Abstract:
Systems and methods are described for controlling the location where a fracture initiates and the fracture direction at the initiation point. A mechanical device is positioned in a main wellbore or partially or fully in a side hole off of the main wellbore. The mechanical device is actuated so as to contact the formation walls and induce stress in the formation. According to some embodiments, fractures are also initiated using the mechanical device. A hydraulic fracturing process is then carried out to fracture and/or propagate fractures in locations and/or directions according to the actuation of the mechanical device.

Description:
FIELD 
       [0001]    The subject disclosure generally relates to reservoir fracturing. More particularly, the subject disclosure relates to an apparatus and method for controlling the location where a fracture initiates and the fracture direction at the initiation point. 
       BACKGROUND 
       [0002]    Hydraulic fracturing (HF) is performed in many hydrocarbon bearing formations to enhance production level. This is particularly useful for reservoirs having low permeability such as shale gas, shale oil, and tight reservoirs. During hydraulic fracturing, a relatively large volume of fluid, typically water with some additives, is injected at a pressure high enough to exceed the mechanical integrity of the rock and fracture it. Some of the energy used in a fracturing operation is used to induce fractures and the remainder is used to propagate the fractures. 
         [0003]    Conventionally, a well is drilled into the formation. This is followed by a set of logging measurements that provide petrophysical and geomechanical information about the reservoir rock as a function of depth. Based on log information, a desired depth zone is chosen to be fractured and it is common to hydraulically isolate this zone from the remaining part of the well by placing mechanical packers on the two limits of the zone, and sealing-off the zone. This is followed by pumping fracturing fluid into the zone to fracture the rock. With this conventional approach, the pumped fluid exerts radial force to the formation and initiates fracture(s) in the formation away from the borehole. The fracture position will be between the two packers; however the location of the fracture cannot be controlled. This is because the formation is already under stress and the fracture starts at a depth (albeit within the zone) where minimum energy is needed. The position (and direction) of the fracture is controlled by the stress distribution in the rock formation. It is common to use perforations to create weak points in the formation and help fracture initiation, but this may not always be successful as there is no control on the amount of stress exerted on the formation. 
       SUMMARY 
       [0004]    This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter. 
         [0005]    According to some embodiments, a system is described for inducing fractures in a subterranean rock formation surrounding a wellbore. The system includes: an expandable mechanical device configured to make physical contact with rock surfaces on the subterranean rock formation and to apply force on the rock surfaces thereby inducing stress in the subterranean rock formation; and a hydraulic fracturing sub-system configured to isolate a subterranean zone including the mechanical device and to pump fluid into the isolated zone thereby inducing further stress in the rock formation such that fractures are induced and/or propagated in the formation. The induced and/or propagated fractures are in a location and/or direction that are based on the force applied from the expandable mechanical device. 
         [0006]    According to some embodiments, a method is described for inducing fractures in a subterranean rock formation surrounding a wellbore. The method includes: positioning an expandable mechanical device in the wellbore; expanding the mechanical device so as to make physical contact with rock surfaces on the subterranean rock formation; further expanding the mechanical device so as to induce stress in the subterranean rock formation; and pumping hydraulic fluid into an isolated zone of the wellbore including the expandable mechanical device so as to induce and/or propagate fractures in the formation in a location and/or direction that is based in part on the force applied from the expandable mechanical device. 
         [0007]    The subject disclosure describes mechanical devices and methods that can be used to help initiate fracturing a reservoir and doing so in a preset depth and initial direction. 
         [0008]    According to some embodiments, mechanical devices are inserted into a reservoir and attached to the formation. The devices have an expansion mechanism to increase the size of the device. Once the size of the device becomes larger than the distance between opposing walls, any further diameter increase induces a stress in the rock formation. The device can be engineered to induce sufficient stress to cause (micro) cracks in the rock. The cracks so produced will act so as to initialize a hydraulic fracturing process and will be the starting point for a fully developed fracture. 
         [0009]    According to some further embodiments, the stress can be reduced to a low enough level that cracks are not generated but the stress is present as a result of the mechanical device and acts to facilitate fracturing the rock at that location. 
