Abstract:
A method for stimulating a sub-commercial geothermal well includes steps of drilling a stimulating well; isolating a corresponding zone in the stimulating well using vertically spaced swell packers that are swellable when contacted by subterraneously heated geothermal brine present in the stimulating well and are resistant to the high temperature of the brine; injecting stimulating fluid into the stimulating well such that it will flow only through a zone of the well that is not isolated; and allowing the stimulating fluid to exit the well from a non-isolated zone located at a desired depth into a surrounding geological formation. The fracture or system of fractures within the formation is thereby hydraulically reopened.

Description:
FIELD 
     The present invention relates to the field of geothermal energy. More particularly, the invention relates to a method and apparatus for stimulating a geothermal well. 
     BACKGROUND 
     In geothermal power plants, hot fluid from a geothermal resource is extracted via a production well from underground to the ground surface. The extracted hot fluid is used for power production either directly when converted to steam and expanded in a turbine, or indirectly by means of a binary cycle power plant whereby the extracted hot fluid is brought in heat exchanger relation with the motive fluid of the power plant, such as an organic motive fluid. The heat depleted geothermal liquid is returned underground via an injection well, which is separated from the production well. The injected geothermal liquid becomes reheated and makes its way back to the production well. 
     As a result of continuous exploitation of the geothermal resource, the enthalpy of the extracted fluid and/or pressure tends to decrease over the course of time, reducing the economic viability of a power plant for producing power from the extracted geothermal resource. It would therefore be desirable to provide a method for enhancing a production well or injection well drilled in or adjacent to a field containing a depleted geothermal resource. 
     US 2012/0181034 discloses a method for stimulating an underground reservoir formation by introducing a particulate diverting agent into a well, to thereby temporarily seal passages within a fracture near the wellbore face and to isolate the fracture from the well. When a stimulation fluid is applied to the well at a sufficient pressure, an additional fracture is produced by hydroshearing such that it is expanded under shear. Rather than causing permanent damage to the permeability of the fractures which will lead to a reduction in economic value of the geothermal resource, the particulate diverting agent is able to degrade over an extended time. One disadvantage of this stimulation method is that it is a one-time operation due to the degradation of the diverting agent. 
     Another drawback of this stimulation method relates to its unpredictability. At times, a fracture will be produced at that unknown subterranean region which is not necessarily to best depth for use in such a developed well. 
     It is an object of the present invention to provide a method for stimulating a geothermal well by reopening a fracture extending thereto within a rock formation at a selected depth. 
     It is an additional object of the present invention to provide a method for stimulating a geothermal well that is repeatable for a plurality of stimulation operations. 
     It is a further object of the present invention to advantageously provide a method for stimulating an injection/production wells so that the amount of power that can be generated from the geothermal fluid can be increased. 
     Other objects and advantages of the invention will become apparent as the description proceeds. 
     SUMMARY 
     The present invention provides a method for stimulating a sub-commercial geothermal well, comprising the steps of drilling a stimulating well; isolating a corresponding zone in said stimulating well by means of a plurality of vertically spaced swell packers that are swellable when contacted by subterraneously heated geothermal brine present in said stimulating well and are resistant to the high temperature of said brine; injecting stimulating fluid into said stimulating well such that it will flow only through a zone of said well that is not isolated; and allowing said stimulating fluid to exit said well from a non-isolated zone located at a desired depth into a surrounding geological formation in order to hydraulically reopen a fracture or a system of fractures within said formation at said desired depth that will be connected with said existing well to be stimulated. 
     In one aspect, said a stimulating well is located within a field containing a geothermal resource and an existing sub-commercial well. 
     In one aspect, said a stimulating well is located adjacent a field containing a geothermal resource. 
     In one aspect, the corresponding zone of the stimulating well becomes isolated by mounting the plurality of vertically spaced swell packers to an outer face of a perforated liner, attaching said liner to a casing of the stimulating well while the plurality of swell packers are separated from a face of the stimulating well, and allowing the plurality of swell packers to swell when contacted by the subterraneously heated brine and to thereby seal a radial interspace between said well face and said outer face of said liner. 
