Classifications

• • E—FIXED CONSTRUCTIONS
  • E21—EARTH OR ROCK DRILLING; MINING
  • E21B—EARTH OR ROCK DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
  • E21B43/00—Methods or apparatus for obtaining oil, gas, water, soluble or meltable materials or a slurry of minerals from wells
  • E21B43/16—Enhanced recovery methods for obtaining hydrocarbons
  • E21B43/24—Enhanced recovery methods for obtaining hydrocarbons using heat, e.g. steam injection
  • E21B43/2406—Steam assisted gravity drainage [SAGD]
• • E—FIXED CONSTRUCTIONS
  • E21—EARTH OR ROCK DRILLING; MINING
  • E21B—EARTH OR ROCK DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
  • E21B43/00—Methods or apparatus for obtaining oil, gas, water, soluble or meltable materials or a slurry of minerals from wells
  • E21B43/30—Specific pattern of wells, e.g. optimising the spacing of wells
  • E21B43/305—Specific pattern of wells, e.g. optimising the spacing of wells comprising at least one inclined or horizontal well

Definitions

• SAGD Steam Assisted Gravity Drainage
• SAGD is a saturated-steam process using two parallel horizontal wells.
• the upper well is a steam injector.
• the lower well is a fluid producer (bitumen + water).
• the process is operated by injecting steam to achieve a target pressure (pressure control) and to produce fluids at an average temperature (T) less than saturated steam T (sub-cool or steam-trap control).
• Leaky reservoirs are bitumen reservoirs that produce an unusual amount of water using SAGD. Leaks may occur from top water, bottom water, or interspersed water lean zones (WLZ). SAGD performance may be seriously impaired in a leaky reservoir.
• SAGDOX Steam assisted gravity drainage with Oxygen
• SAGD is a hybrid, combining SAGD and in situ combustion (ISC).
• SAGDOX has geometry similar to SAGD but adds two (or more) vertical wells to inject oxygen gas and to produce vent gas (combustion non-condensible gases).
• vent gas combustion non-condensible gases.
• SAGD pressure control on steam injection and sub-cool control on production
• SAGDOX adds oxygen injection and vent gas removal as control variables.
• SAGD and SAGDOX EOR proceed through four distinct phases. In order, the phases are 1) start up, 2) growth, 3) decline, and 4) shut down. The transition from growth to decline occurs when the GD (gravity drainage) gas chamber reaches the net pay ceiling.
• the process control objectives and methods are different for each stage of the SAGDOX process and are influenced by the characteristics of a leaky reservoir.
• This invention describes SAGDOX control methods for the SAGDOX stages in a leaky reservoir.
• the Athabasca bitumen resource in Alberta, Canada is unique for the following reasons:
• the in situ EOR estimate is based on SAGD, or a similar process.
• Bitumen reservoirs have a top seal (cap rock) that prevents oil from leaking and contains the resource. Bitumen was formed by bacterial degradation of lighter source oil to a stage where the degraded bitumen is immobile under reservoir conditions. Bitumen reservoirs can be self-sealed (no cap rock seal). If an in situ EOR process hits the "ceiling", the process may not be contained and it can easily be contaminated by water or gas from above the bitumen.
• Bitumen density is close to the density of water or brine. Some bitumens are denser than water; some are less dense than water. During the bacterial- degradation and formation of bitumen, the hydrocarbon density can pass through a density transition and water can, at first, be less dense than the reservoir "oil”. Bitumen reservoir water zones are found above the bitumen (top water), below the bitumen (bottom water), or interspersed in the bitumen net pay zone (water lean zones (WLZ)).
• the matrix permeability can be very high, particularly the vertical permeability. Permeabilities can be as high as about 6D. This is important for gravity drainage processes that rely on high vertical permeability for good drainage rates and high bitumen productivity.
• SAGD is a bitumen EOR process that uses saturated steam to deliver energy to a bitumen reservoir.
• Figure 1 shows the basic SAGD geometry, using twin, parallel horizontal wells (up to about 1000m long) separated by about 5m spacing with the lower well parallel to the net pay basement, 2 to 8 m above the "floor".
• the upper well is in the same vertical plane and injects saturated steam into the reservoir.
• the steam heats the bitumen and the reservoir matrix.
