pure steam (ie SAGD) SAGDOX (100) = pure oxygen

(34) TABLE-US-00003 TABLE 2 SAGDOX production well volumes SAGDOX SAGDOX SAGDOX SAGDOX (0) (9) (35) (100) Bitumen 1 1 1 1 (bbls) produced 3.37 1.80 .71 0 water (bbls) connate water 0 0.31 .31 .31 (bbls) comb. water 0 0.09 .19 .27 (bbls) Total (bbls) 4.37 3.20 1.21 0.58 Assumes: 80% original bitumen saturation All connate water is produced in SAGDOX All combustion water is produced in SAGDOX Nexen case studies

(35) TABLE-US-00004 SAGDOX Reservoir Steam use SAGDOX SAGDOX SAGDOX SAGDOX SAGDOX SAGDOX (0) (9) (35) (50) (75) (100) Avg. 1.180 1.230 1.387 1.475 1.623 1.770 ETOR O.sub.2 (V/V) % 0 9 35 50 75 100 of mix (% of 0 50.0 84.5 91.0 96.8 100.0 heat) (MCF/bbl) 0 1.281 2.442 2.796 3.273 3.688 ETOR 1.18 0.615 0.215 0.133 0.052 0.000 (steam) ETOR 0 0.615 1.172 1.342 1.571 1.770 (O.sub.2) Steam use (bbl/bbl) Steam inj. 2.36 1.230 0.430 0.266 0.104 0.0 Connate steam 0 0.330 0.330 0.330 0.330 0.330 Comb 0 0.024 0.046 0.053 0.062 0.070 steam Reflux 0 0.776 1.554 1.711 1.864 1.960 steam Totals 2.36 2.36 2.36 2.36 2.36 2.36 Reflux % 0 33 66 73 79 83 Where: ETOR = MMBTU/bbl bitumen ETOR is prorated between SAGDOX (0) and SAGDOX (100); assuming ETOR for SAGDOX (100) is 150% ETOR SAGDOX (0) steam use = bbl steam/bbl bitumen injection “steam” is vapor component, assuming 70% Q at sand face all connate water in swept zone is assumed vaporized at 80% initial bit. and 20% residual bit. (for O.sub.2 cases) reflux = plug to make steam totals equal, assuming bitumen productivity < total steam and same productivity for all cases reflux % = reflux as % of total steam used combustion steam = 14% (v/v) of O2 consumed (see Table 3) SAGDOX (0) = pure steam (ie SAGD); SAGDOX (100) = pure O.sub.2 (ie ISC (O.sub.2)) Oxygen combustion heat = 480 BTU/SCF; steam = 1000 BTU/lb

(36) TABLE-US-00005 TABLE 3 Integrated ASU: Cogen Energy Use (MMBTU/bbl) SAGDOX (9) SAGDOX (35) SAGDOX (100) 99.5% O.sub.2 purity Steam 0.683 (73.0) .239 (52.6) .148 (40.7) Electricity 0.065 (7.0)  .124 (27.4) .142 (39.3) Waste 0.187 (20.0) .091 (20.0) .072 (20.0) Total  0.935 (100.0)  .454 (100.0)  .362 (100.0) 95-97% O.sub.2 purity Steam 0.683 (74.7) .239 (57.5) .148 (46.4) Electricity 0.049 (5.3)  .093 (22.5) .107 (33.5) Waste 0.183 (20.0) .083 (20.0) .064 (20.0) Total  0.915 (100.0)  .415 (100.0)  .318 (100.0) Where: (1) ETOR values from Table 2 (2) see text for assumptions (3) lower purity O.sub.2 uses 25% less electricity

