Abstract:
A process for recovery of hydrocarbons in a production fluid from an underground reservoir of said hydrocarbons, the process comprising of: (a) injecting a viscosity reducing solvent of a fraction of said hydrocarbons into said reservoir at a pressure in the reservoir of above a liquid/vapor phase change pressure of a fraction of said solvent; said pressure in said reservoir also being sufficient to cause geomechanical formation dilation or pore fluid compression, and then, (b) allowing said solvent to mix with said hydrocarbons under pore dilation conditions, and then, (c) reducing the pressure in said reservoir to below said liquid/vapor phase change pressure of at least said fraction of said solvent thereby evincing solvent gas drive of said fraction of said hydrocarbons from said reservoir; and then, (d) repeating steps (a) to (c) as required.

Description:
BACKGROUND TO THE INVENTION 
     This invention relates to an in-situ solvent-based process to produce bitumen from oil sand and heavy oil reservoirs. 
     A significant amount of bitumen in Alberta and other parts of the world is located either in thin, bottom water reservoirs or water sensitive sands which are not amenable to exploitation by steam based processes. A potential alternative for extracting these reservoirs is a solvent-based process. The advantages of the solvent-based processes are: little heat loss and limited water handling. The disadvantages are: high solvent cost and inherently low production rate limited by mass transfer of the solvent into the bitumen. 
     In general, many processes and methods utilizing a variety of solvents under a variety of temperature and pressure conditions have been developed to improve solubilization and production of hydrocarbons from reservoirs. 
     Lim et al in Canadian SPE/CIM/Canmet International Conference on Recent Advances in Horizontal Well Application, Mar. 20-24, 1994, disclose the use of light hydrocarbon solvents to produce bitumen for Cold Lake oil sand in three dimensional scaled physical modelling experiments. The results showed that the production rate of bitumen was significantly higher than what could be expected from molecular diffusion of the solvent into the bitumen. The author surmised that other mechanisms, probably solvent dispersion or fingering are important in mass transfer of solvent into bitumen. 
     Lim et al (1995) in Society of Petroleum Engineers paper no. SPE 302981 p. 521-528 discloses cyclic stimulation of Cold Lake oil sand with supercritical ethane through a single horizontal injector/producer well in a model system. Supercritical ethane enhanced the cyclic solvent gas process by improving the early production rate. This article directs the reader towards using supercritical ethane. 
     A problem that remains outstanding is to maximize extraction bitumen from oil sand and heavy oil reservoirs with maximum economy, minimum loss of solvent and to leave minimal residual bitumen in the oil sand and heavy oil reservoirs. A problem unaddressed to date is that of effective solvent distribution in a bitumen reservoir. If the solvent distributes too quickly throughout the reservoir there is a tendency for the solvent to be distributed along long thin solvent fingers penetrating into the reservoir from the point of injection. This leads to ineffective viscosity reduction and poor and difficult recovery of bitumen. If the solvent is insufficiently distributed in short thick fingers then solvent-bitumen contact is too limited to provide efficient bitumen extraction. We have developed an in-situ cyclic solvent-based process to produce bitumen from oil sand and heavy oil reservoirs which has advantages in maximizing solubilization and production rates. 
     SUMMARY OF THE INVENTION 
     We have found that careful choice of a viscosity reducing solvent and cyclic injection of this solvent at a pressure in the reservoir of above the liquid/vapor phase change pressure (saturation pressure) of the solvent, the pressure also being sufficient to cause geomechanical formation dilation or pore fluid compression, followed by mixing of the solvent with reservoir hydrocarbons under pore dilation conditions, followed by pressure reduction to below the liquid/vapor phase change pressure can be used to drive at least a fraction of the reservoir hydrocarbons from the reservoir. 
     The invention therefore provides a process for recovery of hydrocarbons from an underground reservoir of said hydrocarbons, the process comprising of: 
     (a) injecting a viscosity reducing solvent of a fraction of said hydrocarbons into said reservoir at a pressure in the reservoir of above a liquid/vapor phase change pressure of at least a fraction of said solvent; said pressure in said reservoir also being sufficient to cause geomechanical formation dilation or pore fluid compression, and then, 
     (b) allowing said solvent to mix with said hydrocarbons under pore dilation conditions, and then, 
     (c) reducing the pressure in said reservoir to below said liquid/vapor phase change pressure of at least said fraction of said solvent thereby demonstrating solvent gas drive of a fraction of said hydrocarbons from said reservoir; and then, 
     (d) repeating steps (a) to (c) as required. 