• the heated bitumen and condensed steam drain, by gravity, to the lower horizontal well that produces the liquids.
• the heated liquids (bitumen + water) are pumped (or conveyed) to the surface using ESP pumps or a gas-lift system.
• Figure 2 shows how SAGD matures.
• a young steam chamber has bitumen drainage from steep sides and from the chamber ceiling. When the chamber grows and hits the top of the net pay zone, drainage from the chamber ceiling stops and the slope of the side walls decreases as the chamber continues to grow outward.
• bitumen productivity peaks at about 1000 bbls/d when the chamber hits the top of the net pay zone and falls as the chamber grows outward, until eventually the economic limit (10-20 years) is reached. Since the produced fluids are at/near saturated steam temperatures, it is only the latent heat of the steam that contributes to the process in the reservoir. It is important to ensure that steam is high quality as it is injected into the reservoir.
• WRR water recycle ratio
• SAGD operation in a good-quality reservoir, is straightforward.
• Steam injection rate, into the upper horizontal well, and steam pressure are controlled by pressure targets chosen by the operator. If the pressure is below the target, steam pressure and injection rates are increased. The opposite is done if pressure is above the target.
• Production rates from the lower horizontal well are controlled to achieve sub-cool targets in the average temperature of the production fluids.
• the sub-cool is the difference in temperature of saturated steam and the actual temperature of produced liquids (bitumen + water). Produced fluids are kept at lower T than saturated steam to ensure that live steam does not get produced. 20°C is a typical sub-cool target. This is also called steam trap control.
• the SAGD operator has two choices to make— the sub-cool target and the operating pressure of the process.
• Sub-cool is safety issue, but operating pressure is more subtle and usually more important.
• the higher the pressure the higher the temperature— linked by the properties of saturated steam ( Figure 3).
• Figure 3 As operating temperature rises, so does the temperature of the heated bitumen which, in turn, reduces bitumen viscosity.
• Bitumen viscosity is a strong function of temperature ( Figure 4).
• the productivity of a SAGD well pair is proportional to the square root of the inverse bitumen viscosity (Butler (1991)). So the higher the pressure, the faster bitumen can be recovered - a key economic performance factor.
• the SAGD operator usually opts to maximize economic returns, so the operator increases P and T as much as possible. Pressures are usually much greater than native reservoir P. A few operators have gone too far and exceeded parting pressure (fracture pressure) and caused a surface breakthrough of steam and sand (Roche, P., "Beyond Steam", New Tech. Mag., Sept 2011).
• SAGD There also may be a hydraulic limit for SAGD (Figure 5).
• the hydrostatic head between the two SAGD wells is about 8 psia (56 kPa).
• the steam/liquid interface can be "tilted", and it can intersect the producer or injector well ( Figure 5). If the producer well is intersected, steam can break through. If the injector well is intersected, it can be flooded and effective injector length can be shortened.
• SAGD well lengths are limited to about 1000m due to this limitation.
• WLZ Zones with high water saturation are known as WLZ.
• WLZ can be at the top of the bitumen reservoir (top water), at the bottom (bottom water), or interspersed within the pay zone.
• Interspersed WLZ have to be heated so that GD steam chambers can envelop the zone and continue growth of the GD chamber above and around the WLZ blockage.
• a WLZ has a higher heat capacity than a bitumen pay zone.
• Table 3 shows a 25%
• a WLZ also has higher heat conductivity than a bitumen pay zone. For the example in (2), WLZ has more than double the heat conductivity of the bitumen pay zone.
• the interspersed WLZ acts as a thief zone, the problems are most severe.
• the WLZ can channel steam away from the SAGD steam chamber. If the steam condenses prior to removal, the water is lost but the heat can be retained. But, if the steam exits the GD steam chamber prior to condensing, both the heat and the water are lost to the process.
• Interspersed WLZ's can distort SAGD steam chamber shapes, particularly if the WLZ is limited in lateral size. Normal growth is slowed down as the WLZ is breached. This can reduce productivity, decrease efficiency, and limit recovery.
• bottom water zones As best seen in Figure 7, the issues are similar to interspersed WLZ except that 1) bottom water underlies the bitumen and 2) the usual expectation is that bottom water is more active.