(37) TABLE-US-00006 TABLE 4 Energy Efficiencies (%) SAGDOX SAGDOX SAGDOX SAGD (9) (35) (100) 99.5% oxygen Separate 73.8 84.4 91.5 92.7 delivery Integ — 84.4 92.4 94.0 ASU:Cogen 95-97% oxygen Separate 83.8 85.4 92.4 93.8 delivery Integ — 84.5 93.1 94.7 ASU:Cogen Where: (1) heat value of bitumen = 6 MMBTU/bbl (2) see text for energy definition (3) separate delivery case gas boiler 85% + electricity at 55% comb. cycle
Insitu Combustion Chemistry

(38) CH..sub.5=reduced formula for “coke” that is combusted. Ignores trace components (eg S, N . . . ). Doesn't imply molecular structure, only ratio of H/C in large molecules

(39) Best guess of net “reservoir oxidation chemistry”
Oxidation of combustion front (assumes 10% carbon goes to CO)=CH.sub.0.5+1.075O.sub.2.fwdarw.0.9CO.sub.2+0.1CO+0.25H.sub.2O+HEAT

(40) Water gas shift, in reservoir:
CO+0.1H.sub.2O.fwdarw.0.1CO.sub.2+0.1H.sub.2

(41) Net reaction stoichiometry:
CH.sub.0.5+1.075O.sub.2.fwdarw.1.0CO.sub.2+0.1H.sub.2+0.15H.sub.2O

(42) Where: (1) non-condensable gas make (CO.sub.2+H.sub.2)=102% of Oxygen volume (2) combustion water make=14% of oxygen volume (3) hydrogen make=9.3% of oxygen volume (4) produced gas composition (v/v) %

(43) TABLE-US-00007 Wet dry CO.sub.2 80.0 90.9 H.sub.2 8.0 9.1 H.sub.2O 12.0 — Totals 100.0 100.0

(44) Heat release=480 BTU/SCF O.sub.2

(45) Table 3 shows the efficiencies for various SAGDOX mixtures using the assumptions of Table 2. The following points are evident: SAGDOX is more efficient than SAGD

(46) The efficiency improvement increases with increasing oxygen content in SAGDOX mixtures.

(47) For SAGD the energy loss is 26%. This loss for SAGDOX is 16 to 6% depending on oxygen content—an improvement of 10-20% or a factor of 1.6 to 4.3.

(48) If we reduce oxygen purity to say the 95-97% range, energy needed to produce oxygen drops by about 25% and SAGDOX efficiencies increase even more than above (see Table 3)

(49) Oxidation Chemistry

(50) SAGDOX creates some energy in a reservoir by combustion. The “coke” that is prepared by hot combustion gases fractionating and polymerizing residual bitumen, can be represented by a reduced formula of CH..sub.5. This ignores trace components (S, N, O . . . etc.) and it doesn't imply a molecular structure, only that the “coke” has a H/C atomic ratio of 0.5. Let's assume CO in the product gases is about 10% of the carbon combusted Water-gas-shift reactions, occur in the reservoir
CO+H.sub.2O.fwdarw.CO.sub.2+H.sub.2+HEAT

(51) This reaction is favored by lower T (lower than combustion T) and high concentrations of steam (ie SAGDOX). The heat release is small compared to combustion.

(52) Then our net combustion stoichimetry is as follows:

(53) TABLE-US-00008 Combustion: CH.sub.0.5 + 1.075O.sub.2 .fwdarw. 0.9CO.sub.2 + 0.1CO + .25H.sub.2O + HEAT Shift: .1CO + .1H.sub.2O .fwdarw. .1CO.sub.2 + .1H.sub.2 + HEAT Net: CH.sub..5 + 1.075O.sub.2 .fwdarw. CO.sub.2 + .1H.sub.2 + .15H.sub.2O + HEAT

(54) Features are as follows: Heat Release=480 BTU/SCF O.sub.2 Non-condensable gas make=102% of oxygen used (v/v) Combustion water make=14% of oxygen used (v/v) (net) hydrogen gas make=9.3% of oxygen used produced gas composition (v/v %)=

(55) TABLE-US-00009 Wet Dry CO.sub.2 80.0 90.9 H.sub.2 8.0 9.1 H.sub.2O 12.0 — Total 100.0 100.0

(56) Combustion temperature is controlled by “coke” content. Typically combustion T is between about 400 and 650° C. for HTO reactions.

