Abstract:
A SAGD operation is provided, comprising: drilling a SAGD well pair in a bitumen bearing formation, the well pair comprising an injector well and a producer well; introducing an electro-thermal heating element comprising at least one individually controllable heating section in or near either the injector well, the producer well, or both; positioning the at least one individually controllable heating section at or near a sector of either the injector well, the producer well, or both, which sector may require greater thermal heat to establish thermal communication between the injector well and the producer well; and operating the at least one individually controllable heating section at a desired temperature for a period of time sufficient to establish thermal communication between the injector well and the producer well. The electro-thermal heating element having the at least one electrical heating section may remain in the injection well beyond start-up to assist in effectively recovering the hydrocarbons.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention is directed to Steam Assisted Gravity Drainage (SAGD) in general, and, more particularly, to the use of electrical heating elements to both reduce the start-up time of a SAGD operation and control the amount of heat (thermal energy) used in start-up based on the particular characteristics of a SAGD reservoir. The electrical heating elements may be retained in the injection well after start-up is complete and during normal SAGD operations. 
       BACKGROUND OF THE INVENTION 
       [0002]    Alberta, Canada has some of the world&#39;s largest deposits of oils sands. Oil sands are a mixture of sand, water and bitumen, a type of heavy oil that is very thick and thus difficult to flow on its own. Bitumen can be extracted from oil sand by mining oil sands and extracting the bitumen with either hot water or solvents. However, over 80% of oil sands deposits in reservoirs contain oil that is deeper than 75 meters below the earth&#39;s surface, making it very difficult to mine. 
         [0003]    Thus, a number of in situ methods for extracting bitumen from oil sands have been developed. One of these methods, Steam Assisted Gravity Drainage (SAGD) has proven to be very reliable. Generally, SAGD involves drilling two horizontal, parallel wells (well pairs), a top injector well for injecting steam for reducing the viscosity of the bitumen and a bottom producer well to collect the reduced viscosity bitumen which is lifted to the surface. Start-up generally involves a pre-heating stage, wherein steam is injected into the injector well, or circulated through both the injector well and the producer well, with the aim of establishing thermal communication between the well pair. 
         [0004]    However, the pre-heating stage can take several months before thermal communication is established between the well pair and the production of bitumen can begin. Thus, there is a need in the industry for a method to reduce the pre-heating time necessary to establish thermal communication between the SAGD well pair. 
         [0005]    Furthermore, there may be some instances where there exists permeability barriers in sections of the formation surrounding either the injector well, the producer well, or both. Establishing a desireable thermal communication profile between the well pair may be difficult due to the presence of these permeability barriers. In these instances, the pre-heating time may be even greater, as it will take more energy (steam) and time to establish the micro-channels in these permeability barriers which are necessary for fluid to flow during the steam injection and production processes. Thus, there is a need in the industry for a method to also be able to control the amount of heat (thermal energy) used in given areas of the wells in order to also conserve on the energy used during pre-heating of a SAGD well pair. 
       SUMMARY OF THE INVENTION 
       [0006]    The current application is directed to a method for decreasing the start-up time for a SAGD well pair, with the added flexibility of being able to control the amount of heat (thermal energy) used in start-up by controlling the amount of heat used to heat up a given area of the SAGD wells in order to conserve on the energy. It was discovered that by conducting the method of the present invention, one or more of the following benefits may be realized: 
         [0007]    (1) Less time is needed to establish initial thermal communication between a pair of SAGD wells. 
         [0008]    (2) Less energy in the form of steam is required to establish initial thermal communication between a pair of SAGD wells. 
         [0009]    (3) The pre-heating process could be accompanied by fluid injection for enhancing the heat transfer efficiency. 
         [0010]    (4) The material of the electro-thermal heating element used in the present invention may be selected for proper heat-capacity and thermal conductivity so that increasing and decreasing temperature (heating and cooling processes) occurs in a timely manner as required. 
