Method for controlling fluid interface level in gravity drainage oil recovery processes with crossflow

Abstract

In a method for controlling the interface level between a liquid inventory and an overlying steam chamber in a subterranean petroleum-bearing formation, an inflow relationship is developed to predict the vertical position in a gravity field of the interface between two fluids with a density contrast (most commonly a water/oil emulsion and steam), relative to a horizontal producer well. The inflow relationship is applied to producer well completions by designing the completion to raise or lower sand face pressures over the horizontal length of the well. This pressure distribution will affect liquid levels according to the inflow relationship. Axial flow relationships for the liquid inventory may be developed to facilitate estimation of liquid levels at selected locations. Axial flow relationships for the steam chamber may also be developed to estimate the effect of the injector well completion on the steam chamber pressure and, in turn, the liquid level.

Claims

1. A method for characterizing an axial flow relationship relating the conditions at selected first and second axially-separated locations along a horizontal injector well disposed within a petroleum-bearing formation to the axial flow rate through a steam chamber surrounding the injector well, said method comprising the steps of: (a) characterizing the injection performance relationship at the first and second locations; (b) evaluating the axial fluid mobility in the steam chamber at the first and second locations; (c) interpolating to approximate the axial fluid mobility in steam chamber between the first and second locations; and (d) calculating the axial flow rate through the steam chamber as the product of the axial fluid mobility, effective axial pressure gradient, and mean flow area.

2. A method as in claim 1 wherein the axial fluid mobility in the steam chamber between the first and second locations is taken as the average of the axial fluid mobility at the first location and the axial fluid mobility at the second location.

3. A method as in claim 1 wherein when the conditions other than the pressure are approximately equal at the first and second locations, the axial fluid mobility in the steam chamber at the first location is assumed to equal the axial fluid mobility at the second location and, in turn, the axial fluid mobility between the first and second locations.

4. A method as in claim 1 wherein the effective axial pressure gradient between the first and second locations is taken as the difference between the steam chamber pressure at the first location and the steam chamber pressure at the second location, divided by the axial distance between the first and second locations.

5. A method as in claim 1 wherein the injection performance relationship is characterized at a plurality of pairs of axially-separated locations along the injector well, and an axial flow relationship is characterized for each pair of adjacent locations to create a system of axial flow relationships.

6. A method for characterizing the steam chamber pressure distribution produced by an injector completion using (a) the system of axial flow relationships of claim 5, (b) the distribution of steam demand from the steam chamber, (c) hydraulic characterization of the injector completion, and (d) operating injection pressures for the injector completion.

7. A method for characterizing the liquid level distribution produced by a combination of injector and producer completions, said method comprising the steps of: (a) calculating the axial pressure distribution in a steam chamber associated with the injector, using the method of claim 6; (b) creating a system of axial flow relationships relating the conditions at a plurality of selected pairs of axially-separated locations along the producer to the axial flow rate through a liquid inventory surrounding the producer, by performing, with respect to each pair of axially-separated locations, the steps of: characterizing the gravity inflow performance relationship (GIPR) at each of the axially-separated locations; evaluating the axial hydraulic conductivity of the liquid inventory at each of the axially-separated locations; interpolating to approximate the axial hydraulic conductivity of the liquid inventory between the pair of axially-separated locations; and calculating the axial flow rate through the liquid inventory as the product of the axial hydraulic conductivity, effective axial hydraulic gradient, and mean flow area; and (c) calculating a liquid level distribution of a liquid inventory associated with the steam chamber, using: said system of axial flow relationships; said axial pressure distribution; the distribution with which liquid is delivered to the liquid inventory from the steam chamber; a hydraulic characterization of the producer completion; and boundary conditions corresponding the operational controls for the well.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the invention will now be described with reference to the accompanying figures, in which numerical references denote like parts, and in which:

(2) FIG. 1 is a schematic cross-section through a steam chamber within a subterranean oil sands reservoir, in conjunction with a horizontal steam injection well and a horizontal production well.