• SAGD can operate at pressures greater than reservoir pressure as long as the following occurs: 1) pressure drops in the production well (due to flow/pumping) do not reduce local pressures below reservoir P and 2) the bottom of the reservoir, underneath the production well, is "sealed" by high- viscosity immobile bitumen (basement bitumen).
• base bitumen basement bitumen
• basement bitumen will become heated by conduction from the production well. After a few years, this bitumen will become partially mobile and SAGD pressure will need to be reduced to match reservoir pressure. This can be a delicate balance.
• SAGD pressures cannot be too high or a channel may form, (reverse cone) allowing communication with the bottom water.
• SAGD steam pressures cannot be too low either or water will be drawn from the bottom water (cresting). If this occurs, water production will exceed steam injection. The higher the pressure drops in the production well, the more delicate the balance and the more difficult it is to achieve a balance.
• the channel or crests can be partial and the onset of the problem is accelerated.
• top water zones As best seen in Figure 7, again, the issues are similar to interspersed WLZ and bottom water, with the expectation that top water is also an active water supply.
• the problems are similar to bottom water, as above, except that SAGD wells are further away from top water. So, the initial period-when the process can be operated at higher pressures than reservoir pressure-can be extended compared to bottom water. The pressure drop in the production well is less of a concern because it is far away from the ceiling.
• the first problem is likely to be steam breaching the top water interface. If the top water is active, water will flood the chamber and may shut the SAGD process down. i.
• SAGDOX is a process similar to SAGD; however, it uses oxygen gas as well as steam to provide energy to the reservoir to heat bitumen.
• the GD chamber is preserved, but it contains a mixture of steam and hot combustion gases.
• SAGDOX can be considered a hybrid process, combining steam EOR (SAGD) and in situ combustion (ISC). SAGDOX preserves the SAGD horizontal well pair, but the process adds at least two new wells (Figure 8) - one well to inject oxygen gas and a second well to remove non-condensable combustion gases. Compared to SAGD, SAGDOX has the following advantages/features:
• the oxygen content in steam and oxygen mixes (e.g. Table 1) is used as a way to label the process.
• the term mix or mixture doesn't imply that a mixture is injected, or that good mixing is a prerequisite for the EOR process. It is only a convenient way to label the process. In fact, the preferred process has separate injectors for oxygen and steam.
• SAGDOX also has the following features that can be helpful for EOR in impaired bitumen reservoirs:
• the vertical oxygen injector vertical wells and the produced gas (PG) vent wells are small diameter wells - 3 to 4 inches for most SAGDOX operations.
• the wells are inexpensive to drill.
• the oxygen injector can be completed in/near a WLZ (water lean zone) or near a shale barrier to take advantage of residual fuel in the WLZ or hydrocarbon fuel in shales.
• SAGDOX can have average T much higher than SAGD. Combustion occurs at T between 400 and 800°C (HTO), compared to steam T ⁇ 250°C.
• SAGDOX has lower fluid flow rates (bitumen + water) in the horizontal production well. This will lower pressure drops down the length of the well, producing a more-even pressure distribution than SAGD.
• a method to operate a SAGDOX process, in a leaky bitumen reservoir wherein said SAGDOX process comprises a start-up phase, growth phase, decline phase and shut down phase, said method comprising, at the start-up phase:
• Opening at least one PG vent gas well preferably to allow combustion gases to escape and to help control pressure and WRR, until steam injection is substantially limited to the upper horizontal well, oxygen injection has started and PG vent gas removal has started.
• a method to operate a SAGDOX process, in a leaky bitumen reservoir wherein said SAGDOX process comprises a start-up phase, growth phase, decline phase and shut down phase, in the growth phase of the process, preferably to increase/optimize bitumen productivity and preferably to achieve a WRR target for the process, said method comprising: (1) Adjusting oxygen/steam ratios (v/v),
• the end of the growth phase is achieved at the peak of bitumen productivity.
• a method to operate a SAGDOX process, in a leaky bitumen reservoir wherein said SAGDOX process comprises a start-up phase, growth phase, decline phase and shut down phase, in the decline phase of the process, preferably resulting in at least one of improving energy efficiency, reducing costs, maximizing bitumen recovery and achieving a WRR target, comprising:
• a method to operate a SAGDOX process in a leaky bitumen reservoir comprising:
• said SAGDOX process has system pressures less than reservoir parting pressure.
