Patent Publication Number: US-7216708-B1

Title: Reactive stimulation of oil and gas wells

Description:
This application claims the benefit of the filing date of provisional application Ser. No. 60/502,703, entitled “Reactive Stimulation of Oil and Gas Wells,” filed Sep. 12, 2003, the contents of which are incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The present invention relates to methods and devices for stimulating producing formations in oil and gas wells to increase production. 
   BACKGROUND OF THE INVENTION 
   The quantity of oil and gas production from a hydrocarbon bearing strata into a borehole is influenced by many physical factors. Darcey&#39;s flow equation, which defines flow in a well, takes into account the reservoir constants of temperature, viscosity, permeability, reservoir pressure, pressure in the borehole, thickness of the producing strata, and the area exposed to flow. 
   It has long been known that increasing the exposed flow area in a producing well increases production. For example, it is known that drilling a larger diameter hole exposes more of the producing strata and thus increases production. 
   Enlarging the flow areas, in open hole intervals, has been accomplished by using both explosives and chemicals. However, use of these agents is somewhat limited where the producing strata are cemented behind steel casing. In cased applications, the well is “perforated” to create small holes that extend through the steel casing, the annulus cement and the adjacent formation. 
   Prior to the invention of the shaped charge, wells were perforated with multiple, short-barreled guns. The bullets penetrated the casing, the annulus cement, and the producing strata. The shaped charge, with its greater penetration and reliability, though, has largely replaced the so-called “bullet guns.” 
   A shaped charge makes a hole through the casing and into the strata by forming a high speed stream of particles that are concentrated in a small diameter jet. As the high energy particles hit solid material, the solid material is pulverized. Thus, shaped charges can be used to place numerous small perforations where desired in a well. However, the fine material from the pulverized rock and the shaped charge particles can have a detrimental effect on fluid flow in the area around the perforation. Debris from the spent charge as well as fragments and particles from the pulverized formation tend to plug the perforations and obstruct passages in the fractured formation. 
   The formation pressure acts on the small oil droplets in the formation to force the hydrocarbons from the connected pore spaces into the well bore. The magnitude of the area in the formation exposed by the perforations directly affects the amount of flow and/or work required for that production. Accordingly, increasing the exposed flow area by perforation does two favorable things: it increases the flow rate directly, and, it reduces the amount of work required to maintain a given production rate. Increasing the flow area in a well increases the ultimate recovery from the well/reservoir by conserving formation pressure or reservoir energy. 
   The present invention provides a method and apparatus capable of increasing the exposed surface area in a formation when using shaped charges to perforate a well. This apparatus and method augment the use of shaped charges by including the introduction of oxygen rich material into the formation with the explosive. The delivery of an oxygen source to the hydrocarbon-containing formation, in the presence of the explosive reaction, provides sustained explosive burning of the hydrocarbons in the vicinity of the perforation. The burning in the formation continues until the oxygen-rich material is depleted, when the burning self-extinguishes. Thus, the extent of the burning can be controlled by selecting the amount of oxygen-rich material to be introduced into the formation. 
   This significant secondary reaction in the strata has two beneficial effects. In the first place, the reaction will cause a cleaning effect on the fine particles that might otherwise plug the perforation. The cleaning effect occurs when the explosive burning causes high pressure gases to be generated, and these pressurized gases are discharged rapidly back into the borehole or casing. Secondly, the extended burning or explosion in the treated statum causes further fracturing of the formation. This results in further expansion of the exposed flow areas in the formation beyond the initial shaped charge perforation. In addition, in the event the strata being perforated are water bearing, the explosive reaction will not occur; rather, only oil or gas bearing formations will be stimulated. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to apparatus for stimulating production from a hydrocarbon-containing formation in an oil or gas well. The apparatus comprises a container sized to be received and supported in the well at a level adjacent the formation. At least one shaped charge is supported within the container. The shaped charge is adapted, when ignited, to perforate the formation and to initiate a burn of hydrocarbons therein. The apparatus includes a supply of oxygen-rich material supported within the container and adapted to be introduced explosively into the formation with the shaped charge. In this way, the burn of hydrocarbons therein is extendable. The apparatus further includes at least one igniter for detonating the shaped charge. 