     In the context of this invention by solvent we mean a compound that has a liquid/vapor phase change pressure that is below the regularly used injection pressure of the reservoir and so is injected in the liquid phase. Preferably, the liquid/vapor phase change pressure should be close to the initial reserve pressure so that the operating reservoir pressure can easily be raised above the phase change pressure during injection and brought down below the phase change pressure during production. It also should be high enough so that the solvent vaporizes at the reduced pressures used for production so that solvent gas drive can be used to assist production. Suitable solvents include lower hydrocarbons, such as methane, ethane and propane, as well as CO 2 . 
     In the context of this invention by diluent we mean a liquid compound that can be used to dilute the solvent and can be used to manipulate the viscosity of any resulting solvent-bitumen mixture. By such manipulation of the viscosity of the solvent-bitumen (and diluent) mixture, the invasion, mobility and distribution of solvent in the reservoir can be controlled so as to increase bitumen production. 
     The diluent is typically a viscous hydrocarbon liquid, especially a C 4  to C 20  hydrocarbon or mixture thereof, is commonly locally produced and is typically used to thin bitumen to pipeline specifications. Pentane, hexane and heptane are commonly components of such diluents. Bitumen itself can be used to modify the viscosity of the injected fluid, often in conjunction with ethane solvent. 
     In preferred embodiments, the diluent may have an average initial boiling point close to the boiling point of pentane (36° C.) or hexane (69° C.) through the average boiling point (defined further below) may change with reuse as the mix changes (some of the solvent originating among the recovered viscous oil fractions). Preferably more than 50% by weight of the diluent has an average boiling point lower than the boiling point of decane (174° C.). It is more preferred that more than 75% by weight, especially more than 80% by weight, and particularly more than 90% by weight of the diluent has an average boiling point between the boiling point of pentane and the boiling point of decane. In further preferred embodiments, the diluent has an average boiling point close to the boiling point of hexane (69° C.) or heptane (98° C.), or even water (100° C.). 
     In additional preferred embodiments, more than 50% by weight of the diluent (particularly more than 75% or 80% by weight and especially more than 90% by weight) has a boiling point between the boiling points of pentane and decane. In other preferred embodiments, more than 50% by weight of the diluent has a boiling point between the boiling points of hexane (69° C.) and nonane (151° C.), particularly preferably between the boiling points of heptane (98° C.) and octane (126° C.). 
     By average boiling point of the diluent, we mean the boiling point of the diluent remaining after half (by weight) of a starting amount of diluent has been boiled off as defined by ASTM D 2887 (1997) for example. The average boiling point can be determined by gas chromatographic methods or more tediously by distillation. Boiling points are defined as the boiling points at atmospheric pressure. 
     In the context of the invention geomechanical formation dilation means the tendency of a geomechanical formation to dilate when pore pressure is raised to the formation minimum in-situ stress, typically by injecting a liquid or a gas. The formation in-situ stress is typically determined in a well test in which water is injected to the formation at low rates while bottom-hole pressure response is recorded. Analysis of the pressure response would reveal the conditions at which formation failure occurs. Pore fluid compression means just that, compression of a pore fluid (by pressure). In the field, the user can obtain pore fluid compression by multiplying pressure increase by fluid compressibility, which is a fluid property measurable in laboratory tests. Pore dilation refers to dilation of pores in rock or soil and simply means more loosely packed. 
     In a preferred embodiment, ethane is mixed with bitumen and the diluent and co-injected into the reservoir. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 shows a particular embodiment of the Cyclic Solvent Process (CSP) of the invention. 
     FIG. 2 shows scenarios of solvent distribution and mixing with bitumen during injection. FIG. 2A shows widely spaced thin and long fingers; FIG. 2B shows solvent penetration limited by thick fingers; and, FIG. 2C shows preferred fine fingers during formation dilation. 
     FIG. 3 shows fraction of solvent volume injected at or below fracture pressure during the vertical well field test at Cold Lake. 
     FIG. 4 shows oil recovery and pressure variation during CSP laboratory physical modelling experiments. 
     FIG. 5 shows production profile of CSP from laboratory tests. 
     FIG. 6 shows solvent oil ratio profile of CSP from laboratory tests. 
     FIG. 7 shows the produced solvent to oil (PSOR) operating range for CSP from laboratory tests. 
     FIG. 8 shows storage solvent ratio profile of CSP from laboratory tests. 