         [0010]    According to some further embodiments, the mechanical device is engineered to cause stress in a desired direction. The initial direction of a fracture is controlled and facilitated with these mechanical devices. 
         [0011]    The mechanical devices of the subject disclosure function when they are in contact with the formation. Sensing devices are added to the mechanical device to measure properties such as pressure, temperature, mechanical stress directions, etc. 
         [0012]    It is desirable to control the location where the fracture initiates, the fracture direction at the initiation point, and the fracture direction as it propagates away from the borehole. The subject disclosure describes an apparatus and method for controlling the location where the fracture initiates and the fracture direction at the initiation point. 
         [0013]    According to some embodiments, the device can be configured to expand in a rapid accelerative manner thereby inducing a degree of shock to the wellbore wall. This will further induce micro-cracks in the rock and in a direction appropriate for fracture initiation. 
         [0014]    Further features and advantages of the subject disclosure will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0015]      FIG. 1  illustrates a horizontal well with two side holes to aid in fracture initiation, according to some embodiments; 
           [0016]      FIGS. 2-1  and  2 - 2  are side views illustrating an anchor device used for inducing stress and/or initiating fractures, according to some embodiments; 
           [0017]      FIGS. 3-1  and  3 - 2  illustrate side views of a device for inducing stress and/or initiating fractures, according to some other embodiments; 
           [0018]      FIGS. 4-1  and  4 - 2  show side and end views, respectively, of a symmetrical nut used in a device for inducing stress and/or initiating fractures, according to some embodiments. 
           [0019]      FIGS. 4-3  and  4 - 4 , show side and end views, respectively, of an asymmetrical nut used in a device for inducing stress and/or initiating fractures, according to some embodiments 
           [0020]      FIG. 5  is a side view of a radial expander devices used to induce stress and/or initiate fractures, according to some embodiments; 
           [0021]      FIG. 6  is a side view of a radial expander devices used to induce stress and/or initiate fractures, according to some other embodiments; 
           [0022]      FIG. 7  shows examples of T-expanders being used to apply stress in side holes, according to some embodiments; 
           [0023]      FIG. 8  shows example expanders in the main borehole capable of applying stress in the main borehole or in a side hole, according to some embodiments; 
           [0024]      FIG. 9  is a flowchart showing aspects of a hydraulic fracturing process that includes mechanical expander devices used to induce stress and/or initiate fractures, according to some other embodiments; 
           [0025]      FIGS. 10-1  and  10 - 2  show aspect of an expander used to apply stress in a formation having a greatly reduced contact tip, according to some embodiments; and 
           [0026]      FIG. 11  shows details of the shape of spacers according to some embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    The particulars shown herein are by way of example and for purposes of illustrative discussion of the embodiments of the subject disclosure and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the subject disclosure. In this regard, no attempt is made to show structural details in more detail than is useful for the fundamental understanding of the subject disclosure, the description taken with the drawings making apparent to those skilled in the art how the several forms of the subject disclosure may be embodied in practice. Furthermore, like reference numbers and designations in the various drawings indicate like elements. 
         [0028]      FIG. 1  illustrates a section of a horizontal well with two side holes to aid in fracture initiation, according to some embodiments. Although a horizontal well  112  is shown, the embodiments described herein can be applied to vertical or deviated wells according to some embodiments. Two example side holes  114  and  116  are shown. The side holes have a length comparable to the diameter of the main borehole  112  and are intended to help fracture initiation. According to some embodiments, the side holes may be the perforations, although in the example shown in  FIG. 1  the side holes  114  and  116  are drilled. The side hole  114  is drilled perpendicular to the z-direction, as shown by coordinate system  100 . Note that the z-direction is parallel with the longitudinal axis of the main wellbore  112 . The side hole  114  is shown along the x-direction but other side holes can exist which may be along the y-direction or any direction between these two directions. The side hole  116  is drilled at a slanted angle relative to the borehole axis (which is in the z-direction). The double-sided arrows in  FIG. 1  define stress directions within the borehole and inside the side holes. Considering a 3×3 stress tensor in a coordinate system  100  aligned within the wellbore  112 , the diagonal stresses (xx, yy, and zz) are shown in  FIG. 1 . The off-diagonal stress directions are not shown for clarity, but they can be visualized having  FIG. 1  and the coordinate system  100  defined in it. The side holes  114  and  116  help introduce stress to the formation in the zz direction, which is normally not possible along the axis of the well. 