   Still further, the present invention comprises a method for stimulating production from a hydrocarbon-containing formation in an oil or gas well. The method comprises perforating the formation using a shaped charge and introducing an oxygen-rich material to the formation. Thus, the burn of the hydrocarbons is enhanced. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a longitudinal section view of an apparatus in accordance with a first embodiment of the present invention. The apparatus is shown positioned at the level of a target formation in an oil or gas well. 
       FIG. 2  is a schematic diagram illustrating the timing of the sequence of events produced by the apparatus of  FIG. 1 . 
       FIG. 3  is a fragmented sectional view of the target formation shown in  FIG. 1  after completion of the stimulation treatment. 
       FIG. 4  is a longitudinal sectional view of an apparatus in accordance with a second embodiment of the present invention positioned at the level of a target formation in an oil or gas well. 
       FIG. 5  is a section view of a shaped charge made in accordance with one embodiment of the present invention. 
       FIG. 6  is a section view of a shaped charge made in accordance with another embodiment of the present invention. 
       FIG. 7  is a section view of a shaped charge made in accordance with another embodiment of the present invention. 
       FIG. 8  is a section view of a shaped charge made in accordance with another embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
   The Embodiment of FIGS.  1 – 3   
   With reference now to the drawings in general and to  FIG. 1  in particularly, there is shown therein an apparatus constructed in accordance with a first preferred embodiment of the present invention and designated generally by the reference numeral  10 . The apparatus  10  is adapted to stimulate production from a hydrocarbon-containing formation or strata  12  in an oil or gas well  14 . 
   An illustrative well environment is shown in  FIG. 1  and comprises shale zones  16  and  18  above and below the formation  12 . In most instances, the apparatus  10  will be used in a cased interval of the well  14 , and the casing of the well  14  is indicated at  20  with the cement in the annulus designated at  22 . 
   The apparatus  10  comprises a container  24  sized to be received and supported in the well  14  at a level adjacent the formation  12 . Preferably, the container  24  is elongated having first and second ends  26  and  28 . 
   The apparatus  10  further comprises at least one shaped charge supported within the container  24 . The shaped charge is adapted, when ignited, to perforate the formation. Preferably, there is a plurality of shaped charges that can be positioned to perforate different locations in the formation  12 . More preferably, there are three shaped charges, such as the charges  30 . This embodiment may use conventional shaped charges. Accordingly, no detailed description of the shaped charges  30  is provided herein. 
   With continuing reference to  FIG. 1 , an igniter of some sort is provided to detonate the shaped charges  30 . In the preferred embodiment of  FIG. 1 , the igniter comprises an electrical igniter  32  disposed within container  24 . The igniter  32  is electrically connected to a conductor wire  34  which extends from the apparatus  10  to the well head (not shown). As shown here, the conductor wire  34  may be used to suspend the apparatus  10  in the well  14 . 
   Extending from the igniter  32  is a primer cord  38 . Preferably, the primer cord comprises a high order explosive, and is crimped into and made a part of the igniter  32 . The primer cord  38  connects to the shaped charges  30  in series. Thus, when the igniter  32  is initiated by a signal from the surface through the conductor wire  34 , the shaped charges  30  will be ignited by the fast burning primer cord  38 , which runs from the igniter  32  to the uppermost shaped charge  30  in the plurality of charges. 
   Referring still to  FIG. 1 , the apparatus  10  preferably also includes a supply of oxygen-rich material supported within the container  24  and adapted to be introduced explosively into the formation  12  with the shaped charges, such as the charges  30 . This will provide a source of oxygen to support explosive burning of the hydrocarbons in the formation. 