         [0011]    (5) The electro-thermal heating element can comprise one or more individually controllable heating sections which sections can be placed in either the injector well, producer well, or both, at sectors of the well where it is difficult for the steam to reach, e.g., a permeability barrier or localized water pocket. 
         [0012]    (6) The electro-thermal heating element can comprise one or more individually controllable heating sections which sections can be placed in either the injector well, producer well, or both, to preferentially heat the higher bitumen-saturation zones and to preferentially avoid heating the lower bitumen-saturated zones. 
         [0013]    (7) The electro-thermal heating element can comprise a plurality of individually controllable heating sections, whereby the temperature of each section can be separately controlled and may be switched on/off so that a desired pre-heating temperature profile in the reservoir can be achieved. 
         [0014]    (8) The present invention can be used together with a solvent, a surfactant or other additive which changes the wettability or reduces the viscosity of the bitumen present in the oil sand reservoir. 
         [0015]    Thus, in one aspect, a method for pre-heating a SAGD operation is provided, comprising:
   drilling a SAGD well pair in a bitumen bearing formation, the well pair comprising an injector well and a producer well;   introducing an electro-thermal heating element having at least one individually controllable heating section in or near either the injector well, the producer well, or both;   positioning the at least one individually controllable heating section at or near a sector of either the injector well, the producer well, or both, which sector may require greater thermal heat to establish thermal communication between the injector well and the producer well; and   operating the at least one individually controllable heating section at a desired temperature for a period of time sufficient to establish thermal communication between the injector well and the producer well.   
 
         [0020]    It is understood that proper casing and well-completion is adopted to accommodate the high temperature source of the present invention. In one embodiment, the at least one individually controllable heating section is positioned at the toe of the injector well, the producer well, or both. In another embodiment, the at least one individually controllable heating section is positioned at or near the middle of the injector well, the producer well, or both. In one embodiment, steam is also circulated through the injector well, the producer well, or both. 
         [0021]    Following the pre-heating stage, the electro-thermal heating element that is placed in or near the injector well may be left down hole. Thus, when the steam injection into the injector well commences during the steam assisted gravity drainage process, the electro-thermal heating element can assist in controlling the quality of steam in the injector well. For example, steam often condenses at the toe of the well. Thus, the electro-thermal heating element may comprise an individually controllable heating section that is positioned at the toe of the well. It is understood, however, that, in most instances, it is generally undesirable to leave an electro-thermal heating element in the producer well and, hence, the element should be removed prior to commencing the SAGD process. Thus, it is understood that in one embodiment, the electro-thermal heating elements are removable. 
         [0022]    In one embodiment, the electro-thermal heating element comprises a plurality of individually controllable heating sections so that more heat can be provided to one or more specific sections of either the injector well, the producer well, or both, for example, those sections having poor permeability issues or other undesirable reservoir conditions, than other sections of either the injector well, the producer well, or both. In one embodiment, the electro-thermal heating element can be wrapped around the outside of the injector tubing, or liner, of the injector well, the producer tubing of the producer well, or both. Thus, depending on thermal properties of the formation, i.e., heat capacity, thermal conductivity, thermal expansion, and coefficient, an electro-thermal heating element comprising individually controllable heating sections of varying temperatures may be deployed along the injection well, the production well or both to achieve the desired (e.g., uniform) thermal pattern. 
         [0023]    In one embodiment, the electro-thermal heating elements can be controlled from surface of the SAGD and each individually controllable heating section can be separately controlled by controlling or adjusting the current passing through the element and to the individual controllable heating section. In one embodiment, each individually controllable heating section of the electro-thermal heating element has its own thermocouple, DTS sensor, or similar, for measuring the temperature and adjusting the temperature of each section accordingly. 
         [0024]    It is further contemplated to use this technique for infill wells between the SAGD well pairs. The invention will allow for the faster communication of the infill well to steam chambers so that production of the infill well can begin sooner than would be possible without this technique. 