         [0029]    In  FIG. 1 , the stress directions are relative to the borehole direction. However, the geo-stresses have their own directions that define the stress frame of reference of the formation. According to some embodiments, matrix algebra can be used to transform stresses from one frame of reference to those in the other, if needed. 
         [0030]    According to some embodiments, logging tools such as borehole imaging (e.g. FMI Fullbore Formation MicroImager, provided by Schlumberger™) and sonic logging tools are used to help determine the stress directions. When a borehole is drilled in the formation, the drilling induced stresses perturb the space around the borehole (the damaged zone) leading to a stress distribution that may be different from the natural geo-stresses. One of the reasons for drilling the side holes, according to some embodiments, is to bypass the damage zone which allows desired stresses to be applied at a location closer to the formation that is controlled by the geo-stresses. The side holes also make it possible to apply stress in the well direction (zz) which cannot be done very effectively by operating in the main well. 
         [0031]    According to some embodiments, a side hole is drilled perpendicular to or at a predetermined angle relative to the direction of the main well bore to a depth which is comparable to the diameter of the main well, such as the case shown in  FIG. 1 . The side hole is made deep enough that the stress variation caused by the well drilling operation is by-passed, or at least reduced. A mechanical device is then inserted inside the hole. The mechanical device is designed to apply stress to the hole wall and initiate a mini (local) fracture, or at least, induce stress in the formation and keep the stress active so that when the hydraulic fracturing fluid is pumped into the well, the presence of this added stress helps to initiate the fracture at that point. As a result, the fracture will initiate at the location where the mechanical device is placed (a pre-determined depth). In addition, some or all of the energy that is normally used to initiate the fracture in a normal fracturing operation is saved. According to some embodiments, the side hole(s) are created using a perforation tool. 
         [0032]    According to some further embodiments, a small drill bit (smaller than the drill bit used to drill the well) is used along with a drill motor to drill the side hole. Commercial logging tools are available to perform this operation such as the Cased Hole Dynamics Tester (CHDT) tool offered by Schlumberger™. According to some embodiments the side holes are drilled using a side wall coring tool. In such cases, the cut core may be brought to the surface and used for other studies. 
         [0033]    According to some embodiments, the mechanical device is chosen to have an appropriate diameter for insertion into the newly drilled side hole. Once at the desired location and orientation, the device is expanded to make contact with the borehole wall and then further expanded to apply stress to the formation. 
         [0034]    According to some embodiments, a class of mechanical devices known as an anchor is used to provide radial stress. Anchors are mechanical devices that fit into a hole, and are expanded to make contact with the hole wall. These devices are used for attaching objects on a surface. The attached objects apply a pull on the anchor; however, the radial force prevents the anchor from moving axially. For the application according to some of the embodiments described herein, there is no axial pull, and the radial force is used instead to induce stress on the formation and facilitate fracture initiation. The anchors are available commercially, for example, from Concrete Fastening System in Cleveland, Ohio. Anchors vary in design but as long as they are an appropriate size to fit into the side hole and are made mechanically strong to induce the desired level of stress they may be used for the applications described herein. 