   In the embodiment of  FIG. 1 , the oxygen-rich material  40  in the container  24  is external to and surrounds the shaped charges  30 . Preferably, the oxygen-rich material  40  is potassium nitrate. However, the other materials such as ammonium nitrate may be utilized in addition to or instead of potassium nitrate. As used herein, “oxygen-rich material” denotes any material capable of releasing oxygen when activated. 
   To propel the oxygen-rich material  40  through the perforations behind the shaped charges  30 , the apparatus is provided with separate delivery explosives in the form of end charges  44  and  46 . The end charges  44  and  46  preferably are composed of a slow burning (low order) explosive and may be positioned at the first and second ends  26  and  28 , respectively, of the container  24 . When thus arranged, it is convenient to attach the primer cord  38  to the end charges  44  and  46 , as shown in  FIG. 1 . Thus, a single signal on the conductor wire  34  to the igniter  32  will ignite the end charges  44  and  46  as well as the shaped charges  30  via the primer cord  34 . 
   The end charges  44  and  46 , positioned at each end of the supply of oxygen-rich material  40 , will create very high pressures momentarily inside the container  24  and the well casing  20 . This pressure will force the oxygen-rich material  40  out through the perforations in the casing  20 , the annulus cement  22 , and into the surrounding formation  12  immediately behind the shaped charges. This in turns causes explosive burning of the hydrocarbons in the formation  12  that is supported by the oxygen being released by the oxygen-rich material  40 . 
   The operation of the apparatus of  FIG. 1  is explained with reference to the diagram in  FIG. 2 . At Time Zero, the signal from the conductor wire  34  triggers the igniter  32  ( FIG. 1 ), which in turn initiates the explosive reaction in the fast burning primer cord  38  that runs the length of the container  24 . The reaction time of the igniter  32  is shown at  50  on the time graph in  FIG. 2 . The spike has a duration of about 0.0500 milliseconds, and the total reaction time of the igniter is about 0.200 milliseconds. 
   The igniter  32  initiates the reaction in the fast burning primer cord  38 . Being a fast burning explosive, the cord  38  burns from the igniter to the cord end very rapidly, for a duration of about 0.500 milliseconds indicated at  52  in  FIG. 2 . The preferred primer cord  38  burns at about 20,000 feet per second. Thus, the primer cord  38  could travel a 10-foot string of 40 shaped charges, for example, in only about 0.500 milliseconds. 
   The primer cord  38  ignites the shaped charges  30 , the oxygen-rich material  40 , and the low order explosives in the end charges  44  and  46 . Due to fast burning (high order) explosives in the shaped charges  30 , the shaped charges burn rapidly for about 0.100 milliseconds as indicated at  54 . However, the much slower burning oxygen-rich material  40  and the end charges  44  and  46  burn for a much longer duration, about 4.000 milliseconds and about 5.000 milliseconds at  56  and  58 , respectively. 
   Referring still to  FIG. 2 , the secondary reaction in the formation comprising the sustained burning of the hydrocarbons lasts until the oxygen-rich material  40  is depleted, as indicated at  60 . The total duration of the reactive explosion of hydrocarbons and oxygen in the formation, therefore, begins shortly after the introduction of oxygen in the perforated hole and into the formation and expires as the pyrotechnic reactions stop for lack of oxygen or other reagents. 
   The effect of the operation of the apparatus  10  is illustrated in  FIG. 3 , to which attention now is directed. This drawing illustrates the condition of the well after ignition of the apparatus  10 . The container  24  and its components are substantially destroyed, leaving perforations  62  corresponding to the positions of the shaped charges  30 . The sustained, explosive burn of the hydrocarbons in the formation surrounding the perforations  62  has substantially increased the surface area for production by fracturing and cleaning the formation. 
   The Embodiment of FIGS.  4 – 6   
   Shown in  FIG. 4  is another preferred embodiment of the apparatus of the present invention. In this embodiment, the apparatus  10 A comprises an elongated container  24 A having first and second ends  26 A and  28 A. The container  24 A is suspended by a conductor wire  34 A similar to the corresponding components of the apparatus  10  of  FIG. 1 . An electrical igniter  32 A, which may be similar to the igniter  32  of the previous embodiment, is supported near the first end  26 A of the container  24 A. 