• said SAGDOX process has system pressures greater than, or substantially equal to, native reservoir pressure.
• vent gas removal in said SAGDOX process is subject to the following constraints:
• the dry gas composition of the vent gas contains less than 5% (v/v) oxygen
• the wet gas composition of the vent gas (at surface) contains less than 20% (v/v) steam, and
• the WRR target is between 0.5 and 1.2, more preferably between 0.8 and 1.2, and most preferably between 0.9 and 1.2.
• the SAGDOX process has a sub cool target between 5°C and 30°C.
• the SAGDOX process further comprises adjusting combustion conformance by adjusting individual PG vent well rates.
• the leaky bitumen reservoir comprises a top water zone
• said process further comprises adjusting PG vent well production, creating a top zone from non-condensable gases, acting as an insulator and thus minimizing GD chamber vertical growth and maximizing horizontal growth.
• efficiency of the process is monitored by ETOR.
• bitumen is a liquid hydrocarbon with API ⁇ 10 and viscosity > 100,000 cp.
• said leaky bitumen reservoir has a WRR outside the range 0.9 to 1.1, after 200 days or more of SAGDOX operation.
• bitumen reservoir is determined to be leaky by a cold water injection testing, preferably performed prior to start-up.
• bitumen reservoir is determined to be leaky based on geological or geophysical data/interpretation prior to start-up.
• Figure 1 depicts a typical SAGD well configuration
• Figure 2 depicts the stages in a SAGD operation
• FIG. 3 depicts Saturated Steam Properties
• Figure 4 depicts Bitumen and heavy Oil Viscosities versus Temperature
• FIG. 6 depicts Interspersed Bitumen Lean Zones
• Figure 7 depicts Top and bottom Water Zones
• Figure 8 depicts a SAGDOX well configuration
• Figure 9 depicts a SAGD Simulation
• Figure 12 depicts Case 2(a)
• Figure 16 depicts Cumulative Performances of Cases 1 through 3
• Figure 17 depicts Cumulative Performances of Cases 1, 4 and 5
• Figure 18 depicts Dual Well Pair Production and Performance of Case 1 and 2
• Figure 19 depicts Pressure Control Performance of Connected Well Pairs of Case 1 and 2
• Figure 20 depicts the WRR performance with crossflow in connected well pairs of Case 3
• Figure 21 depicts the WRR performance of connected well pairs of Cases 1 and 3
• Figure 22 depicts the SOR Performance of Case 1 and 3
• Figure 23 depicts Individual Well Pair Bitumen Production of Case 3
• Figure 24 depicts Well Pair Bitumen Production Rates of Case 1 and 3
• Figure 25 depicts the WRR Performance for a Homogeneous Reservoir with Contained
• Figure 26 depicts Steam volumes occupying Bitumen voidages and the SOR
• Figure 27 depicts a Schematic of a well Pair Cross-flow Model
• FIG 28 depicts Water lean zone Bitumen recovery
• the subject of this invention is SAGDOX operations in a "leaky" bitumen reservoir.
• SAGDOX operations in a "leaky" bitumen reservoir.
• Bitumen production has limited ceiling drainage and the slope of the lateral walls declines.
• Figure 9 shows the predicted performance of the homogeneous contains (no leaks) reservoir.
• Figures 10 to 24 show the performance of dual well pairs connected by a WLZ for the various sensitivities studied (leaky reservoir).
• Figures 20, 21, and 25 show how the WRR varied for the simulations.
• WRR is the volume ratio of water produced to steam injected, both measured as condensed water.
• WRR is a measurement that can indicate whether or not water is leaking into or out from a SAGD pattern volume.
• a "leaky” pattern is one that produces an unusual amount of water in a bitumen reservoir (i.e. a leaky bitumen reservoir).
• the pattern may have water leaks in/out of the pattern volume to other portions of the reservoir; it can have water leaks to/from an adjacent reservoir pattern; or, it can produce unusual water volumes from WLZ within the reservoir.
• the WRR will be used as an indicator (the volume ratio of produced water to steam injected, where steam is measured as a water- volume equivalent)
• Figure 25 shows the expected WRR behaviour.
• WRR is between 0.90-0.95.