         [0025]    In one embodiment, a solvent, a surfactant or other additive which changes the wettability or reduces the viscosity of bitumen is added to either the injector well, the producer well, or both, either prior to or during pre-heating. By being able to control the temperature of individual sectors of a well, one can also control how much solvent or other wettability additive will diffuse into the formation at a particular sector. Useful solvents can be hydrocarbon solvents such as naphtha, butane, propane and the like. It is understood, however, that non-hydrocarbon solvents could also be used. 
         [0026]    Surfactants that could be used in this process could be either non-ionic surfactants such as some alcohols (e.g., fatty, cetyl, stearyl) or ionic surfactants such as sulfates, sulfonates, phosphates or carboxylates. Generally, many ionic surfactants are not preferable, due to potential implications downstream in the refining process, but bitumen does have some natural carboxylates that could be enhanced biologically, using the same microbes that are used to help clean up oil spills. In another embodiment, depending on reservoir conditions, surfactants having cationic head groups such as primary, secondary and tertiary amines, which are pH dependent, or quaternary ammonium cations can be used. The mineral characteristics of the reservoir should be taken into account in all cases when choosing a surfactant. 
         [0027]    Thus, the present invention reduces the circulation time of steam in traditional SAGD operations by reducing the start-up time from a few months to a few weeks. In particular, a midpoint temperature between the injector well and the producer well of 70-195° C. may be reached within 60 days or less. In one embodiment, the electro-thermal heating element comprises an electrical cable, such as an ESP cable, at least one temperature gauge and the at least one heating section comprises an electro-thermal heating cable. In another embodiment, the heating sections are spaced along the electrical cable. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0028]    Referring to the drawings wherein like reference numerals indicate similar parts throughout the several views, several aspects of the present invention are illustrated by way of example, and not by way of limitation, in detail in the figures, wherein: 
           [0029]      FIG. 1  is a drawing showing one embodiment of a SAGD operation of the present invention. 
           [0030]      FIG. 2  is a drawing showing another embodiment of a SAGD operation of the present invention. 
           [0031]      FIGS. 3A ,  3 B and  3 C shows the temperature distribution after 90 days of circulation of steam through both the injector well and producer well ( 3 A), 60 days of steam circulation through both the injector well and producer well with a heater of the present invention at the toe of the injector well ( 3 B) and 60 days of steam circulation through both the injector well and producer well with a heater of the present invention at the toe of both the injector well and the producer well ( 3 C). 
           [0032]      FIG. 4  is a drawing showing another embodiment of steam-assisted gravity drainage (SAGD) production of the present invention comprising two SAGD well pairs and an infill well therebetween. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0033]    The detailed description set forth below in connection with the appended drawings is intended as a description of various embodiments of the present invention and is not intended to represent the only embodiments contemplated by the inventor. The detailed description includes specific details for the purpose of providing a comprehensive understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced without these specific details. 
         [0034]      FIG. 1  shows one embodiment of steam-assisted gravity drainage (SAGD) production of the present invention. The SAGD well pair  10  is drilled into a formation  18  (e.g., an oil sands formation) comprising an overburden  12 , and underburden  16  and a pay zone  14 . Injector well  20  comprises a vertical section  24  and a horizontal section (or leg)  26  having a toe  28  at its far end. Producer well  22  also comprises a vertical section  32 , vertically spaced proximate to vertical section  24  of the injector well  20 , and a horizontal section (or leg)  34  having a toe  30  at its far end. Generally, horizontal sections  26  and  34  are parallel to one another and are spaced about 4 to 10 meters from each other. Although drawn vertically in  FIG. 1 , the wells may be drilled directionally as determined by practitioners skilled in the art of drilling. 
         [0035]    In conventional SAGD, the injector well  20  injects steam to heat the space between and above the two wells and establish thermal communication between the two wells. With continuous injection of steam, hydrocarbon (e.g., bitumen) present in the pay zone  14  is heated, mobilized and collected in producer well  22 . 