(57) The Importance of Steam

(58) For SAGD heat transfer is dominated by steam. For SAGDOX we add heat transfer from hot combustion gas. Compared to hot non-condensable gases, steam has 2 significant advantages:

(59) Including latent heat when steam condenses, a fixed volume of steam will deliver more than twice the amount of heat available from the same volume of hot combustion gases When steam condenses, it creates a transient low pressure zone that draws in more steam—ie a heat pump without the plumbing

(60) For SAGDOX and SAGD we expect stream use/creation to be a dominant factor for productivity.

(61) Steam Use in SAGDOX

(62) As we add oxygen to steam we expect less steam in the reservoir, as more and more of the heat injection comes from combustion. So, if everything else was equal, we would expect decreasing productivity or increasing ETOR for constant productivity. But, oxidation processes offer 3 ameliorating factors: Some extra steam is produced as a product of combustion Some extra steam is produced by vaporizing connate water in combustion swept zones Some extra steam is produced when hot gases or hot bitumen vaporizes condensed water (i.e. reflux)

(63) So we expect, if SAGDOX is to have the same productivity as SAGD, to inject more energy than SAGD (to compensate for reduced steam inventory) and to have significant reflux of steam, accounting for extra steam sources. Table 2 shows one such balance—but there may be several and each reservoir may be different.

(64) SAG Performance

(65) With some assumptions, we can compare SAGDOX performance with SAGD. Nexen has simulated SAGD under the following assumptions: a homogenous sandstone bitumen reservoir generic properties for LLK bitumen 25 m, clean, homogeneous pay zone 800 m, SAGD well pair at 100 m spacing, with 5 m separation between steam injector and bitumen/water producer 10° C. sub cool for production control 2 MPa pressure for injection control 4 mos. start-up period, using steam circulation discretized well-bore model

(66) The simulation production results are shown in FIG. 3.5. The economic limit is taken at SOR=9.5, at the end of year 10. The results for SAGD can be summarized as follows: bitumen recovery=333.6 km.sup.3=2.099 MMbbl average bitumen production=575 bbl/d peak bitumen rate (end yr. 2)=159.2 m.sup.3/d=1002 bbl/d steam used=1124.9 km.sup.3=7.078 MMbbl=2.477×10.sup.12 BTU average steam rate=1939 bbl/d peak steam (end yr. 4)=456.7 m.sup.3/d=2874 bbl/d average SOR=3.37 (average ETOR=1.180) recovery factor=63.4% OBIP OBP in pattern=3.31 MMbbl

(67) We will use this simulation as the basis for SAGDOX production comparisons.

(68) SAGDOX Performance

(69) Mechanisms

(70) SAGDOX has 2 separate sources of reservoir heat delivery—steam condensation, and oxygen combustion of residual bitumen. Before we develop comparisons to SAGD, lets look at a simulation of SAGD so we can understand the mechanisms that are important. FIG. 3 presents the results of a simulation of a SAGDOX process using a combustion kinetic model and a modified STARS simulator. The plot is for a “mature” process after several years of operation, taking a horizontal slice half-way up the pay zone and half-way down the length of the horizontal well pair. The plot is for bitumen saturation as a function of lateral distance from the vertical plane of the horizontal well pair. Looking at the plot, we see the following process features, as we move outward from the central plane: A combustion-swept zone with zero residual bitumen and zero residual water; A combustion front, indicated by a share increase in bitumen saturation; A bank of hot bitumen, partially fractionated (stripped of light ends) and partially upgraded by pyrolysis from hot combustion gases. The bitumen bank temperatures are higher than saturated steam, so bitumen draining is hot and can reflux steam as it meets condensed water below the plane; A steam swept zone made up of 2 parts—superheated zone with no steam condensate and a saturated-steam zone with condensed water; The cold-bitumen: saturated-steam interface where steam condenses to heat bitumen; Bitumen drains downward (and inward) from 2 areas—the hot bitumen bank near the combustion front and heated bitumen, near the cold bitumen interface. (Most of the bitumen produced comes from the later zone); Water also drains from 2 areas—the saturated steam zone and near the bitumen interface. (Most of the water drained comes from the later zone).
Kinetics/Productivity

