Patent Publication Number: US-2016222772-A1

Title: Pseudoelastic Materials as Additives to Enhance Hydraulic Fracturing

Description:
BACKGROUND 
     1. Field of the Invention 
     This invention relates to hydraulic fracturing in the hydrocarbon industry. More specifically, this invention relates to systems and methods of improving fluid conductivity of fractures within a hydrocarbon formation. 
     2. Description of Background Art 
     Hydraulic fracturing is a common method of stimulation for hydrocarbon reservoir formations. During common methods of hydraulic fracturing, a viscous fluid is pumped through a well and injected into the reservoir formation to create a fracture. After the fracture has been created, a higher viscosity fluid with suspended particles of proppant or sand can be pumped into the well. The proppant is generally spherically shaped particles with a fixed volume. The fluid is used to transport the proppant into the created fractures. After pumping stops, the pressure of the fluids in the well decreases and a high overburden or closure stress, which is a result of the weight of the earth above the fracture, can force the fracture to close. The proppant in the fracture will help to resist this closure stress and to keep the fracture open. 
     In practice, however, the proppants may degrade, crush, or embed into the rock formation surrounding the fracture due to the high closure stresses and the high temperature environment, thereby resulting in a deterioration of the ability of the proppant to support the fracture and to keep the fracture open. This deterioration results in reduced permeability and conductivity of the propped fracture, and therefore a reduction in the ability of production fluids in the reservoir formation to reach the well and be produced. 
     Some current systems for maintaining an open fracture include injecting the proppant-containing fracturing fluid in alternating stages. For example, the composition of the proppant-containing fracturing fluid can alter, the pumping rate can be adjusted and the density of the proppant can be changed. In other current systems, conventional fibers or shape memory fibers and shape memory alloys (“SMA”) can be used to initiate the aggregation of the proppant grains. 
     In some current systems using shape memory fibers and SMA, the shape memory effect is a temperature dependent response. Therefore, the shape memory materials must complete the phase transformation and return to their original shapes before the bottom-hole temperature recovers its original value, which sometimes is very challenging to control when deploying the shape memory proppants into the fracture under the field condition. Additionally, transformation of the shape memory materials must not have occurred prior to proper placement of the shape memory materials. This requirement presupposes that the transformation temperature is not exceeded prematurely. Therefore, the procedure is very temperature dependent and meticulous control of the temperatures within the well and the reservoir formation is critical to the operation of the system. Also, shape memory polymer fibers may have insufficient stiffness and strength to resist the high closure stresses acting on the fracture. 
     SUMMARY OF THE DISCLOSURE 
     Embodiments of this disclosure provide systems and methods of maintaining an open fracture using shape memory materials such as SMA and, in particular, making use of the pseudoelastic feature of the SMA. This pseudoelastic feature is exhibited by many SMAs in response to stress-induced transformation and is distinct from a shape memory effect application which instead uses controlled temperature variation to cause the shape memory to change shape. Embodiments of this application provide a soluble container that applies the stress and removes the stress on the SMA and dissolves by a combination of the thermal and the chemical environment of the reservoir formation. Embodiments of this disclosure utilize the pseudoelasticity effect, and not a heated shape memory effect, in order to return a shape memory filter to its expanded filter shape. Therefore it is the soluble container that undergoes a change due to the thermal and chemical environment, and not the SMA itself, and there is not a need for precise temperature control. Use of SMAs for forming shape memory filters in accordance with embodiments of this disclosure will enhance the permeability and conductivity of fractures in the reservoir formation. 
     In an embodiment of this disclosure, a method for enhancing hydraulic fracturing productivity for recovery of a reservoir fluid from a reservoir formation having an open fracture includes providing a shape memory filter that is pseudoelastically deformed and contained within a soluble container. The shape memory filter and soluble container are pumped into the open fracture so that the soluble container dissolves and the shape memory filter returns to an expanded filter shape. A proppant is pumped into the open fracture so that the proppant is trapped by the shape memory filter and forms a column across the open fracture. 