         [0035]    Anchors are designed such that the expansion is typically caused by turning a bolt that moves the inner parts of the device relative to each other which causes the diameter to increase.  FIGS. 2-1  and  2 - 2  are side views illustrating an anchor device used for inducing stress and/or initiating fractures, according to some embodiments. The anchor device  200  includes a plurality of rods each having two or more pieces that are hinged together and to the body of the anchor. Two rods  212  and  222  are visible in the side views of  FIGS. 2-1  and  2 - 2 , but more may be present. Each of the rods (e.g.  212  and  222 ) is made of three smaller rods. Each of the rods (e.g.  212  and  222 ) is attached to the end pieces  226  and  228  via hinges. One of the ends, in this case end piece  228  is not threaded (has a through hole) and remains fixed while the other end piece  226  is threaded and can be moved by turning the bolt  229  in the direction that shortens the length of the rods  212  and  222 , causing the hinges  224  and  225  to move out and away from the central axis of the device  200 . The expansion in this example can be caused by turning the bolt  229 , which pulls the end piece  226  and reduces the distance from  226  to  228 .  FIG. 2-2  shows the same anchor  200  where the bolt  229  has been tightened to some extent causing the rods  212  and  222  to expand as described. In using a device such as anchor  200  shown in  FIGS. 2-1  and  2 - 2 , the device  200  in its non-expanded state (as shown in  FIG. 2-1 ) is inserted in the side hole (such as side holes  114  and/or  116  in  FIG. 1 ) and oriented to the desired direction, with the bolt head accessible from the main well and can be turned to expand the anchor. The anchor is then expanded in place and the expanded rods ( 222  and  212 ) exert radial stress to the formation (e.g. formation  110  in  FIG. 1 ). 
         [0036]    According to some embodiments, the mechanical devices described herein are also equipped with one or more sensors. Sensors  250  and  252  shown in  FIGS. 2-1  and  2 - 2 , for example, can be stress measuring sensors that are placed on the part of the mechanical device that touches the formation. According to some embodiments, the sensors  250  and  252  are designed to measure the stress in three orthogonal directions (e.g. in x-, y-, and z-directions). In this way, the stress variation can be monitored as a function of time and in particular before and after the fracture initiation. According to some embodiments, sensors  250  and  252  can be powered by batteries (not shown) and data can be saved in memory (not shown) for future download. According to some other embodiments, the sensors  250  and/or  252  can include other types of measurement devices such as to measure temperature, pressure, acoustic, and/or flow. In this way, further details as to the mechanism of fracturing can be studied. According to some embodiments, the data from the sensors  250  and  252  are transmitted wirelessly to the surface during the fracturing operation and the data is monitored to optimize the fracturing process in real time. 
         [0037]      FIGS. 3-1  and  3 - 2  illustrate side views of a device for inducing stress and/or initiating fractures, according to some other embodiments. A bolt  332  is attached to a nut  334 , which is used to expand the device  300 . The device has two or more rods of which two  316  and  336  are visible in  FIGS. 3-1  and  3 - 2 . The rods are attached at one end via hinges to end piece  338 , while the other end of each rod is loosely held together (not shown in  FIGS. 3-1  and  3 - 2 ) before the expansion. The expansion is caused by the shape of nut  334  which is designed to be of a higher diameter at one end. When the nut  334  is moved into the space between the rods  316  and  336 , as a result of tightening bolt  332 , it causes the rods  316  and  336  to expand by moving away from each other. The expansion increases as the nut  334  approaches the end piece  338 . An expanded form of the device  300  is shown in  FIG. 3-2 . According to some embodiments, one or more sensors  250  and/or  252  are included such as described with respect to  FIGS. 2-1  and  2 - 2 , supra. 
         [0038]    In the design shown in  FIGS. 3-1  and  3 - 2 , the nut  334  may or may not be circularly symmetric. If the nut  334  is symmetric, it has no preferred orientation, and as a result it can expand the rods to the same extent and induce stress on the wall of the side hole in a non-preferential way. 
         [0039]    According to some embodiments, the expansion nut can be asymmetric.  FIGS. 4-1  and  4 - 2  show side and end views, respectively, of a symmetrical nut  334 , according to some embodiments.  FIGS. 4-3  and  4 - 4 , show side and end views, respectively, of an asymmetrical nut  444 , according to some embodiments. The asymmetrical nut  444  has been designed to have a higher expansion capability in a selected direction causing the induced stress to be higher in that direction. This embodiment is useful in causing fracture initiation in one preferred direction. Also shown in  FIGS. 4-2  and  4 - 4  are cross sections of rods  316  and  336  as well as rods  416  and  436 . As can be seen from  FIG. 4-4 , the asymmetry in the shape of nut  444  will cause greater expansion, and therefore greater induced stress, by rods  316  and  336  than by rods  416  and  436 . 