     In one aspect, the corresponding zone of the stimulating well also becomes isolated by lowering a cement float collar to a depth of a lowermost swell packer into the simulating well and causing said stub-in float collar to become bonded with an inner face of the liner at the depth of said lowermost swell packer. The Stub-in float collar and Tag-in adapter prevent downward flow of the injected stimulating fluid, whereby to urge the stimulating fluid to exit the stimulating well via the perforations of the liner which are located within an inter-packer zone between two adjacent swell packers. 
     In one aspect, a downwardly extending injection tube and the Tag-in adapter is embedded within, and passes through a bottom face of the float collar. The stimulating fluid is injected within the injection tube, whereby to urge the stimulating fluid to exit the stimulating well via the perforations of the liner which are located within a toe zone between the lowermost swell packer and a well bottom. 
     In one aspect, the temperature for which each of the plurality of swell packers is resistant ranges from about 180° F. to about 420° F. 
     The present invention is also directed to apparatus for stimulating a sub-commercial geothermal well, comprising a perforated liner attachable to a casing of a stimulating well drilled within a field containing a geothermal resource and an existing sub-commercial well, a plurality of vertically spaced swell packers mounted to an outer face of said liner, and a Stub-in float collar bonded with an inner face of said liner at a depth of a lowermost swell packer, wherein each of said plurality of swell packers is swellable when contacted by subterraneously heated geothermal brine present in said stimulating well to isolate a corresponding zone in said stimulating well and is resistant to the temperature of said brine, stimulating fluid injectable into said stimulating well thereby being caused to exit said stimulating well from a non-isolated zone located at a desired depth into a surrounding geological formation in order to hydraulically stimulate a fracture or a system of fractures within said formation at said desired depth that will be connected with said existing well to be stimulated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIG. 1  is a schematic plan view of a field containing a geothermal resource, illustrating an example of the relative location of a stimulating well for inducing or opening a fracture in an adjoining rock formation; 
         FIGS. 2 and 3  are schematic illustrations of two units, respectively, of an exemplary geothermal power plant, the power output of which can be significantly increased by stimulating a well; 
         FIG. 4  is a vertical cross sectional view of a stimulating well according one embodiment of the present invention, through which stimulation fluid is injectable; 
         FIG. 5A  is a front view of a swell packer according to one embodiment of the invention, shown in a non-swollen state; 
         FIG. 5B  is a front view of the swell packer of  FIG. 5A , shown in a swollen state; 
         FIG. 6  is a vertical cross sectional view of the stimulating well of  FIG. 4 , showing one method, according to an embodiment of the present invention, for injecting stimulating fluid therethrough in order to hydraulically produce a fracture at a selected depth of a subterranean region; and 
         FIG. 7  is a vertical cross sectional view of the stimulating well of  FIG. 4 , showing another method, according to a further embodiment of the present invention, for injecting stimulating fluid therethrough in order to hydraulically produce a fracture at a different selected depth; and 
         FIG. 8  is a front view of a mechanical packer according to one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention provides a novel method for stimulating a geothermal well which can lead to an increase in the amount of power that can be generated by fluid extracted from a field containing a geothermal resource. In such cases, the geothermal resource is generally in the form of a reservoir containing hot water and steam trapped within permeable and porous rocks under a layer of impermeable rock. Over the course of time, the output of a production well supplying the geothermal fluid to be extracted tends to decline, due to depletion of the resource or clogging of a fracture extending to the production well. In order to increase the output of the production well, a new well is then drilled within the field at a location which is relatively close to the resource. The newly drilled well (hereinafter the “stimulating well”) is caused to be partially isolated, so that stimulation fluid injected through the well will flow only through a zone that is not isolated to hydraulically produce a fracture or stimulate an existing fracture at a desired depth. The newly produced or stimulated fracture may extend from the stimulating well to the geothermal reservoir, enabling additional geothermal fluid to be in fluid communication with the production well and to thereby increase output of the production well. 
     As shown in  FIG. 1 , the location of a stimulating well  10  through which stimulation fluid is injectable is advantageously carefully selected within field  15  containing a geothermal resource and advantageously also a geothermal power plant so as to be relatively close to the resource. In such a manner, when a fracture is induced by the stimulating fluid, a hydrological connection is made between the fracture and a subterranean permeable region not intersected by well  10 , facilitating an increase in the amount of fluid that can be reinjected into field  15 , an increase in the amount of geothermal fluid extractable by a production well. Consequently, thereby an increase in the amount of electricity produced by the existing power plant will be achieved. 