     In alternate embodiments, containing the shape memory filter in the soluble container can cause a stress-induced transformation of the shape memory filter from an austenitic alloy to a martensitic alloy. The shape memory filter can undergo pseudoelastic transformation from a martensitic alloy to an austenitic alloy when the soluble container dissolves, and can transform without a change in temperature of the shape memory filter. 
     In other alternate embodiments, when the shape memory filter is contained within the soluble container the soluble container can apply a confining stress on the shape memory filter, the confining stress maintaining the shape memory filter in a martensitic state. As the soluble container dissolves, a release of the confining stress can allow the shape memory filter to transform to an austenitic state. The shape memory filter can be formed of a nickel titanium alloy and the soluble container can be a dissolvable tube, an enclosed capsule, or a dissolvable tablet. 
     In another embodiment of this disclosure, a method for enhancing hydraulic fracturing productivity for recovery of a reservoir fluid from a reservoir formation having an open fracture includes forming an austenitic alloy into a shape memory filter with an expanded filter shape. The shape memory filter is contained in a soluble container to cause a stress-induced transformation of the shape memory filter from the austenitic alloy to a martensitic alloy with a contracted shape. The shape memory filter and soluble container are pumped into the open fracture so that the soluble container dissolves and the shape memory filter returns to an austenitic alloy with the expanded filter shape. A proppant is pumped into the open fracture so that the proppant is trapped by the shape memory filter and forms a column across the open fracture. 
     In alternate embodiments, the column can resist high closure stresses acting on the open fracture. The method can also include pumping a plurality of the shape memory filters and soluble containers into the open fracture, pumping a proppant into the open fracture so that the proppant is trapped by the plurality of shape memory filters and forms a plurality of columns across the open fracture, and forming channels of fluid conductivity within the open fracture with the plurality of columns. 
     In other alternate embodiments, the shape memory filter can be formed with a material that returns to an austenitic alloy with the expanded filter without a change in temperature of the shape memory filter. The shape memory filter can be formed of a nickel titanium alloy. 
     In yet another embodiment of this disclosure, a system for enhancing hydraulic fracturing productivity for recovery of a reservoir fluid from a reservoir formation having an open fracture includes a shape memory filter. A soluble container contains the shape memory filter when the shape memory filter is pseudoelastically deformed. The soluble container can dissolve after being pumped into the open fracture so that the shape memory filter returns to an expanded filter shape. A proppant is trapped by the shape memory filter and forms a column across the open fracture. 
     In alternate embodiments, the shape memory filter is formed of a material that can undergo pseudoelastic transformation from a martensitic alloy to an austenitic alloy when the soluble container dissolves. The shape memory filter can transform without a change in temperature of the shape memory filter. The soluble container can be formed of a material operable to apply a confining stress on the shape memory filter, the confining stress maintaining the shape memory filter in a martensitic state. The shape memory filter can be formed of a nickel titanium alloy. The soluble container can be a dissolvable tube, an enclosed capsule, or a dissolvable tablet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the features, advantages and objects of the invention, as well as others which will become apparent, are attained and can be understood in more detail, more particular description of the invention briefly summarized above may be had by reference to the embodiment thereof which is illustrated in the appended drawings, which drawings form a part of this specification. It is to be noted, however, that the drawings illustrate only a preferred embodiment of the invention and are therefore not to be considered limiting of its scope as the invention may admit to other equally effective embodiments. 
         FIG. 1  is a schematic section view of a reservoir formation with an open fracture, in accordance with an embodiment of this disclosure. 
         FIG. 2  is a section view of a shape memory filter in a contracted shape and contained within a soluble container, in accordance with an embodiment of this disclosure. 
         FIG. 3  is a section view of a shape memory filter in a contracted shape and contained within an alternate soluble container, in accordance with an embodiment of this disclosure. 