   At least one and preferably three shaped charges  70  are supported inside the container  24 A. As in the previous embodiment, the shaped charges  70  preferably are connected in series to a primer cord  38 A, which is connected to the igniter  32 A. Generally, it is desirable to average about four shaped charges per foot. 
   The apparatus  10 A also includes a supply of oxygen-rich material. However, in this embodiment, the oxygen-rich material is contained in the shaped charges  70 . 
   One preferred embodiment for the “oxygenated” shaped charge  70  of this invention is shown in  FIG. 5  and designated as  70 A. The shaped charge  70 A comprises a body of high explosive  72  formed to have a conically shaped frontal recess  74 . 
   A detonator is included in the shaped charge  70 A to ignite the body of explosive  72 . The detonator may be the primer cord  38 A running therethrough. 
   A liner  76 , usually of copper, is included. The liner  76  is shaped to line the frontal recess  74  in the body of explosive  72 . Thus, the liner  76  in this configuration is conical. 
   Still further, a layer of oxygen-rich material  78  is included in the shaped charge  70 A. In the preferred form, the oxygen-rich layer  78  is positioned between the conical copper liner  76  and the conical frontal recess  74  of the body of explosive  72 . The conically shaped oxygen-rich material  78  and the conically shaped copper liner  76  thus form a bimetallic liner for the shaped charge  70 . 
   After the primer cord  38 A ignites the high explosive  72 , the rapid burning of explosive  72  will convert the conically shaped copper liner into a rapidly moving jet that will perforate the casing and the formation (neither shown in this Figure). At the same time, the conically shaped oxygen-rich layer  78  will also be converted into a slower moving slug of oxygen-rich material. This slower moving slug follows the rapidly moving jet into the formation where, in the presence of the jet and the hydrocarbons in the formation, the oxygen-rich slug will support an extended burn of the hydrocarbons. 
   Shown in  FIG. 6  is another embodiment of a shaped charge in accordance with the present invention designated as  70 B. In this embodiment, the shaped charge  70 B comprises a conically shaped body of fast burning explosive  80 . The recess  82  is also conical in shape. A detonator is included, such as the primer cord  38 A, to ignite the fast burning explosive  80 . 
   The shaped charge  70 B further comprises a conically shaped insert  84  of slower burning (lower order) explosive. The insert  84  is shaped to conform to and be received in the frontal recess  82  of the body  80 . Thus, the insert  84  in the embodiment shown is conically shaped. Further, the insert  84  is shaped to have a planar front  86 . 
   Referring still to  FIG. 6 , the shaped charge  70 B comprises a disc shaped layer  88  of fast burning explosive. The fast burning layer  88  has a front  90  and a rear  92 . The rear  92  is fixed to the planar front  86  of the insert  84 . 
   Still further, the shaped charge  70 B includes a disc shaped layer  98  of elastic material molded at high pressure to contain an oxygen-rich material, such as potassium nitrate fixed on the front of the fast burning layer  88 . 
   It is now seen that, when the shaped charge  70 B is detonated, the oxygen-rich disk  98  will be propelled through the casing  20  and cement annulus  22 . The initial movement of the disc of oxygen-rich material  98  will be ahead of the shaped charge jet. However, the shaped charge jet will quickly pierce the disc of oxygen-rich material  98  and will proceed to make the perforation through the casing  20  and cement annulus  22 . The solid oxygen-rich disk  98  becomes a projectile that follows the jet into the perforation tunnel. The disk  98  supports the combustion of hydrocarbons in the formation ignited by the jet for the selected duration. 
   Turning now to  FIG. 7 , another embodiment of the “oxygen-loaded” shaped charge will be described. This embodiment, designated generally by the reference numeral  70 C, comprises a first body  100  of fast burning explosive formed to have a frontal recess  102 . Preferably, the frontal recess  102  is generally conical in shape and the apex is curved or domed instead of pointed. 