     FIG. 9 shows instantaneous produced solvent oil ratio from the ethane injection horizontal well field test at Cold Lake. 
     FIG. 10 shows phase diagram of diluent-ethane-Cold Lake bitumen system at 5 MPa and 21° C. 
     FIG. 11 compares bitumen recoveries from pure ethane injection test and diluent-ethane-bitumen mixture injection. 
     FIG. 12 shows greater pressure drawdown for fluid in-flow by diluent circulating or co-injection. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     This invention discloses an in-situ cyclic solvent-based process to produce bitumen from oil sand and heavy oil reservoirs. 
     With reference to FIG. 1, the present invention comprises of a single well method for cyclic solvent stimulation, the single well preferably having a horizontal wellbore portion and a perforated liner section with intake of an artificial lift located at the toe of the liner. 
     In FIG. 1 a vertical wellbore  1  comprising an outer sleeve  2  and an inner bore  3  driven through overburden  4  into reservoir  5  is connected to a horizontal wellbore portion  6 . The horizontal wellbore portion  6  comprises a perforated liner section  7  and an inner bore  8 . An isolation packer  9  preferably is located at or near the heel  10  of the horizontal wellbore portion where it joins the vertical wellbore. Another packer  16  located downstream of isolation packer  9  provides a means for diverting diluent to the adjacent reservoir during production. At or near the toe  11  of the horizontal wellbore portion is a down hole pump  12 . 
     In operation solvent or viscosified solvent is driven down outer sleeve  2  to perforated liner section  7  where it percolates into reservoir  5  and penetrates reservoir material to yield a reservoir penetration zone  13 . Oil dissolved in the solvent or viscosified solvent flows down and collects at or around the toe  11  and is pumped by down hole pump through inner bores  8  and  3  through a motor at the wellhead  14  to a production tank  15  where oil and solvent are separated and the solvent commonly recycled as shown. 
     In the practice of this invention, the viscosity reducing solvent is injected at high pressure into the reservoir through the horizontal well. The reservoir accommodates the injected solvent by dilation of its pore space and by compression of pore fluids. The solvent mixes with the reservoir bitumen and the mixture is then produced from the same well. Fluids are driven to the production well by formation re-compaction, fluid expansion and gravity. 
     The fluid rates decline with time. The injection and production procedures are repeated until the produced solvent to oil ratio (PSOR) is so high that the incremental production becomes uneconomical. 
     Design of an Injection Procedure 
     One of the key elements of the invention is in the design of an injection procedure which achieves sufficient solvent mixing with bitumen in the reservoir. A balance in solvent penetration and bitumen contact is essential for the most effective viscosity reduction. 
     If the solvent is distributed too widely during injection, there will be insufficient viscosity reduction to yield economic production rates. An example of this scenario is uncontrolled hydraulic fracturing in which solvent is distributed sparsely over a wide region of the reservoir. Another example would be if solvent were distributed through fingering via relatively few long thin fingers (FIG.  2 A). In this case, the solvent finger, being thin, would resaturate rapidly along its length during production, trapping a large fraction of the solvent away from the wellbore and behaving like uncontrolled hydraulic fracturing. 
     If the solvent is not dispersed during injection, most of the solvent remains near the wellbore or in only a few thick solvent fingers. In this case, the contact between solvent and bitumen is too limited to have the desired effect FIG.  2 B). 
     The ideal scenario is to achieve reservoir penetration, which results in good solvent mixing without dispersing the solvent too far as to be ineffective at reducing viscosity. To achieve such a balance, reservoir pressure is raised with the solvent injection to levels approaching the minimum in-situ stress. The fraction of solvent injected at or above this pressure is limited to be half to three quarters of the solvent injected. Ideally, poro-elastic behaviour under such conditions provides a large pore dilation which permits solvent to be distributed as numerous pore-scaled fingers to maximize solvent mixing over a large reservoir volume (FIG.  2 C). Once the solvent is mixed with bitumen under the pore dilation conditions, the injected solvent is continually thickened as a fraction of bitumen is extracted and dissolved into the solvent. This in-situ viscosifying of solvent reduces the interfacial tension contrast between the displacing and displaced fluids and helps to minimize adverse tendencies of solvent fingering to be limited to a few large fingers. 