         [0040]    The anchors described above in  FIGS. 2-1 ,  2 - 2 ,  3 - 1 ,  3 - 2 ,  4 - 1 ,  4 - 2 ,  4 - 3  and  4 - 4  can be applied to the side hole  114  of  FIG. 1 , causing stresses in the yy or zz (or any combination thereof) directions as seen in  FIG. 1 . When applied to the side hole  116 , the anchor can be applied inline with the axis of the side hole, which causes the stress direction to not align with the borehole defined axis system thereby creating off-diagonal elements. According to some embodiments, a tilted direction (as opposed to inline) may be adapted for the anchor (or by using other devices described herein below) if desired. 
         [0041]    According to some embodiments, the stress can be induced within the borehole. In a non-limiting example, the anchors of  FIGS. 2-1 ,  2 - 2 ,  3 - 1 ,  3 - 2 ,  4 - 1 ,  4 - 2 ,  4 - 3  and  4 - 4  can be made proportionally larger so that they can operate in the main borehole. In such cases, the anchor designs described above can be used to induce the desired stress in a radial direction perpendicular to the axis of the well, rather than to the side hole. These anchors can also be used to apply force in one particular radial direction by limiting the number of rods to two, or by limiting the rod expansion to a selected sub set of rods, for example, by using the asymmetric nuts of  FIGS. 4-3  and  4 - 4 . 
         [0042]    According to some embodiments, another class of mechanical device that may be used is a radial expander. These devices can be used to induce stress in a direction perpendicular to the borehole (or the side hole) wall.  FIG. 5  is a side view of a radial expander devices used to induce stress and/or initiate fractures, according to some embodiments. The expander device  500  is made of a bolt  552  and a nut  554 . The total length of this device  500  can be varied by turning the nut  554  in a clockwise direction to reduce the length of the device or a counter clockwise direction to increase the length of the device. A suitably long radial expander can be held perpendicular to the borehole axis and the nut  554  can be turned clockwise to increase the device length causing it to contact the borehole wall. The tightening can be continued leading to induced stress on the borehole wall. According to some embodiments, one or more sensors  250  and/or  252  are included such as described with respect to  FIGS. 2-1  and  2 - 2 , supra. 
         [0043]    A feature of the radial expander is that it can be mounted on the borehole wall on one side and the deep end of a side hole  114  on the other end, as in the case of expander  502  depicted in  FIG. 8 , infra. The radial extender of  FIG. 5  is turned to set and apply stress to the formation. The turning motion is in a plane perpendicular to the long axis of the expander; the plane of turn is perpendicular to the expansion direction. The embodiments using this type of expander are particularly well suited to main borehole applications where there is good access to the expander, making it practical to turn one of the two sides and set the expander in place. 
         [0044]      FIG. 6  is a side view of a radial expander devices used to induce stress and/or initiate fractures, according to some other embodiments. The radial T-expander  600  is similar to expander  500  of  FIG. 5 , except that it is expanded by turning a nut at a plane that is parallel to the direction of expansion. As depicted in  FIG. 6 , the T-expander is “T” shaped. In these embodiments, the turning motion is transferred to the two parts of the expander through a 90° joint. The two nut parts  614  and  624  are used to press on the formation and apply the stress. These two nut parts are pushed in or out when the two screws  612  and  622  are turned. These screws are attached to gears  610  and  620  which are engaged to a gear  618 . The gear  618  turns as a result of rotating the nut  602 . The nut  602  is used to set the device in place. As the nut  602  is turned, its rotational motion is transferred to the rotational motion of screws  612  and  622  which in turn move the parts  614  and  624  closer together or away from each other, depending on the direction of rotation of nut  602 . One of the design parameters of the T-expander is the gear ratio between gear  618  and gears  610  and  620 . A smaller gear  618  can be turned easier, with less torque, than a larger gear  618 . The penalty of course is the number of turns but a smaller gear  618  can be used to apply a relatively larger force on the formation wall. According to some embodiments, one or more sensors  250  and/or  252  are included such as described with respect to  FIGS. 2-1  and  2 - 2 , supra. 