(3) FIG. 2 is an enlarged cross-section through a production well and adjacent regions as in FIG. 1.

(4) FIG. 3 is a flow chart illustrating steps in one embodiment of a method for establishing an inflow performance relationship for a production wellbore in accordance with the present disclosure.

(5) FIG. 4 is a graph illustrating the variability of inflow rate into a production well with changes in inflow temperature.

(6) FIG. 5 is a flow chart illustrating steps in one embodiment of a method for establishing an axial flow relationship for a liquid inventory surrounding a production wellbore in accordance with the present disclosure.

(7) FIG. 6 is a flow chart illustrating steps in one embodiment of a method for establishing an axial flow relationship for a steam chamber surrounding an injection wellbore in accordance with the present disclosure.

DETAILED DESCRIPTION

(8) FIG. 1 schematically illustrates a horizontal well pair (i.e., injector and producer) in a typical SAGD bitumen recovery installation in a bitumen-laden subterranean oil sands formation 30 underlying an overburden layer 20 extending to the ground surface 10, and overlying an underburden formation 40, all in accordance with prior art knowledge and well within the understanding of persons of ordinary skill in the art. Steam under high pressure is introduced into injector well 50 from a connecting well leg (not shown) extending to ground surface 10. Injector 50 has a slotted or orificed liner such that steam exits injector 50 through the liner slots or orifices and permeates oil sands formation 30 to create a steam chamber 70 within formation 30. In this context, the term “steam chamber” may be understood to mean a volume within formation 30 in which steam remains present and mobile, at least for so long as steam injection into formation 30 continues. For analytical purposes, it is assumed that regions of formation 30 outside steam chamber 70 are essentially uninfluenced by the steam injected through injector 50.

(9) The pattern of steam migration within formation 30, and thus the configuration of steam chamber 70, will vary with a variety of factors including formation characteristics and steam injection parameters. However, as represented by the idealized configuration shown in FIG. 1, a typical steam chamber 70 for a SAGD well can be considered or modeled as being generally wedge-shaped in cross-section, surrounding injector well 50, with a “roofline” 72 and sloping side boundaries 74 converging downward toward a lower limit 76. Steam migrating to steam chamber side boundaries 74 condenses due to the lower temperature of the surrounding region of formation 30, which creates the “demand” for steam to flow from the injector into the steam chamber. As the steam condenses, it transfers energy to the bitumen, increasing its temperature and thus decreasing its viscosity such that it becomes flowable, whereupon the mobile bitumen and condensate flow downward and accumulate as a liquid inventory 80 within a lower region of steam chamber 70, below injector 50. A fluid interface 85 is thus formed between liquid inventory 80 and the overlying region of steam chamber 70. Based on theory and field observation, the level of fluid interface 85 is assumed for analytical purposes to be lowest (i.e., closest to producer 60) at a point 85X directly above producer 60.

(10) A producer well 60 is installed at a selected depth below and generally parallel to injector 50, such that it can be expected to lie within the zone of liquid inventory 80 upon formation of steam chamber 70. Producer well 60 has slots or other suitable orifices to allow the bitumen/condensate mix in liquid inventory 80 to enter producer 60 for production to the surface 10. For this purpose, producer well 60 typically has a liner with narrow slots or other orifices that allow liquid flow into producer 60 while substantially preventing sand or other contaminants from entering producer 60 or clogging the slots or orifices in the liner.

(11) FIG. 2 provides an enlarged illustration of liquid inventory 80 and producer well 60 within a lower region of steam chamber 70. Also indicated in FIG. 2 is an analysis boundary 90 surrounding producer well 60, with analysis boundary 90 being an empirically defined or selected parameter for purposes of predictive methods in accordance with the present disclosure. In accordance with a preferred embodiment of these predictive methods, analysis boundary 90 is assumed to be circular in cross-section and centered around producer well 60, with a radius corresponding the distance from the center of producer 60 to point 85X on fluid interface 85. However, alternative configurations of analysis boundary 90 may be appropriate to satisfy case-specific physical and/or analytical constraints.