         [0036]    In the embodiment of  FIG. 1 , electro-thermal heating elements  36 ,  38  comprising electrical cables  46  and  48 , respectively, can be inserted into the injector well  20  and the producer well  22 , respectively. It is understood, however, that electro-thermal heating element may be inserted in only one of the injector well  20  and the producer well  22 . In this embodiment, electro-thermal heating elements  36 ,  38  each further comprise an individually controllable heating section,  40 ,  42 , respectively, to thermally heat the formation, i.e., the pay zone  14 , to establish thermal communication between the two wells. In one embodiment, each individually controllable heating section  40  and  42  may comprise a temperature gauge (not shown) or a distributed temperature sensing (DTS) device, e.g., an optoelectronic temperature sensing device, and an electro-thermal heating cable (not shown). In  FIG. 1 , individually controllable heating section  40  is located in the middle portion of the horizontal leg  26  of injector well  20  and individually controllable heating section  42  is located near the toe  30  of the horizontal leg  34  of producer well  22 . 
         [0037]    A power supply and control system  50  can control the temperature of each individually controllable heating section  40 ,  42  by increasing/decreasing the amount of current through electrical cables  46 ,  48 , respectively. For example, the sector of the pay zone surrounding the individually controllable heating section  40  may be a sector of the well where it is difficult for the steam to reach, e.g., it is near a permeability barrier. Therefore, while individually controllable heating section  40  will eventually heat up the entire area surrounding the horizontal leg  26 , the majority of the heat is initially focused on the area within the permeability barrier. In this way, energy can be used in the most efficient way possible to form the thermal communication between the two wells. 
         [0038]      FIG. 2  shows another embodiment of steam-assisted gravity drainage (SAGD) production of the present invention. The SAGD well pair  200  is drilled into a formation  218  (e.g., an oil sands formation) comprising an overburden  212 , and underburden  216  and a pay zone  214 . Injector well  220  comprises a vertical section  224  and a horizontal section (or leg)  226  having a toe  228  at its far end. Producer well  222  also comprises a vertical section  232 , vertically spaced proximate to vertical section  224  of the injector well  220 , and a horizontal section (or leg)  234  having a toe  230  at its far end. 
         [0039]    In the embodiment of  FIG. 2 , electro-thermal heating elements  236 ,  238  can be inserted into both the injector well  220  and the producer well  222 , respectively. In this embodiment, electro-thermal heating element  236  comprises five individually controllable heating sections,  240   a,    240   b,    240   c,    240   d  and  240   e,  to thermally heat the formation to establish thermal communication between the two wells. Similarly, electro-thermal heating element  238  comprises five individually controllable heating sections,  242   a,    242   b,    242   c,    242   d  and  242   e,  to thermally heat the formation to establish thermal communication between the two wells. In this embodiment, the thermal properties of a plurality of vertical sectors along the entire width of the pay zone  214  have been determined and each individually controllable heating section is positioned accordingly. Thus, each of the individually controllable heating section is operated according to the thermal properties surrounding it. Power supply and control system  250 , together with, optionally, temperature gauges, can control the temperature of each individually controllable heating section by increasing/decreasing the amount of current through the electro-thermal heating elements  236 ,  238 , respectively, and by other means known in the art. 
         [0040]    By way of example, in the injector well  220 , three of the individually controllable heating sections  240   a,    240   b  and  240   c  (i.e., closest to the toe  228 ) may be operated at a much higher temperature than the two individually controllable heating sections  240   d  and  240   e.  This would be due to the fact that the sectors of pay zone  214  surrounding horizontal section  226  has a variable geological profile along its width, with the right half having a lower permeability than the left half. However, with respect to the producer well  222 , the pay zone  214  surrounding the horizontal section  234  is much more uniform. Thus, individually controllable heating sections  242   a,    242   b,    242   c,    242   d  and  242   e  are all operated at the same temperature, albeit at a higher temperature that the two left individually controllable heating sections  240   d  and  240   e  of injector well  220 , as the permeability is lower in the area surround the horizontal section  234  of the producer well  222  than that surrounding the left half of the horizontal section  226  of the injector well  220 . 