     The above mixing behaviour was demonstrated in a field study whereby a model solvent was injected through a vertical well located at a Cold Lake oil sand reservoir. Bottom hole pressure was monitored during the injection where rate of injection varied from 20 to 150 m 3 /d. The study was performed over 5 injection/production cycles. Figure shows the fraction of solvent volume injected at or below the fracture pressures which was predetermined in a formation stress test prior to the solvent injection. Significant volume of the solvent was injected at or below the measured fracture pressure and was within the targets set for the test. The ensuing mixing behaviour achieved by formation dilation during the injection contributed significantly to achieving the bitumen production and solvent usage that were expected. 
     Minimization of Solvent Gas Production and Produced Solvent to Oil Ratio (PSOR) 
     Good mixing of solvent with bitumen during injection ensures significant oil and solvent production during the production phase. Production is carried out from the same well at a controlled pressure decline rate such that any “gas coning” effect is minimized. This effect occurred when the pressure declined rapidly. The accompanying high solvent gas production was detrimental to oil production. 
     A laboratory study was conducted in a three dimensional physical model packed with Cold Lake oil sand. The tests were performed to assess the physics of cyclic stimulation, measure production rate and solvent usage under the conditions of interest relating to the Cold Lake oil sand deposits. During the tests, ethane was injected into a horizontal well placed along one of the lower corners of a 50×50×27 (h) cm model, which was packed with Cold Lake bitumen and sand. The model was placed inside a sealed pressure vessel. The annulus between the model and the pressure vessel was pressurized with nitrogen to exert a confining pressure on the model and to prevent the sand pack from bulging during injection. 
     The laboratory tests demonstrated that by increasing reservoir pressure above the saturation pressure of ethane during injection, followed by decreasing pressure below the saturation pressure during production, incremental bitumen was produced. By repeating the cycles 13 times, 50% of the bitumen in the model was recovered in one of the experiments. FIG. 4 shows variation in the reservoir pressure from 4.5 MPa at the end of injection to 2.5 MPa at the end of production, and the accompanying oil recovery obtained from the experiment. Higher injection pressure in the field application would help formation dilation and promote pore-scale fluid mixing. Note that most of the oil was produced during the solvent phase transition, demonstrating the important role that solution gas drive played in the process. 
     A characteristic production profile of CSP is shown in FIG.  5 . The initial rate is typically high and declines in early cycles; after reaching a minimum, it then rises rapidly to a peak value before declining again in the late cycles. 
     The accompanying produced solvent-oil ratio (PSOR) profile shown in FIG. 6 indicates that PSOR is low and close to the solubility limit in early cycles, implying that the solvent is fully utilized in mobilizing the bitumen. Solvent utilization in the subsequent cycles is relatively effective, indicated by the value of PSOR being less than twice the equilibrium PSOR for ethane in Cold Lake bitumen at 4 MPa. As bitumen recovery approaches 30%, PSOR begins to increase rapidly and reaches the value of 2-3. High PSOR in late cycles implies low effectiveness of the injected solvent for oil mobilization. For commercial application, this means the process has reached an economic threshold beyond which the incremental production will be offset by a higher compression cost for recycling the produced solvent. 
     To further elucidate the importance of PSOR, a plot of production rate versus PSOR from the same experiments is shown in FIG.  7 . Note that high production rate occurs when PSOR is between 0.5, which is the solubility limit of liquid ethane in bitumen at 4 MPa, and 1.5. As PSOR increases beyond the threshold value of 3, production rate drops significantly. The results of the laboratory study indicate the necessity for monitoring PSOR as part of a production strategy in the field. The measured PSOR should be used for proportioning the casing vent gas producing rate relative to the liquid pumping rate. High vent gas production can often lead to “gas coning” effect, and high PSOR and is detrimental to crude oil production. 
     Maximization of Solvent Usage Efficiency and Minimization of Solvent Storage Ratio 