   Also included is a body of oxygen-rich material  104 , such as potassium nitrate, formed to be received in the frontal recess  102  of the first body of explosive  100  and to have a frontal recess  106 . The frontal recess  106  has a cylindrical center portion  108  and a frusto-conical forward portion  110 . 
   Still further, the shaped charge  70 C comprises a second body  112  of fast burning explosive shaped to conform to and be received in the cylindrical center  108  of the recess  102  in the body of oxygen-rich material  104 . The second body  112  is also shaped to have a conical front recess  114  continuous with the frusto-conical forward portion  110  of the frontal recess  106  in the body of oxygen-rich material  104 . In this way, the frontal recess  114  of the second body of explosive  112  and the frusto-conical portion  110  of the frontal recess  106  in the oxygen-rich material  104  form a complete cone. 
   The charge  70 C includes detonators, such as the primer cords  38 A and  38 B, adapted to ignite the first body of fast burning explosive  100  and the second body of fast burning explosive  112 . A conically shaped metal liner  118  is positioned inside the complete cone formed by the frontal recess  114  of the second body of explosive  104  and the frusto-conical portion  110  of the frontal recess  106  in the oxygen-rich material  104 . 
   The primer cords  38 A and  38 B ignite the first and second bodies of fast burning explosives  100  and  112 . Then, second body of high order explosive  112  will collapse the liner  118  to form a high velocity jet which will penetrate the casing, cement, and formation. Concurrently, the first body of high order explosive  100  propels the oxygen rich material  104  into the perforation tunnel in time to support the reaction of the jet and the hydrocarbons in the formation. 
   With reference now to  FIG. 8 , yet another embodiment of a shaped charge will be described. This shaped charge, designated generally as  70 D, comprises a body of fast burning explosive  120 . The body of explosive  120  is formed to have a stepped frontal recess  122  with a conical center portion  124  and a frusto-conical forward portion  126 . The narrowest diameter of the forward portion  126  forms a step  128  between the center portion  124  and the forward portion  126 . 
   The charge  70 D further comprises a body of oxygen-rich material  130  formed to be received in frusto-conical forward portion  126  of the frontal recess  122  of the body of explosive  120 . The narrowest diameter of the body of oxygen-rich material  130  is substantially the same as the widest diameter of the center portion  124  of the frontal recess  122  of the body of fast burning explosive  120 . Thus, the conical center portion  124  of the frontal recess  122  of the body of explosive  120  and the body of oxygen-rich material  130  form a complete cone. 
   A detonator, such as the primer cord  38 A is adapted to ignite the body of fast burning explosive  120 . Also, included is a conically shaped liner  132  positioned inside the conical center portion  124  of the frontal recess  122  in the body of fast burning explosive  120 . 
   The primer cord  38 A ignites the body of fast burning explosives  120 . Then, the liner  132  and a small part of the oxygen rich material  126  will collapse into a high velocity jet that will penetrate the casing, cement, and formation. The remaining oxygen rich material  126  will form a slower moving slug that will enter the perforation tunnel in time to support the reaction of the jet and the hydrocarbons in the formation. 
   In accordance with the method of the present invention, there is provided a method for stimulating the hydro-carbon containing strata in an oil and gas well. First, preferably using one of the above described apparatus, the formation is perforated using a shaped charge. An oxygen-rich material, such a potassium nitrate, is introduced into the formation to support a sustained burn of the hydrocarbons therein. 
   Whether the apparatus  10 A with the shaped charge  70 B is employed or the shaped charge  70 A is utilized or the apparatus  10  of  FIG. 1  is used, the oxygen-rich material is forced into the formation following the shaped charge jets. In all cases, though, a supply of oxygen-rich material is dispersed through the altered formation in the presence of ignited hydrocarbons so that a sustained burn can occur. This effectively increases the exposed surface area and enhances production from the altered formation. 
   Changes can be made in the combination and arrangement of the various parts and elements described herein without departing from the spirit and scope of the invention as defined in the following claims.