(71) Let's first look at SAGD (steam gravity drainage). The process is complex with many steps, as follows: steam is injected at the sand face; steam enters the reservoir, in a steam-swept zone, at (near) saturated steam temperature; as the steam moves through the reservoir heat losses reduce steam quality, but T is relatively constant; when steam reaches the cold bitumen interface, it condenses (to water) and releases its latent heat; the latent heat is conducted in the interface and heats the matrix rock and the reservoir fluids (bitumen and connate water); the heated bitumen drains downward and inward to the horizontal production well, about 5 m underneath the steam injector well—(drainage distances are ≦50 m); condensed water also drains to the same well; the bitumen/water mixture is pumped/conveyed to the surface.

(72) Productivity (rate of bitumen production) is determined by the cumulative rate of all of these steps. The slowest step (rate-limiting step) is usually considered to be bitumen drainage to the production well (step (6)). Drainage rates are dependent on the drainage distance, the matrix permeability and the viscosity of the heated bitumen. Bitumen viscosity is the key variable and it is a strong function of temperature.

(73) SAGDOX has a similar geometry to SAGD, but the process is more complex. The mechanisms for steam (SAGD) EOR are still active, but the combustion component adds the following steps: ignition occurs at the combustion front, where oxygen reacts with residual fuel (coke); hot combustion gases fractionate residual bitumen, in (or near to) the steam-swept zone, and pyrolyse bitumen to prepare residual fuel (coke) for combustion; connate water, in the steam-swept zone, is vaporized to steam; hot combustion gases superheat steam; hot bitumen and hot combustion gases vaporize (reflux) condensed steam; at the cold bitumen interface, some heat is transferred directly from hot combustion gases to cold bitumen, connate water and matrix rock; a hot bitumen bank is formed downstream of the combustion front; This hot bitumen drains downward and inward to the horizontal production well; Temperatures are greater than saturated steam temperatures; Heat exchange (reflux) from the hot bitumen in (G) and (H) to condensed water draining to the production well.

(74) So SAGDOX has all the mechanisms/steps of SAGD plus the additional steps arising from combustion processes. It is not obvious, for productivity and kinetics, what is the rate-limiting step for SAGDOX.

(75) Preferred Range of Oxygen Content in SAGDOX Gases

(76) Below about 5% oxygen in a steam+oxygen mixture combustion may become unstable and it becomes difficult to keep oxygen flux rates to sustain HTO. It also becomes difficult to vaporize and mobilize all connate water.

(77) Above about 50% oxygen in steam, the reflux rates to sustain productivity are more than 70% of the total steam. This may be difficult in practice. Also, above this limit the bitumen (“coke”) fuel that is consumed starts to be greater than the residual fuel left behind in the steam-swept zone. Also, above this limit it isn't possible to produce steam/electricity mixes from an integrated cogen: ASU for SAGDOX. Compared to SAGD, SAGDOX (50) may have lower recoveries.

(78) So the preferred range is 5 to 50 (v/v) % oxygen in the steam+oxygen mixture injected. If we are more concerned about safety factors, a range of 10 to 40 (v/v) % oxygen, may be preferred.

(79) Based on an economic study the preferred oxygen content is about 35% (v/v) % or a range of 30 to 40% (v/v).

(80) Synergies

(81) A synergy is an “unexpected” benefit. The cumulative benefits of the steam-oxygen mix are more than the benefits of the stand-alone components.