(12) Gravity Inflow Performance Relationship (Gravity IPR)

(13) FIG. 3 schematically illustrates one embodiment of a procedure for developing a “gravity IPR” for use in evaluating the stability of liquid inventory 80. In this context, the stability of liquid inventory 80 relates to the stability of the vertical distance from producer 60 to point 85X on fluid interface 85 at given points along the horizontal length of producer 60 (which for purposes of FIG. 2 corresponds to the radius of circular analysis boundary 90). Procedural and analytical steps shown in FIG. 3 are summarized below:

(14) Stage 101—Temperature Measurements:

(15) Measure temperatures within steam chamber 70 and the vertical temperature gradient in liquid inventory 80. Define the temperature drawdown to be the difference between the steam chamber temperature and the inflow temperature (i.e., temperature of produced fluids flowing into producer well 60). For this purpose: Temperature drawdown=steam chamber temperature—inflow temperature.
Stage 102—Define Analysis Boundary: Consider a cross-section of producer wellbore 60 and the surrounding liquid inventory 80 in a plane perpendicular to the axis of the wellbore. Define analysis boundary 90 such that it encompasses producer wellbore 60 and contacts fluid interface 85 between liquid inventory 80 and the overlying steam chamber 70. The distance between producer wellbore 60 and fluid interface 85 (i.e., the liquid level) is given by the temperature drawdown and the vertical temperature gradient. For this purpose: Liquid level=temperature drawdown/vertical temperature gradient.
Stage 103—Temperature Mapping: Map the measured steam chamber temperature and vertical temperature gradient onto the area enclosed by analysis boundary 90. For this purpose: The temperature at liquid-vapor interface 85 is assumed to equal the steam temperature. The temperature at locations within analysis boundary 90 is calculated from the vertical temperature gradient and the distance below the liquid-vapor interface 85.
Stage 104—Solution: Specify the pressure conditions at analysis boundary 90 and producer wellbore 60. Define the pressure drawdown to be the difference between the steam chamber pressure and the wellbore pressure. Using numerical or analytical methods known to persons of ordinary skill in the art, determine the relationship between the pressure drawdown and the flow rate into wellbore 60. For this purpose: The pressure at liquid-vapor interface 85 is assumed to equal the pressure within steam chamber 70 (which is taken to be the saturation pressure corresponding to the measured steam chamber temperature). The total head (i.e., the sum of the pressure head and the elevation head) is assumed to be constant along analysis boundary 90. A skin factor is included to account for near-wellbore pressure losses that are measured in the field but not captured by conventional equations for flow through porous media (e.g., Darcy's Law). “Skin factor” in this context is a term well understood in the field (see, for example, the definition of skin factor in the Schlumberger Oilfield Glossary: www.glossary.oilfield.slb.com). Flow chart blocks 110 and 120 in FIG. 3 represent additional criteria taken into consideration in the solution stage 104: Block 110—The analysis boundary represents a uniform head (i.e., a flow isobar), and flow normal to the boundary integrated around the perimeter of the boundary defines the inflow to the wellbore. In its simplest form, it is a cylindrical boundary centered on the producer wellbore and touching the lowest part of the fluid interface. Other shapes for the analysis boundary can be incorporated to reflect better conformance to a different fluid level interface, if additional refinement to reflect a changing steam chamber shape with time is desired. Block 120—Reservoir and fluid properties are calculated over the range of temperatures considered inside the analysis boundary. Relative permeability properties are incorporated and in combination with the temperature field and fluid portions in determining the pressure gradients that are integrated to arrive at the inflow characterization.
Stage 105—Stability Assessment: Determine the relationship between the pressure drawdown and inflow rate at various temperature drawdowns. Plot inflow rate as a function of inflow temperature for a constant pressure drawdown, as shown in FIG. 4. The slope of the plotted curve(s) is negative in the stable range of inflow temperatures. Within the stable range of inflow temperatures, an increase in liquid level (resulting when the delivery rate into liquid inventory 80 exceeds the inflow rate into producer well 60) will cause the inflow rate to increase. The liquid level will rise until it reaches an equilibrium position at which the inflow rate matches the delivery rate. A decrease in liquid level (resulting when the inflow rate exceeds the delivery rate) causes the inflow rate to decrease. The liquid level will drop until it reaches an equilibrium position at which the inflow rate matches the delivery rate. Outside the stable range of inflow temperatures, an increase in liquid level will cause the inflow rate to decrease, allowing the liquid level to “run away.” For certain combinations of pressure drawdown, fluid properties, and reservoir properties, the slope of the curve(s) will be positive for all inflow temperatures, indicating that there is no stable range of inflow temperatures. A decrease in liquid level will cause the inflow rate to increase, potentially leading to steam breakthrough into producer 60.
Practical Application of Gravity IPR