     Prior to injecting fluid into stimulating well  10 , a set of geological, geophysical and geomechanical surveys are made of field  15  to determine at which depth or depths are located the highest density of old natural fractures. The stimulating fluid is then delivered within well  10  to the selected depth, as will described hereinafter, in order to open a fracture at the selected depth. 
     In the exemplary field  15  located at the Desert Peak geothermal reservoir, Nevada, USA, four artesian production wells, three of which,  17 - 19 , are shown for producing two-phase geothermal fluid containing steam and liquid, three pumped production wells, one of which,  21  is shown for producing geothermal brine, and injection wells  23 - 24  have been in use for producing power by e.g. means of binary cycle power plant  30  shown in  FIGS. 2 and 3  comprising advantageously geothermal energy conversion units  31  and  32 . The outlet of production wells  17 - 19  and  21  are in fluid communication with two flash separators, to ensure that the separated geothermal steam and liquid are suitably delivered to the power plant. Stimulating well  10  is drilled in the vicinity of injection wells  23 - 24  so that the induced or reopened fracture will be connected to the existing geothermal resource so that geothermal fluid flows to fractures through which geothermal fluid is flowable to one or more of the production wells. By drilling simulating well  10  through which geothermal fluid is injectable, the total power capacity of power plant  30  can be considerably increased, for example from about 13 MW gross to about 15 MW gross. 
     It will be appreciated that the stimulating well can also be a production well. After a fracture is induced or opened thereby, the fracture will receive geothermal fluid from an injection well or from another fracture. 
       FIG. 4  illustrates a cross sectional view of a stimulating well  10  through which stimulation fluid is injectable according to one embodiment of the present invention. Wellbore  3  is formed by using a drill bit that is lowered at a lower end of a drill string. After a predetermined depth is drilled, the drill string and bit are removed, and the earthen well face at that depth is then lined with casing  4 , which is cemented to the upper well face. This procedure is repeated until well bottom  9  is drilled to a desired depth. The lower earthen well face  7  below the lowermost casing  4  is formed with a smaller diameter than the inner diameter of the lowermost casing, e.g. the inner diameter of the lowermost casing is 13⅜″ while that of the lower earthen well face is 12¼″. 
     A cylindrical slotted steel liner  6  for preventing solid material from entering wellbore  3  while permitting fluid from exiting the wellbore is lowered into the well and is attached to the lowermost casing  4  by means of an oblique and outwardly extending sealing liner hanger  8 . Before liner  6  is lowered into the well, two vertically spaced, annular swell packers  11  and  12  are attached to the outer face of the liner. Liner  6  extends downwardly to substantially the edge of well bottom  9 . When liner  6  contacts well bottom  9 , the latter may apply a reactive force to ensure liner immobilization. 
     Rubber swell packers  11  and  12  have elastomeric polymer sealing elements that are adapted to swell to about twice their size, when exposed to the high temperature of geothermal brine, generally ranging from about 180° F. to 500° F. 
     Prior art swell packers, for example those that are used in the oil and gas well industry, which are resistant to high pressures but not to high temperatures characteristic of geothermal brine, in contrast tend to burst or otherwise deteriorate when exposed to the high temperature of geothermal brine or characteristic of geothermal brines. 
     Swell packers  11  and  12  are made of a material that is resistant to the high temperatures of the brine. In one embodiment, the material of the swell packer is selected to swell when exposed to the specific composition of brine found in the simulating well. In other embodiments, the swell packer is selected to swell when exposed to any one of a range of brine compositions. Swell packers  11  and  12  begin to swell 1-2 days after being exposed to the brine that is present in the well, thereby exerting a pressure on both liner  6  and well face  7 . After a period of approximately 12 days, the swell packers develop a pressure which is sufficient to adequately seal and isolate the annulus between liner  6  and well face  7 . Slotted liner  6  to which swell packers  11  and  12  are attached is sufficiently thick and rigid to resist deformation despite the pressure applied by the swell packers. 