         [0045]      FIG. 7  shows examples of T-expanders being used to apply stress in side holes, according to some embodiments. In the implementation of  FIG. 7 , the nut  602  of T-expander  600  is accessible easier from the borehole  112  and can be turned to achieve the desired stress on the formation  110 . In another example, another T-expander  650  is being deployed in side hole  116  to apply stress and/or initiate fractures at the location and in the direction shown. 
         [0046]      FIG. 8  shows example expanders in the main borehole applying stress in the main borehole or in a side hole, according to some embodiments. In the case of  FIG. 8 , an expander  500  is deployed as shown for applying stress on the main borehole  112  as shown. According to some embodiments, a longer sized expander  502  is shown with one end on the wall of the main borehole  112  and the other end on the deep end of a side hole  114  as shown. In yet another example, a T-expander  600  is being deployed in the main borehole  112 . 
         [0047]    According to some other embodiments, one or more spacers can be used to reduce the length requirement for the mechanical device. Shown in  FIG. 8 , another T-expander  810  is used for applying pressure to the side hole  116  and main hole  112  at the same time, similar to the application shown that uses radial expander  502 . In this case, however, a shorter T-expander  810 , along with spacers  660  and  662  are used to achieve the objective. The T-expander  810  does not have to be long enough to span the total distance rather the spacers  660  and  662  are used in conjunction with it to increase its effective length. The effective length can be made even longer if greater numbers of spacers are used.  FIG. 11  shows further detail of the shape of spacers  660  and  662 , according to some embodiments. 
         [0048]    According to other embodiments, spacers such as shown in  FIG. 11  can be combined with other types of expanders. For example, using the design of the hydraulic jack used in many applications, the gear system used in T-expander can be replaced with a hydraulic piston similar to what is done with a conventional hydraulic jack. The new expander can be called H-expander where H refers to its hydraulic mode of activation. Some of the hydraulic jacks have telescopic extensions allowing them to expand to more than twice their length. According to some embodiments, the H-expanders are used with any of the features described herein with respect to T-expanders. A conventional hydraulic jack used in automobile applications, is designed such that one side expands. The same type of asymmetric expansion can be used for the H- or T-expanders described herein. In this case, the nut  602  of the T-expander, and corresponding nut for the H-expander will be at the same distance from the borehole wall which simplifies the design of the nut driver needed to implement the expanders. 
         [0049]    In a further embodiment of an H-expander, the hydraulic fluid can be replaced by a slow burn charge that is activated by a detonator or electrical impulse. This slow burn charge expands a fluid or gas, allowing the device to accelerate to the borehole surface thus imparting a force and a shock to induce micro-fractures in the rock. 
         [0050]    In a further embodiment of an H-expander, the hydraulic circuit can be designed so as to impart a vibrational force on the wall of the borehole, in a similar fashion to a jack-hammer. This will further enhance the cracking of the rock. 
         [0051]    A further embodiment of an H-expander allows for the placement of a reactive fluid—an oxidizer, acid or chelating agent—that can be pumped out of the mechanical or hydraulic device in a predetermined manner and direction. The direction of this flow of reactive fluid can create weakpoints in the rock by dissolving or reacting with the rock grains to create chemically weakened zones that either further evolve into micro-cracks or sufficiently reduce the rock mechanical strength allowing for easier fracture initiation. 