(16) When coupled to a wellbore hydraulic model, the gravity IPR enables the performance of a production well to be evaluated by measuring the inflow temperature along the well to determine when the liquid level is reaching critical levels (i.e., when fluid level rise in portions of the well compromises production efficiency, or when fluid level drop in portions of the well compromises well integrity). More specifically, the gravity IPR provides a basis for: Configuring producer well completions to deliver a pressure distribution that is within the range of self-balancing performance over the life of the well. Evaluating how pump intake subcool should be controlled to maintain hydraulic conditions within the self-balancing range of operation over the entire well. Evaluating production rate capacities for specific completion options and field applications. Using inflow temperature distributions for evaluating completion configuration changes to match reservoir variations and maintain performance within the self-balancing range over the entire well. Using temperature fall-off logs for evaluating completion configuration changes to match reservoir variations and maintain performance within the self-balancing range over the entire well. Using temperature measurements to set “smart well” controls for production wells and maintain performance within the self-balancing range over the entire well. Positioning or repositioning tubing intake points to maintain performance within the self-balancing range over the entire well. Adjusting chokes on gas lift tubing based on intake temperature to maintain performance within the self-balancing range over the entire well. Determining where fluid conditions approach water saturation, leading to flashing, which in turns chokes flow to automatically regulate inflow. By using flow conditions in the GIPR assessment, determining locations where pore throat water flashing may produce scaling and inflow restrictions. If options exist for modifying the steam chamber pressure distribution with the injector completion, the GIPR assessment can be used to determine the steam chamber pressure variation required to control the liquid level of the liquid inventory.

(17) The gravity IPR also provides a basis for determining reservoir delivery distribution over the length of the steam chamber: For producer wells operating in the self-balancing range, the delivery distribution can be calculated from temperature fall-off logs and inflow distributions using distributed temperature measurements under static inflow conditions. For wells operating in the dynamic range, the reservoir delivery distribution can be calculated from the inflow rate to the well and the transient behaviour of the fluid level. Transient plugging development (for example, plugging of slots/orifices in the liner, or plugging in the formation itself by way or pore throat plugging) can be determined using temperature measurements and the gravity IPR. Producer well configuration updates can be evaluated to: Assess the likelihood of maintaining the well in the self-balancing performance envelope and the reconfiguration requirements to maintain stability. Determine a production intervention schedule to maintain an efficient production distribution under dynamic fluid level control.

(18) Other analytical methods for describing the inflow performance of the SAGD or any other gravity process can be calibrated using methods in accordance with the present disclosure. For example a conventional IPR inflow performance relationship can be calibrated by determining the drainage radius in the basic IPR equation as a function of inflow temperature. This can provide an even simpler basis for evaluating SAGD inflow performance. One example of such an application would be wellbore hydraulics programs used for analyzing and optimizing completions for SAGD production.