     The solvent for the process is designed by matching its phase behaviour properties with reservoir conditions. Phase change of solvent from a liquid state during injection to a vapour state during production is beneficial to the process in two respects. First, it provides important drive energy through solution gas drive. Second, if gaseous solvent replaces the voidage in the reservoir as fluids are depleted, this minimizes the amount of solvent remaining in the reservoir, thus increasing the efficiency of solvent usage for the process. A particular solvent is said to be efficient when the storage solvent ratio (SSR) is low. The ratio is the volume of solvent expressed in liquid form remaining in the reservoir to the cumulative oil volume produced from the reservoir. Part of the solvent replaces the oil produced and occupies the voidage as a vapour, and the rest is mixed in bitumen not yet produced. FIG. 8 shows that the ratios obtained from two separate laboratory tests drop steadily to below 0.4. The ratio is reduced further to below 0.1 by blowdown at the end of the process. It has been shown that a process that injects pure ethane and recycles all the produced solvent is economical if the storage solvent ratio is kept below 0.4 as the process reaches the threshold oil recovery of 30%. For the above reasons, light hydrocarbons that are effective viscosity reducing agents such as ethane or propane are preferred for the process. They are relatively inexpensive compared with the heavier hydrocarbon solvents. 
     One method of lowering storage solvent ratio is to add methane to the injected solvent mixture. This is particularly effective in later cycles when formation voidage is large after substantial amount of bitumen has been produced. In this case, the voidage would be occupied by gaseous methane during injection thus reducing the amount of more expensive solvents such as ethane or diluent required for the process. 
     Another CSP field test was conducted by injecting ethane to a short horizontal well in an oil sand reservoir at Cold Lake, Alberta. One of the objectives of the test was to study field scale mixing behaviour of ethane during injection. FIG. 9 shows the producing solvent-oil ratio during one of the production cycles in the study. Integrating the ratio over the cycle period produces a cycle PSOR value that is quite comparable to that observed from the laboratory tests shown in FIG.  5 . With reference to FIG. 9, PSOR was high initially due to production of the injected ethane from the near wellbore region. As the ethane at the near wellbore area became depleted, the PSOR dropped. As the production continued, it rose and levelled off at a constant value of about one, indicating that ethane was well mixed with bitumen within the reservoir and both were produced back at a fairly constant ratio. The field test results show that the character of the ethane usage is consistent with good solvent utilization. If the ethane injected was sparsely distributed by thin long fingers, the total bitumen produced would be small and ethane recovery would be very poor ( 2 A). If the ethane were distributed in a few thick fingers, high ethane recovery would result with the PSOR never reaching low values ( 2 B). The benefit of many small fingers to achieve good mixing has been realized ( 2 C). 
     Optimization of Solvent Mixing 
     In the event that solvent mobility is too high and formation dilation is not possible, solvent viscosity can be increased by dissolving a viscous liquid into it at the ground surface. A suitable liquid for this is the upper solvent-rich phase of an ethane/bitumen mixture. A schematic for recycling a small stream of produced bitumen into the ethane is shown in FIG.  1 . Laboratory phase behaviour tests where ethane was mixed with Cold Lake bitumen show that the ethane-rich phase has a ten-fold increase in viscosity over pure ethane. The addition of a small volume of bitumen will provide the ideal blend viscosity for mobility control, which helps minimize adverse thick finger solvent fingering, enhance formation dilation and increase solvent/bitumen mixing and contact. 
     Addition of small quantity of diluent to an injected solvent will further improve the phase behaviour of the solvent system. The diluent used in the test has an IBP (initial boiling point) of 20° C., an average boiling point (as defined above) of 75° C. and a FBP (final boiling point) of 460° C. Results of hydrocarbon blending tests shown in FIG. 10 indicate that the diluent-ethane-bitumen system can be a very effective solvent mixture for CSP. The tests reveal an optimal CSP solvent design for single-phase recovery of bitumen as indicated by the mixing path line AB in FIG. 10 that is tangent to the two-phase boundary. This path provides the leanest diluent solvent composition C that will form a one-phase liquid at 5 MPa with any proportion of bitumen. While the solvents of composition along line AB behave like first contact miscible solvents, solvents in the shaded region to the left of line AB are multiple contact and near miscible solvents. These solvents, though not readily miscible with bitumen initially, would become miscible after multiple contact with bitumen in the reservoir. The dissolved diluent in bitumen will decrease the viscosity at low pressures compared to pure ethane and allow the reservoir pressure in a CSP production cycle to be drawn down much further than that possible with pure ethane. This solvent design assures single phase oil displacement in the high pressure region (&gt;3.6 MPa) which permits better mixing and desirable flow behaviour (all solvent components staying together during mixing in porous media). 