         [0052]    According to some embodiments, the contact surface between the mechanical device and the borehole wall can be tuned to achieve different outcomes. For example, if the contact surface is wide, as shown in  FIGS. 2-1 ,  2 - 2 , and  5 - 8 , the induced stress by the mechanical device is less likely to cause a failure in the formation. With this approach relatively large stress levels can be induced in the formation in preparation for the subsequent hydraulic fracturing operation.  FIGS. 10-1  and  10 - 2  show aspect of an expander used to apply stress in a formation having a greatly reduced contact tip, according to some embodiments. The contact surface of rod  1094  is made into a sharp chisel point  1010 . The T-expander  1002  has a wide surface  1092  on one side which can be used as the base contact with the borehole wall. It also has a sharp surface  1010 , mounted to end piece  1014  which is a chisel like structure and with appropriate amount of force can penetrate into the rock and create a mini-fracture.  FIG. 10-2  shows the top view of the sharp surface  1010 . In this case, application of the mechanical device leads to a relatively short fracture in the formation even before the hydraulic fracturing operation has started. The relatively short fracture will act as the initiation point for the final hydraulic fracture. 
         [0053]      FIG. 9  is a flowchart showing aspects of a hydraulic fracturing process that includes mechanical expander devices used to induce stress and/or initiate fractures, according to some other embodiments. In block  910 , a side hole is drilled perpendicular to or at a predetermined angle relative to the direction of the main well bore to a depth which is comparable to the diameter of the main well, such as the case shown in  FIG. 1 . The side hole(s) are made deep enough that the stress variation caused by the well drilling operation is by-passed, or at least reduced. According to some embodiments, a small drill bit (smaller than the drill bit used to drill the well) is used along with a drill motor to drill the side hole. Commercial logging tools are available to perform this operation such as the Cased Hole Dynamics Tester (CHDT) tool offered by SCHLUMBERGER™. According to some embodiments the side holes are drilled using a side well coring tool. In such cases, the cut core may be brought to the surface and used for other studies. According to some embodiment, no side hole is drilled in block  910 , and the mechanical device is positioned to apply stress and/or induce fractures within the main borehole. 
         [0054]    In block  912 , a mechanical device is then inserted inside the borehole. A mechanical device is chosen to have an appropriate diameter for insertion into the newly drilled side hole. Once at the desired location and orientation, in block  914  the device is expanded to make contact with the borehole wall and/or side hole wall. The mechanical device is designed to apply stress to the hole wall and initiate a mini (local) fracture, or at least, induce stress in the formation and keep the stress active so that when the hydraulic fracturing fluid is pumped into the well, the presence of this added stress helps to initiate the fracture at that point. As a result, the fracture will initiate at the location where the mechanical device is placed (a pre-determined depth). In addition, some or all the energy that is normally used to initiate the fracture in a normal fracturing operation is saved. According to some embodiments, no side hole is drilled in block  910 , and the mechanical device is positioned to apply stress and/or induce fractures within the main borehole. 
         [0055]    In block  916 , if the mechanical device includes one or more sensors such as sensors  250  and  252  shown in  FIGS. 2-1  and  2 - 1 , those sensors are used to measure the stress in three orthogonal directions (e.g. in x-, y-, and z-directions), and or make other types of measurements (e.g. temperature, pressure, acoustic, and/or flow). According to some embodiments, one or more of the measurements are taken during the expansion process of the mechanical device and/or during the hydraulic fracturing process of block  918 . The measurements are recorded in the device and/or transmitted to another tool or the surface for analysis. In the case where data from the sensors  250  and  252  are transmitted during the fracturing process the data is monitored in real-time so as to optimize the fracturing process in real-time. 
         [0056]    In block  918 , the hydraulic fracturing process is carried out. A zone that includes the location of the mechanical device is hydraulically isolated using packers. According to some embodiments, at least one of the packers (the one farther away from uphole) is positioned prior to the mechanical expansion (block  914 ) such that the fluid can be pumped into the isolated zone soon after the expansion of the mechanical device. According to some embodiments, the mechanical device remains in place while the fracturing fluid is pumped into the well. Following the completion of the hydraulic fracturing, in block  920 , the mechanical device is contracted and removed. 
         [0057]    Although a few example embodiments have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the example embodiments without materially departing from this invention. Accordingly, such modifications are intended to be included within the scope of this disclosure as defined in the following claims. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures. It is the express intention of the applicant not to invoke 35 U.S.C. §112, paragraph 6 for any limitations of any of the claims herein, except for those in which the claim expressly uses the words ‘means for’ together with an associated function.