(19) Axial Flow Relationship

(20) FIG. 5 schematically illustrates one embodiment of a procedure for developing an axial flow relationship for use in predicting the axial flow rate through liquid inventory 80. In FIG. 5, reference numbers 101-105, 110, and 120 correspond to the same reference numbers in FIG. 3, specifically in the context of a first location along a producer well. Reference numbers 201-205, 210, and 220 similarly correspond to flow chart blocks 101-105, 110, and 120 in the context of a second location along the producer well. Procedural and analytical steps shown in FIG. 5 are summarized below:

(21) Characterization of Gravity IPR at Two Axial Locations:

(22) Characterize the gravity IPR at two axial locations along producer well 60: Measured or estimated conditions at the two locations (for example, steam chamber temperature, vertical temperature gradient, fluid properties, or reservoir properties) will be used to approximate conditions in the liquid inventory between the two locations. The greater the distance between the two locations, the greater the uncertainty in this approximation. An analysis boundary suitable for characterization of the gravity IPR may not be appropriate for characterization of the axial flow relationship. When liquid flows radially from fluid interface 85 to producer well 60, the pressure gradient is largest near producer well 60, where the flow area is smallest and the fluid viscosity is highest (because the temperature decreases from fluid interface 85 to producer well 60). Consequently, conditions in the part of liquid inventory 80 near producer well 60 will have a greater influence on the gravity IPR than conditions in other parts of liquid inventory 80. By contrast, the axial flow relationship will be most strongly influenced by conditions in the part of liquid inventory 80 near fluid interface 85, where the temperature is highest and the fluid is most mobile. Therefore, for characterization of the axial flow relationship, analysis boundary 90 should be expanded to include the part of liquid inventory 80 near fluid interface 85. For purposes of characterizing an axial flow relationship, the axial hydraulic conductivity may be calculated at numerous points in liquid inventory 80 and analysis boundary 90 defined according to an axial hydraulic conductivity criterion. For example, the analysis boundary may be drawn along a contour of constant axial hydraulic conductivity to encompass only the part of the liquid inventory where the axial hydraulic conductivity is greater than a specified minimum value. The axial hydraulic conductivity criterion may alternatively be expressed in terms of an axial hydraulic conductivity ratio—for example, the ratio of the local axial hydraulic conductivity to the maximum axial hydraulic conductivity.
Evaluation of Axial Hydraulic Conductivity of Liquid Inventory—Block 300: Evaluate the axial hydraulic conductivity of the part of liquid inventory 80 enclosed by analysis boundary 90 at both axial locations, using numerical or analytical methods known to persons of ordinary skill in the art. The axial hydraulic conductivity is the proportionality constant relating the axial flow velocity and the axial hydraulic gradient. Interpolate to approximate the axial hydraulic conductivity of liquid inventory 80 between the two axial locations. For this purpose: The axial hydraulic conductivity of liquid inventory 80 between the two axial locations is taken as the average of the axial hydraulic conductivity at the first location and the axial hydraulic conductivity at the second location. When conditions other than the liquid level (for example, the steam chamber temperature, vertical temperature gradient, fluid properties, and reservoir properties) are approximately equal at the two locations, the axial hydraulic conductivity of liquid inventory 80 at the first location may be assumed to equal the axial hydraulic conductivity at the second location and, in turn, the axial hydraulic conductivity between the two locations. By extension, when conditions other than the liquid level are approximately uniform along producer well 60, the axial hydraulic conductivity of liquid inventory 80 need only be evaluated at one axial location. Variations in the liquid level will shift the mobile part of liquid inventory 80 vertically but will not significantly affect the axial hydraulic conductivity.
Calculation of Axial Flow Rate—Block 310: Calculate the axial flow rate through liquid inventory 80 as the product of the axial hydraulic conductivity, effective axial hydraulic gradient, and mean flow area. For this purpose: The effective axial hydraulic gradient between the two locations is taken as the difference between the liquid level at the first location and the liquid level at the second location, divided by the axial distance between the two locations. The effective axial hydraulic gradient may account for variations in the axial hydraulic gradient with distance from producer well 60 due to radial flow from fluid interface 85 to producer well 60. The mean flow area is taken as the average of the areas enclosed by analysis boundary 90 at the two locations.
Practical Application of Gravity IPR with Crossflow