     The benefits of injecting the above solvent mixture were demonstrated by comparing the results of two physical modelling experiments as shown in FIG.  11 . These experiments were conducted in the same model at two separate occasions with pure ethane injected in the first experiment and an ethane/diluent/bitumen mixture in the second. The composition of the solvent mixture for the second test was that of point C in FIG.  10 . Due to significantly higher net oil production, the second test achieved higher bitumen recovery for the same time duration and with fewer number of cycles, as shown in FIG.  11 . Moreover, the storage solvent oil ratio and producing solvent oil ratio of the second test were lower than those of the first test. 
     Circulating a Diluent to Increase Drawdown and Improve Wellbore Inflow 
     For a solvent-based process with pure ethane injection, production pressure drawdown may be limited by the ethane&#39;s saturation pressure. In this case, little oil will be produced when production pressure drops below 2.5 MPa due to excessive free gas production and high bitumen viscosity due to reduced solubility of the ethane at low pressure, as shown in FIG.  12 . Note that most of the oil was produced during ethane phase transitional period which occurs at about 3.8 MPa at room temperature. As pressure drops below the phase transition, gas evolves in the reservoir and forms a connecting path to the wellbore. This reduces the effectiveness of gas in mobilizing oil during low-pressure production. 
     To overcome instances where the viscosity of produced fluids will limit the inflow into the wellbore, another element of the present invention is to circulate a hydrocarbon diluent down the well casing to the horizontal portion where it is diverted into the adjacent reservoir with the assistance of a diverting packer. The diluent mixes with the reservoir fluid near the well and reduces formation fluid&#39;s viscosity enabling it to enter the tail section of the horizontal liner easily. Because the near well pressure will decrease, the flow of reservoir fluids will increase. As observed from the experiments, the added diluent lowered the phase transitional pressure to 2.8 MPa and allowed for a greater pressure drawdown that significantly improved production. The lowering of the phase transitional pressure is the result of the change in solvent composition from adding diluent. Another benefit of this technique is that the diluent will absorb free methane and reduce its tendency to interfere with the pump efficiency. 
     The diluent added to the wellhead would be the portion of the diluent usually required for diluting bitumen for pipelining purpose and would therefore incur little extra cost except for the cost of non-recovered diluent. The diluent is bled into the wellbore and hence does not migrate very far into the formation so that it is therefore expected that this diluent loss would be small. 
     The diluent used in the diverting packer would be from the same source as the diluent used in the injection solvent mixture. 
     Improving Lifting Efficiency of Produced Oil 
     Another preferred element of the present invention is an artificial lift method in which the pump/lift intake is located at the toe of the horizontal well. This is different from conventional methods where the intake is typically located at the heel of the horizontal well. Either gas lift or a screw pump that can handle fluids of high gas content is quite appropriate for the application. Due to the high deviation of the wellbore, a downhole driver with a screw pump is preferred. Any free gas that exits in the horizontal well has a tendency to move upstream into the casing annulus while the liquid moves toward the pump intake. Lifting efficiency is significantly improved as a result of the down-hole gas separation since gas is known to impede pump efficiency. Another benefit of this pump configuration is for cases when the horizontal liner is plugged with viscous bitumen, cleaning of the liner can be done with a greater degree of success by circulating diluent to the casing annulus while pumping it out through the tubing or by reversing this circulation direction periodically to access both sides of any restriction. 
     Reducing Hydrate Formation 
     In the presence of connate water in reservoirs, light hydrocarbons are prone to form hydrates under very low temperature (&lt;10° C.) and/or high pressure (&gt;5 MPa) conditions. Hydrate is formed when water molecules form a crystalline structure that is stabilized in the presence of hydrocarbon gas. The conditions of the oil sand reservoirs are such that hydrates are less likely to form in the reservoir during injection or production phases. The hydrates, however, have a tendency to form in tubings or flowlines when gas expansion reduces fluids to sufficiently low temperatures. This happened in a number of occasions during the ethane injection field study. These hydrates blocked the tubing and affected production. Injection of a small quantity of hydrate inhibitor such as methanol to the casing annulus at the surface during the production phase was carried out and found to be quite effective in preventing such occurrences. Other chemicals such as ethanol, glycerin or salts, though not tested in this field experiment, are reportedly effective hydrate inhibitors. 
     Preferred Operating Ranges and Best Mode 
     The preferred operating ranges used in the practice of the invention known to date are: 
     