         FIG. 4  is a section view of a shape memory filter in an expanded filter shape with trapped proppant, in accordance with an embodiment of this disclosure. 
         FIG. 5  is a section view of an open fracture of  FIG. 1 , shown with a plurality of shape memory filters in expanded filter shapes with trapped proppant and located within an open fracture forming channels of fluid conductivity, in accordance with an embodiment of this disclosure. 
         FIG. 6  is a phase diagram of the transformation of a shape memory filter, in accordance with an embodiment of this disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     While the invention will be described with several embodiments, it is understood that one of ordinary skill in the relevant art will appreciate that many examples, variations and alterations to the apparatus and methods described herein are within the scope and spirit of the invention. Accordingly, the exemplary embodiments of the invention described herein are set forth without any loss of generality, and without imposing limitations, on the claimed invention. 
     Looking at  FIG. 1 , a subterranean well  10  can extend into and through reservoir formation  12 . Reservoir formation  12  can include reservoir fluids, such as hydrocarbons, including oil and gas, which are to be produced by subterranean well  10 . The produced fluids would travel upwards through subterranean well  10 , where they would be further directed and controlled through wellhead assembly  14 . 
     Traditional hydraulic fracturing procedure can be used to create open fractures  16  that extend from subterranean well  10  into reservoir formation  12 . Such fractures may be formed, for example, by the pumping and injecting of high pressure fluids into subterranean well  10 . 
     Turning to  FIGS. 2-5 , in order to further enhance hydraulic fracturing productivity for recovery of the reservoir fluid from reservoir formation  12 , a shape memory filter  18  can be used to assist in maintaining open fracture  16  open and forming channels of fluid conductivity  20 , as will be further discussed herein. 
     Looking at  FIG. 4 , shape memory filter  18  is formed of a shape memory material, and can be, for example, an SMA such as a nickel titanium alloy. As an example, the nickel titanium alloy can be that known as NiTi or nitinol as well as a multitude of variants such as NiTiX. While NiTi also has the added benefit of chemical stability, any shape memory alloy subject to the aforementioned transformation temperature requirement may potentially serve as a filter material. As an alternate example, the shape memory filter  18  could alternately be formed of CuZnAl alloys, or other pseudoelastic material. The shape memory material is formed into a filter shape to create shape memory filter  18 . Shape memory filter  18  can include, for example, a series of martensitic alloy wires that are formed into a general rounded conical form with cross members for trapping proppant particles  22 . When formed, shape memory filter  18  will have the expanded filter shape as shown in the example embodiment of  FIG. 4 , and will be in an unstressed martensitic state. 
     Turning to  FIGS. 2-3 , shape memory filter  18  can then be contained within soluble container  24 . Containing shape memory filter  18  within soluble container  24  will cause a stress-induced transformation of shape memory filter  18  from an austenitic alloy to a martensitic alloy. Soluble container  24  will apply a sufficiently large confining stress on shape memory filter  18  to initiate a stress-induced, pseudoelastic phase change into a desirably deformed configuration. When shape memory filter  18  is contained within soluble container  24 , shape memory filter  18  will have changed from being in an expanded filter shape ( FIG. 4 ), to being in a contracted shape ( FIGS. 2-3 ). 
     Soluble container  24  is formed of an expendable material that is able to apply a confining stress on shape memory filter  18 , so that the confining stress is sufficient to maintain shape memory filter  18  in a martensitic state. Soluble container  24  can also be formed of a material that is able to apply a sufficient confining stress on shape memory filter  18  to initiate the transition of shape memory filter  18  to the martensitic state. As an example, soluble container  24  could be formed of a polyvinyl alcohol. Soluble container  24  can be, as an example, a dissolvable tube ( FIG. 2 ), an enclosed capsule, and a dissolvable tablet ( FIG. 3 ). As an example, soluble container  24  can be a tube like member and shape memory filter  18  can be pulled into the tube, shape memory filter  18  being changed to a contracted shape and transitioned to a martensitic state as shape memory filter  18  is pulled into the tube ( FIG. 2 ). Alternately, soluble container  24  can be a dissolvable tablet ( FIG. 3 ) in which shape memory filter  18  is embedded. As the dissolvable tablet dissolves, the shape memory filter  18  is free to transition from martensite to austenite. 