(23) The gravity IPR may be characterized at a plurality of axial locations along the producer well and axial flow relationships developed for each pair of adjacent locations to create a system of axial flow relationships, or axial flow “network”. When included in a wellbore hydraulic model coupled with the gravity IPR, an axial flow network enables improved estimation of liquid level variations over time, based not only on an imbalance between the inflow distribution and delivery distribution, but also on the axial redistribution of liquid from locations with a higher liquid level to locations with a lower liquid level.

(24) Practical applications of an axial flow network include: estimation of the liquid level above blank (i.e., unslotted or unscreened) sections of the producer liner, where liquid must flow axially through the liquid inventory before flowing radially into a slotted section of the liner; and estimation of the liquid level above locations of formation damage, where a reduction in the near-wellbore permeability causes liquid to flow preferentially in the axial direction.
Method for Controlling Steam Chamber Pressure

(25) FIG. 6 schematically illustrates one embodiment of a procedure for developing an axial flow relationship for use in controlling the pressure in steam chamber 70. Procedural and analytical steps shown in FIG. 6 are summarized below:

(26) Characterization of Injection Performance Relationship at Two Axial Locations:

(27) Characterize the injection performance relationship at two axial locations along injector well 50 using numerical or analytical methods known to persons of ordinary skill in the art. The injection performance relationship relates the pressure difference between injector well 50 and steam chamber 70 to the flow rate out of injector well 50. Characterization of the injection performance relationship for injector well 50 is significantly simpler than characterization of the gravity IPR for producer well 60 because the density of steam is negligible relative to the densities of bitumen and condensed water, and because the temperature in the steam chamber is approximately uniform. The effect of gravity may be neglected, and the fluid viscosity may be assumed to be spatially uniform. The pressure gradient associated with flow from injector well 50 into steam chamber 70 is largest near injector well 50, where the flow area is smallest and the flow velocity is highest. Consequently, conditions in the part of steam chamber 70 near injector well 50 will have a greater influence on the injection performance relationship than conditions in other parts of steam chamber 70.
Evaluation of Axial Fluid Mobility in Steam Chamber Evaluate the axial fluid mobility in steam chamber 70 at both axial locations, using numerical or analytical methods known to persons of ordinary skill in the art. The axial fluid mobility is the proportionality constant relating the axial flow velocity and the axial pressure gradient. Interpolate to approximate the axial fluid mobility in steam chamber 70 between the two axial locations. For this purpose: The axial fluid mobility in steam chamber 70 between the two axial locations is taken as the average of the axial fluid mobility at the first location and the axial fluid mobility at the second location. When conditions other than the pressure (for example, the fluid properties and reservoir properties) are approximately equal at the two locations, the axial fluid mobility in steam chamber 70 at the first location may be assumed to equal the axial fluid mobility at the second location and, in turn, the axial fluid mobility between the two locations. By extension, when conditions other than the pressure are approximately uniform along injector well 50, the axial fluid mobility in steam chamber 70 need only be evaluated at one axial location. Variations in the pressure will affect the temperature in steam chamber 70, and in turn the fluid viscosity, since temperature is a function of pressure for saturated steam; however, in many practical applications, the temperature variations and resulting fluid mobility variations will be negligible.
Calculation of Axial Flow Rate Calculate the axial flow rate through steam chamber 70 as the product of the axial fluid mobility, effective axial pressure gradient, and mean flow area. For this purpose: The effective axial pressure gradient between the two locations is taken as the difference between the pressure in steam chamber 70 at the first location and the pressure in steam chamber 70 at the second location, divided by the axial distance between the two locations. The effective axial pressure gradient may account for variations in the axial pressure gradient with distance from injector well 50 due to radial flow from injector well 50 into steam chamber 70. The mean flow area is taken as the average of the cross-sectional area of steam chamber 70 at the first location and the cross-sectional area of steam chamber 70 at the second location. The boundary of steam chamber 70 is characterized by a change in temperature, from the water saturation temperature in steam chamber 70 to a temperature below the water saturation temperature outside of steam chamber 70. The size, shape, and cross-sectional area of steam chamber 70 may thus be estimated from temperature measurements (obtained, for example, from vertical “observation” wells drilled near the SAGD well pair). The boundary of steam chamber 70 is additionally marked by a change in fluid density, from the density of water vapour in steam chamber 70 to the much higher density of water condensate outside of steam chamber 70. This change in density is associated with a change in the acoustic properties of the formation, and so seismic surveys may also be used to estimate the cross-sectional area of steam chamber 70.
Practical Application of Method for Controlling Steam Chamber Pressure