     Note that this is the way, as shown in  FIG. 4 , the well will look like after the stimulation is done. 
       FIGS. 5A-B  illustrate an exemplary structure of a swell packer  11 . Swell packer  11  is an elongated element having a central core  41  about which a tubular piece  43  of swellable rubber is mounted. A protection ring  44  having a diameter substantially equal to the unexpanded rubber piece  43  shown in  FIG. 5A  is provided at each longitudinal end of rubber piece  43  and is in abutting relation therewith, to prevent damage to the rubber and prevent longitudinal expansion. From each protection ring  44  longitudinally extends a corresponding attachment rod  47  of a significantly smaller diameter, for attachment to the slotted liner. As rubber piece  43  is not radially constricted, it is free to radially expand when in contact with geothermal brine.  FIG. 5B  schematically illustrates to what extent rubber piece  43  is expandable with respect to attachment rod  47 . Rubber piece  43  retains a vertically straight annular profile when expanded and is of a sufficient long length to apply a sufficiently high wide area radial pressure onto the well face, which may be made of rough or fractured rock, in order to maintain a seal therewith when high pressure geothermal brine is injected into the stimulating well. 
     Referring now to  FIGS. 6 and 7 , a depth at which a fracture can be induced or opened can be selected by employing a bridge plug in the form of a Stub-in float collar  52  and injection tubing  56  embedded therewith. After slotted liner  6  has been set in position and the swell packers have been sufficiently expanded after being exposed to the geothermal brine present within well  10 , Stub-in float collar  52  is lowered into wellbore  3  to the depth of the lowermost swell packer  12  and is bonded with the liner. 
     By virtue of float collar  52 , the annular space between liner  6  and injection tubing  56  is sealed. When it is desired to induce or open a fracture F 1  located at a relatively shallow depth of surrounding rock formation R, as shown in  FIG. 6 , the presence Stub-in float collar  52  in wellbore  3  prevents the downward passage of stimulating fluid  59  injected into wellbore  3 , causing stimulating fluid  59  to exit well  10  via the slots of liner  6  located within the inter-packer zone  58 . The stimulating fluid is injected at a sufficiently high pressure to produce fracture F 1  in a subterranean region adjacent to inter-packer zone  58 . 
     The injected stimulating fluid is preferably geothermal brine, e.g. geothermal brine discharged from a geothermal power plant, the use of which helps prevent depletion of the geothermal resource and ensures subterranean passage of geothermal brine back towards a production well for increased well output and resulting power production; however, it is envisioned that any other geologically compatible stimulating fluid such as high pressure water may also be used. 
     Alternatively or in addition, a relatively deep fracture F 2  may be opened or induced by injecting stimulating fluid  59  through the interior of injection tubing  56 , as shown in  FIG. 7 . Since injection tubing  56  passes through Stub-in float collar  52  having a Tag-in adapter, the stimulating fluid  59  discharged from the lower end  57  of injection tubing  56  is caused to exit well  10  via the slots of liner  6  located within toe zone  61  between lowermost packer  12  and well bottom  9 . Once the stimulation has been completed, usually cement is drilled out. 
     At exemplary operating conditions when swell packers  11  and  12  of a length of 156 inches are positioned at a depth of about 4500 feet and 5300 feet, respectively, brine is injected at a rate of up to 36 BPM, whether into inter-packer zone  58  or into toe zone  61 . 
     In addition, while the above description of the present invention and its embodiments refers to swell packers and there use in stimulating a geothermal well in the presence of high temperature geothermal brine, other packers can also be used in accordance with the present invention. E.g. mechanical packers can be used to isolate regions or levels in a stimulated geothermal well (see  FIG. 8 ). 
     Furthermore, the Applicant would like to point that while the description refers to the present invention and its embodiments with relation to an existing geothermal resource, the present invention can be carried out also in environments where little or no geothermal resource is present. In such a case, geothermal brine or brine would have to be brought to the site of well stimulation. 
     While some embodiments of the invention have been described by way of illustration, it will be apparent that the invention can be carried out with many modifications, variations and adaptations, and with the use of numerous equivalents or alternative solutions that are within the scope of persons skilled in the art, without exceeding the scope of the claims.