(82) How does Oxygen Help Steam EOR Benefits?

(83) surface steam demand (water use) is directly reduced; extra steam is created directly in the reservoir by combustion of coke; heat of combustion vaporizes connate water to increase steam in the reservoir; heat of combustion results in vaporization of condensed steam (water reflux); in situ combustion can increase avg. T in the steam/combustion swept zones beyond the saturated steam T limit; the use of oxygen improves overall energy efficiency; non-condensable gases produced from combustion insulates the top of the pay zone to reduce energy losses and increases lateral vapour chamber growth rates. This can be beneficial if the reservoir has top water or top gas because SAGDOX steam+oxygen mixes cost less than pure steam for the same energy content, we can extend production beyond the economic limit for steam-only and increase ultimate recovery/reserves if some CO.sub.2 is retained in the reservoir or if some CO.sub.2 is captured and sequestered off-site, we can reduce CO.sub.2 emissions compared to steam only.
How does Steam Help Combustion? steam pre-heats the reservoir, so oxygen gas will ignite to start combustion (this is now the accepted method for ISC); in the presence of increased T (400-600° C. range) and a solid rock matrix, steam adds OH and H radicals (ions) to the combustion zone. This improves combustion kinetics (analogy to smokeless flares); steam added (and created) acts as an efficient heat transfer medium to convey heat from the combustion zone to the cold bitumen interface. This improves EOR kinetics; Steam stimulates increased combustion completeness, even for lean mixes (ie more CO.sub.2 less CO); Steam stabilizes combustion (HTO is more likely than LTO); Steam supplies some direct heat.
Energy Efficiency

(84) Lets define EOR energy efficiency as:

(85) $\left\{\frac{\left(\begin{array}{c}\mathrm{energy}\mathrm{produced}\\ \mathrm{in}\mathrm{bitumen}\end{array}\right)-\left(\begin{array}{c}\mathrm{energy}\mathrm{used},\mathrm{on}\mathrm{surface}\\ \mathrm{to}\mathrm{produce}\mathrm{bitumen}\end{array}\right)}{\left(\mathrm{energy}\mathrm{in}\mathrm{produced}\mathrm{bitumen}\right)}\right\}\times 100\%$

(86) For SAGD (SAGDOX (0)), if we assume that the energy content of bitumen (heating valve) is 6MMBTU/bbl, and that the net efficiency of steam production and delivery to the sand face is 75% (85% in a boiler and 10% loss in distribution); then our SAGD efficiency is:

(87) $\left(\frac{6-\mathrm{ETOR}/\mathrm{.75}}{6}\right)\times 100\%$

(88) For our simulation (4.2) our average ETOR is 1.180 MMBTU/bbl and our SAGD efficiency is 73.8%.

(89) For SAGDOX our energy calculation is more complex. The steam component will have a similar factor (ETOR (steam)/0.75), but oxygen will be different. If we assume our oxygen ASU oxygen use is 390 kWh/tonne O.sub.2 (for 99.5% pure oxygen) and that electricity if produced on-site from a gas-fired, combined-cycle power plant at 55% efficiency, for every MMBTU of gas used to produce power, oxygen in the reservoir releases 5.191 MMBTU of combustion energy. Using these, our SAGDOX efficiency is:

(90) $\left\{\underset{\_}{\left(\frac{6-\mathrm{ETOR}\left(\mathrm{Steam}\right)}{0.75}-\frac{\mathrm{ETOR}\left(O_2\right)}{5.191}\right)}\right\}\times 100\%$
Why is SAGDOX an “Invention”?

(91) To qualify as a true invention the proposal/process/equipment has to be not obvious to one “skilled in the art”. SAGDOX meets this criteria for the following reasons:

(92) It is no obvious that there should be limits on preferred oxygen concentration ranges for SAGDOX injection gases. On the low end, the stability of combustion in situ at low oxygen levels in steam has not been widely studied nor reported. On the high end, the idea that steam use or steam inventory is the deciding factor in bitumen productivity, has not been widely proposed nor published. The specific range and rationale has not been claimed by others.