       
         
               
               
               
             
           
               
                   
               
               
                 Parameter 
                 Preferred Range 
                 Most Preferred Range 
               
               
                   
               
             
             
               
                 Temperature (° C.) of 
                 10-50 
                 13-30 
               
               
                 injected materials 
               
               
                 Maximum injection 
                 Minimum formation 
                 Minimum formation 
               
               
                 pressure (Mpa) of 
                 stress × (100% ± 20%) 
                 stress × (100% ± 1%) 
               
               
                 injected materials 
               
               
                 Minimum Production 
                 0.5-3.0 
                 1.5-2.5 
               
               
                 pressure (Mpa) 
               
               
                 Injected materials 
                 Ethane + Diluent + 
                 Ethane + Diluent + 
               
               
                   
                 Bitumen + Methane 
                 Bitumen + Methane 
               
               
                 Diluent average 
                 21-400 
                 36-174 
               
               
                 boiling point ° C. 
               
               
                 Bitumen viscosity, cp. 
                 1,000-1,000,000 
                 10,000-200,000 
               
               
                   
               
             
          
         
       
     
     In Cold Lake, the minimum formation stress is about 9 Mpa. 
     EXAMPLE 
     Commercial Scale Application of Cyclic Solvent Process 
     An example of how the process can be applied to produce Cold Lake bitumen on a commercial scale is described below. Field scale prediction from the simulation indicates that a commercial well of 750 m long can produce approximately 50 m3/d of bitumen on the average from a Cold Lake thin reservoir. Sixty such wells (see FIG. 1) on production operations would be required for a continuous bitumen production of 3,000 m3/d at a central plant. Each well would operate 9 years and recover about 25-30% of the bitumen in place. New wells would have to be drilled and started up to replace those that are approaching the end of the well life. 
     To reduce cost and environmental impact, ten wells are drilled from the same surface pad location. The horizontal sections of the 5 wells are oriented in parallel in the same direction in the reservoir while the other five wells are oriented in the opposite direction. Spacing of the wells in parallel in the reservoir is approximately 160 m apart. The wellheads of the 10 wells at the same pad are tied to manifolds which are connected to injection and production trunk lines to and from the central plant. Metering and well testing facilities are built in a satellite building at the pad for monitoring pressures, injection and production volumes. Several pads of similar facility design and well configuration are built in the field vicinity as necessary to meet production requirement. 
     Produced fluids from each production well flow through the manifold and are pumped to the central plant for processing. Each production well is tested at the pad site on a daily basis for bottom hole pressure, production volume and PSOR. The data on PSOR is used for optimizing pressure decline and production operations. Production of a well would be terminated if its PSOR is high (&gt;3.0) and bottom hole pressure low (&lt;1.5 MPa). 
     The produced fluids contain ethane, bitumen, diluent and small amount of connate water. Ethane is separated in the central plant through a series of high and low pressure separators and reused for injection. A small amount of diluent, supplied to the plant through a pipeline, is added to the produced fluids free of ethane to aid in the separation of water and oil. The “water-free” oil is finally trimmed with additional diluent to meet pipeline specification and shipped for marketing. In oil sand industry, the term “dilbit” is used to designate oil that contains diluent and bitumen. 
     Ethane is viscosified for injection at the central plant. Prior to the blending, ethane from a supply line and the recycle stream is compressed to a liquid state at 3.9 MPa and 20° C. At the upstream of an in-line mixer, the liquid ethane is mixed with a small stream of dilbit from the production stream. Sufficient dilbit is added to the mixture until viscosity of the blend reaches about 0.4 cp. A small stream of hydrate inhibitor such as methanol is also added. The injectant is then delivered to the various injection wells at the field through the injection trunk line. The injection is carried out at a constant discharge pressure of an injection pump located in the central plant. The pump discharge pressure is set a slightly above the formation minimum in-situ stress, i.e. 9 MPa, taking into account friction loss along the injection line. The injection to a well would gradually slow down and eventaully stop as the bottom-hole pressure rises and approaches the formation minimum in-situ stress of 9 MPa. Comparing to production cycle, injection is typically quite short and lasts several days to a week given sufficiency pump and solvent capacities.