         [0041]    Thus, with the embodiment shown in  FIG. 2 , any designed heating pattern can be achieved by using an electro-thermal heating element of the present invention comprising a plurality of individually controllable heating sections on an electro-thermal cable, with locations of the individual heating sections being tailored to reservoir conditions. 
         [0042]    With reference now to  FIG. 4 , another embodiment of steam-assisted gravity drainage (SAGD) production of the present invention is illustrated. Two SAGD well pairs  400   a  and  400   b  are drilled into a formation  418  (e.g., an oil sands formation) comprising an overburden  412 , and underburden  416  and a pay zone  414 . Each well pair  400   a  and  400   b  comprises an injector well,  420   a  and  420   b,  respectively, and a producer well,  422   a  and  422   b,  respectively. An infill well  460  is drilled into the formation  418  in between well pairs  400   a  and  400   b.  Infill well  460  also comprises a vertical section  462  and a horizontal section (or leg)  464 . In one embodiment, an electro-thermal heating element  436  can be inserted into the infill well  460 , the electro-thermal heating element  436  comprising one or more individually controllable heating sections  440   a,    440   b,    440   c,    440   d,  to thermally heat the formation, i.e., the pay zone  414 , between the two well pairs  400   a  and  400   b,  to establish thermal communication between the two well pairs. 
       EXAMPLE 1 
       [0043]    Electrical elements of approximately 100 m were installed at the toe of the injector only or at the toes of both injector and producer of a SAGD well pair. Generally, circulation start up involves establishing inter well communication by circulating steam through the injector and producer, The steam would flow through the tubing string to the toe of each well. The rate of heat transfer and fluid convection into the reservoir formation determine how communication is established along the length of the well pair. In this example, an electrical cable is located at the toe of injector or both the injector/producer, at a maximum temperature of 350° C.; steam is also injected into both the injector and producer. 
         [0044]    When steam is added, it was discovered through simulation, that the electric cable can heat the condensed steam at the toe of the well and further transfer the heat generated to the mid section of the well, reducing the start up time and creating uniform steam chamber development along the entire well length. The minimum temperature at the midpoint between the two wells of 70 to 100° C. is used as an indicator for terminating steam circulation and switching to SAGD production mode. 
         [0045]    Simulation results indicate that having electrical heaters at the toe of the well can improve the heat transfer, which can lead to a favorable development of uniform temperature between the well pairs and faster start up. 
         [0046]      FIG. 3  shows temperature distribution after 60-90 days of steam injection. On the right hand side of  FIG. 3  is a scale showing temperatures in the ranges of 7-41° C. (dark blue), 41-75° C. (medium blue), 75-110° C. (light blue), 110-144° C. (turquoise), 144-178 (dark green), 178-212° C. (medium green), 212-246 (light green), 246-281° C. (yellow), 281-315° C. (orange), and 315-349° C. (red). In particular,  FIG. 3A  shows the temperature distribution after 90 days of circulation with steam only in both the injector well and producer well.  FIG. 3B  shows temperature distribution after only 60 days of circulation with steam where a 100 m heater is located at the toe of the injector well only.  FIG. 3C  shows temperature distribution after only 60 days of circulation with steam where a 100 m heater is located at the toe of both the injector well and the producer well. The results of the simulation show a reduction in steam circulation time and uniform temperature between the injector and producer. The temperature distribution profiles show that focused electrical heating could reduce both the steam required and start up time. 
         [0047]    From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention. However, the scope of the claims should not be limited by the preferred embodiments set forth in the examples, but should be given the broadest interpretation consistent with the description as a whole.