(93) The synergistic benefits of oxygen and stream are no well-known, not obvious and not published (to my knowledge).

(94) The well configurations for SAGDOX are unique. No one else has tried SAGDOX.

(95) The fact that SAGDOX can also result in extended well lengths, has not been appreciated elsewhere.

(96) No one else has proposed/contemplated an integrated Cogen: ASU plant.

(97) Hydrogen gas production has been noted in some ISC projects for heavy/medium oil, but there is no experience in bitumen. Reservoir conditions in SAGDOX should be ideal for hydrogen production.

(98) The advantages of SAGDOX in inhomogeneous reservoirs and leaky reservoirs are intuitive. No field tests have been conducted.

(99) What Aspects of Invention can be Altered and Still Accomplish Object/Goals?

(100) O.sub.2 content in mix, within range claimed; Geometry of well configurations; Method of supplying steam and oxygen gas; Purity of oxygen (but no more than ˜5% impurities and impurities are “inert”); Length of horizontal wells (up to hydraulic limit).
1.2 Gas Mixture Delivery Invention

(101) SAGDOX is a bitumen EOR process that uses mixtures of steam and oxygen gas in the preferred range of 5 to 50 (v/v) % oxygen in steam. To control corrosion, it is preferable to inject separate streams of oxygen and steam and allow mixing in the reservoir to create the desired mix. We can provide these gases using separate facilities—steam boilers to generate steam and cryogenic air separation units (ASU) to produce oxygen gas. The boilers require a fuel-natural gas is preferred and the ASU requires electricity.

(102) If we integrate steam and oxygen facilities, on site, we can use a cogen plant to produce steam and electricity. We can then dedicate the electricity to the ASU (FIG. 4). Other integration benefits can occur. For example air compression can also be combined, to supply compressed air as a feedstock for the ASU and compressed air for combustion to the gas turbines in the cogen plant.

(103) On a net basis, the integrated plant would consume natural gas and produce oxygen and steam for SAGDOX. A typical high-efficiency modern gas turbine has efficiencies in the range of 40-45%. On the low-side turbine efficiencies are about 20-25%. As we will show these limits if applied, would limit SAGDOX gas concentrations to about 25-30% oxygen on the low side or 50-55% on the high side. In order to extend the low side to the preferred SAGDOX range we can simply add a conventional steam boiler as shown in FIG. 5.

(104) The advantages of an integrated approach include: (1) lower capex (2) less energy; higher energy efficiency (3) reduced footprints
1.3 Invention Analysis Lets assume: (1) cogen plant is a gas-fired gas-turbine generator followed by a heat recovery steam generator (HRSG) (2) cogen has 20% heat losses (ie 80% efficiency) (3) E=total ETOR demand, in reservoir (4) x=fraction of E due to oxygen ETOR (oxygen) (5) (1-x)=fraction of E due to steam ETOR (steam) (6) 10% distribution losses for steam (7) Two oxygen cases—99.5% purity; 390 kwh/tonne and 95-97% purity; 292.5 kwh/tonne (Z=kwh/tonne O.sub.2)

(105) Then at the cogen plant, steam demand=1.111 E (1-x) MMBTU/bbl bit oxygen demand=xE MMBTU in the reservoir from combustion=0.0002717 xEZ MMBTU(e) at the cogen plant.