• the GD steam chamber is forming and the GD area is heating up. An inventory of liquid water is created in the reservoir.
• WRR increases gradually from about 0.96 to 0.99. If the bitumen voidage is occupied by steam only, one would expect WRR to be greater than 0.99 ( Figure 1 1).
• bitumen production is small and the WRR approaches the 0.99 value ( Figure 25).
• a reasonable target for WRR for a perfectly contained SAGD GD chamber and a homogeneous reservoir during the peak period of SAGD (500-1500 days) is about 0.97.
• the pattern may be designated as “leaky” or potentially “leaky”.
• SAGD typically has measurements of the following:
• Sub cool equals the difference between saturated steam T in the reservoir (near the production well) and produced fluid T.
• SAGDOX includes all the measurements and parameters as with SAGD, but with the following suggested additions:
• a non-leaky bitumen reservoir is one that has no substantial water leaks, independent of process operating pressure.
• Non-leaky reservoirs are not necessarily homogeneous. They may contain shales and other non-leaky impairments.
• SAGDOX operation in non-leaky bitumen reservoirs has the following elements.
• Start-up - the early objective in this phase is to start-up the two horizontal wells in the SAGD mode and to form a GD steam chamber. This objective is accomplished, similar to SAGD, by circulating steam in each of the horizontal wells until the wells communicate. Steam circulation pressures cannot exceed reservoir parting pressure. After communication is established, the lower horizontal well is converted to a fluid producer, and the upper well is converted to a steam injector. In a conventional SAGD process, this procedure can take from about 3 to 6 months.
• a secondary objective is to connect the oxygen injection well and the PG vent wells ( Figure 8) with the steam chamber and/or with the horizontal wells, so a transition to SAGDOX can be effected as early as possible.
• the objective is accomplished by circulating steam in each of the oxygen injector and PG vent wells.
• oxygen injection is started, preferable at low oxygen/steam ratios (0.05 to 0.15(v/v)) for safety reasons. The ratio can then be adjusted upwards to achieve target ratios.
• PG vent gas wells are opened to allow vent gas production and to control GD chamber pressures, subject to the following constraints:
• the dry gas composition of the PG vent gas should not contain more than 5.0% (v/v) of oxygen, preferably the PG vent gas should not contain more than 1.0% (v/v) of oxygen.
• the gas composition of the PG vent gas should not contain more than about 20% (v/v) of steam.
• PG vent well constraints can be attained by adjusting PG vent volumes, adjusting volume ratios for more than one PG vent well or by adjusting oxygen injection rates.
• oxygen/steam ratio to increase the energy supplied by oxygen (per unit energy delivered to the reservoir, oxygen is about a third the cost of steam).
• a limit of oxygen/steam is about 1.0 (i.e. 50% oxygen) to retain enough steam for effective heat transfer.
• Residual steam/water in the reservoir can still be used for good heat transfer.
• a leaky bitumen reservoir is one that produces an unusual amount of water based on either actual WRR performance, prior geological knowledge or a cold-water injectivity test prior to EOR start-up.
• bitumen reservoir concerns - top water, bottom water, or interspersed WLZ.
• SAGDOX operation in leaky reservoirs has the following elements:
• SAGDOX can be used to alter the shape of the GD gas chamber and increase bitumen recovery prior to top water breakthrough.
• the PG vent wells can be adjusted to increase retention of a non-condensable combustion gas near the top of the reservoir. This can slow down vertical growth rates and change the aspect ratio of GD chamber growth to accelerate lateral growth. The chamber shape will be stretched laterally and delay top water break through.
• SAGDOX can change conformance by adjusting PG vent well production rates to keep the GD chamber away from the WLZ.
• Influx rates from water lean zones can be reduced by matching pressures or controlling to a WRR target.
• SAGDOX can retain good bitumen productivity at low pressures because combustion temperatures are independent of pressure.
• Process T can be higher than saturated steam T.
• Figure 28 depicts 1) how the process will make use of bitumen in WLZ and 2) the net bitumen in the WLZ recovered.
• the break-even point for the process is at 5.5% bitumen in WLZ.
• Pay zone 35% porosity with 80% bitumen saturation
• Case 3 same as 2, but shift to volume control (drop P control) after 1 yr.

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