(28) The injection performance relationship may be characterized at a plurality of axial locations along the injector well and axial flow relationships developed for each pair of adjacent locations to create an axial flow network for the steam chamber. When included in a wellbore hydraulic model, an axial flow network for the steam chamber enables estimation of the pressure distribution in the steam chamber, which is useful when it is only practical to measure the steam chamber pressure at a limited number of axial locations, or when the pressure gradients in the steam chamber are too small to detect with available instrumentation. An axial flow network for the steam chamber may be further coupled to an axial flow network for the liquid inventory and, in turn, to a wellbore hydraulic model for the producer well to create a flow network for the injector-producer well pair.

(29) Practical applications of a flow network for the injector-producer well pair include: estimation of the pressure distribution in the steam chamber corresponding to a specified steam demand distribution; optimization of the injector completion to provide a pressure distribution in the steam chamber that leads to a favourable (usually uniform) liquid level along the length of the well pair, including: optimization of the size and position of tubing strings in the injector; optimization of the design and placement of tubing-conveyed flow control devices, including ported tubing strings and tubing-conveyed packers or baffles; optimization of the design and placement of liner-conveyed flow control devices; and/or optimization of the length and position of blank (i.e., unslotted or unscreened) sections of the injector liner; and optimization of the injector control strategy to provide a steam chamber pressure distribution that leads to a favourable (usually uniform) liquid level, including optimization of the steam injection split between tubing strings terminating at different depths in the injector.

(30) It will be readily appreciated by those skilled in the art that various modifications of methods in accordance with the present disclosure may be devised without departing from the scope and teaching of the present invention. It is to be especially understood that the subject methods are not intended to be limited to any described or illustrated embodiment, and that the substitution of a variant of a claimed element or feature, without any substantial resultant change in the working of the methods, will not constitute a departure from the scope of the invention.

(31) In this patent document, any form of the word “comprise” is to be understood in its non-limiting sense to mean that any item following such word is included, but items not specifically mentioned are not excluded. A reference to an element by the indefinite article “a” does not exclude the possibility that more than one of the element is present, unless the context clearly requires that there be one and only one such element.

(32) Relational terms such as “parallel”, “horizontal”, and “perpendicular” are not intended to denote or require absolute mathematical or geometric precision. Accordingly, such terms are to be understood in a general rather than precise sense (e.g., “generally parallel” or “substantially parallel”) unless the context clearly requires otherwise.

(33) Wherever used in this document, the terms “typical” and “typically” are to be interpreted in the sense of representative or common usage or practice, and are not to be understood as implying invariability or essentiality.