(106) Table 3 shows an analysis of the above, using ETOR values in Table 2. We have defined energy efficiency as:

(107) $\left\{\frac{\left(\begin{array}{c}\mathrm{energy}\mathrm{produced}\\ \mathrm{in}\mathrm{bitumen}\end{array}\right)-\left(\begin{array}{c}\mathrm{energy}\mathrm{used}\mathrm{on}\mathrm{surface}\mathrm{to}\\ \mathrm{produce}\mathrm{bitumen}\end{array}\right)}{\left(\begin{array}{c}\mathrm{energy}\mathrm{produced}\\ \mathrm{in}\mathrm{bitumen}\end{array}\right)}\right\}\times 100\%$

(108) Table 4 compares efficiencies. The following comments are noteworthy. (1) Surface energy use is less than reservoir energy ETOR, because oxygen delivers much more heat via combustion than it takes to make oxygen. (2) The integrated system has higher efficiencies than separate delivery for all cases except SAGDOX(9) at 95-97% oxygen purity (We assumed a stand alone steam boiler was 85% efficient c/w cogen at 80%).
2. What can be Changed and Still Accomplish Goals? (1) SAGDOX gas mix in 5-50% range O.sub.2 (2) Reservoir P
Advantages of the Invention

(109) An integrated Cogen:ASU plant to produce separate streams of steam and oxygen gas reduces overall cost of oxygen and steam/capex and opex improves energy efficiency reduces (eliminates) reliance on outside (grid) power reduces surface footprint for on-site generation of steam and oxygen
2.2 SAGD Performance

(110) We have simulated a SAGD process in a typical Athabasca reservoir—25 m.net pay, 800 m. SAGD wells separated by 5 m., 2 MPa pressure. This acts as a base case for SAGDOX comparison. The results are shown in FIG. 3.5. Bitumen recovery is 2.099 MM bbls after 10 years, avg. SOR=3.37 (ETOR=1.18), the recovery factor was 63.4% OBIP. [ETOR=MMBTU of energy/bbl bit.]

(111) 2.3 SAGDOX Performance

(112) FIG. 3 shows bitumen saturation as a function of distance from the central vertical plane, about half way in the net pay zone, for SAGDOX in a mature project. The simulation was for a generic Athabasca bitumen reservoir using a combustion kinetic model and the STARS simulator. —Looking at the plot we see, as we move outward= (1) a combustion swept zone with no residual bitumen (2) a combustion front (3) a hot bitumen bank of oil (4) a steam swept zone (5) the cold bitumen interface

(113) Bitumen drains both from the bitumen bank and from the cold bitumen front. Water drains from the saturated-steam zone and from the bitumen front.

(114) SAGDOX is a complex process—more complex than SAGD. We are not sure what is the rate-limiting step for SAGDOX, but we believe steam use and steam inventory are key factors. Steam is an ideal fluid to effect heat transfer. Compared to hot combustion gases, steam has 2 big advantages. A fixed volume of steam will deliver a least twice as much heat when it condenses compared to hot combustion gases, and, when steam condenses, it creates a transient low pressure zone that draws in more steam. Steam in a gas chamber acts like a heat pump, to the cold walls, with the plumbing.

(115) Despite lower heat transfer rates than steam, combustion has some decided advantages. Combustion will vaporize connate water, reflux some condensed steam and produce some steam as a product of combustion. These will all add to the steam inventory and aid transfer. But, as the oxygen content, in SAGDOX injection mix, increases the amount of steam injection decreases, for constant energy injection rates. Table 1 shows the properties of steam-oxygen mixtures.

(116) We expect that for SAGD productivity, we will need to inject more energy than SAGD (ie higher ETOR values), increasing as oxygen levels increase. Table 2 shows this for several SAGDOX mixtures.

(117) The preferred range of O.sub.2 concentration is between 5 and 50 (v/v) %. Below 5% oxidation may be unstable and there is little extra heat to ensure connate water evaporation and steam reflux. Above 50%, we start to oxidize bitumen that we could otherwise produce and it may be difficult to sustain water reflux rates to maintain productivity.

(118) As many changes therefore may be made to the embodiments of the invention without departing from the scope thereof. It is considered that all matter contained herein be considered illustrative of the invention and not in a limiting sense.