     After shape memory filter  18  is contained within soluble container  24 , shape memory filter  18  and soluble container  24  can be pumped down subterranean well  10  and into open fracture  16  of reservoir formation  12 . Shape memory filter  18  and soluble container  24  can be pumped down subterranean well  10  within a fluid, such as, for example, a polymer-based fracturing fluid or water containing a drag reducer. Soluble container  24  can be formed of a material and in a shape that will allow soluble container  24  to dissolve in a controlled manner within the chemical and thermal environment of reservoir formation  12 . The material of soluble container  24  can be a material that dissolves in the ambient thermal and chemical environment of subterranean well  10  and reservoir formation  12  during the pumping of shape memory filter  18  and soluble container  24  down subterranean well  10 , but material of soluble container  24  can also be stable before such pumping process. The material of shape memory filter  18  is conditioned such that the austenite finish temperature is lower than the anticipated ambient temperature during and following the pumping of shape memory filter  18  and soluble container  24  down subterranean well  10 . Soluble container  24  can be coated with a material which will adhere to the fracture face of open fracture  16 . 
     As soluble container  24  dissolves, a release of the confining stress allows shape memory filter  18  to undergo the reverse transformation to austenite and recover its expanded filter shape. Shape memory filter  18  undergoes a pseudoelastic transformation as it transforms from a martensitic alloy to an austenitic alloy. This transformation from a martensitic alloy to an austenitic alloy as soluble container  24  dissolves can take place without a change in temperature of shape memory filter  18 . It is the reduction of the confining stress, not change in temperature, that causes the transformation of shape memory filter  18  back to an austenitic alloy. Depending on the materials used, the temperature, and the chemical environment, the time required for soluble container  24  to dissolve can vary. As an example, if soluble container  24  is formed of a polyvinyl alcohol, soluble container  24  could take in the range of five to twenty-five minutes to dissolve. 
     Looking at  FIGS. 4-5 , after shape memory filter  18  has returned to an expanded filter shape, proppant particles  22  can be pumped into open fracture  16  so that proppant particles  22  are trapped by shape memory filter  18 . Before the proppant particles  22  are pumped into open fracture  16 , the soluble container  24  should have dissolved and the shape memory filter  18  should have deployed. Shape memory filter  18  will span the thickness of open fracture  16  so that it contacts both fracture faces. This contact with the two faces should hold open fracture  16  in place, allowing it to catch proppant particles  22  as proppant particles  22  are pumped into open fracture  16 . The trapping of proppant particles  22  by shape memory filter  18  leads to a localized buildup of the proppant particles  22  and the formation of agglomerate, pillar or column  26  across open fracture  16 . Shape memory filter  18  can be coated with a material to make it adhere to the fracture face of open fracture  16  after shape memory filter  18  is released from soluble container  24 . The coating on shape memory filter  18  can help trap proppant particles  22 , thereby enhancing the buildup of column  26 . Proppant particles  22  should not be too small relative to the size of the gaps in the shape memory filter  18 . If proppant particles  22  are too small, then proppant particles  22  would pass through the gaps in shape memory filter  18  without being trapped by the shape memory filter  18 . 
     Column  26  will resist high closure stresses action on open fracture  16 . The closure stress is a result of the weight of the earth above open fracture  16 , and can force open fracture  16  to close. A plurality of columns  25  throughout open fracture  16  will form channels of fluid conductivity  20  within open fracture  16 . Columns  26  will therefore help to keep open fracture  16  open and provide a network of open channels of fluid conductivity  20 , which surround columns  26 , for the production fluid to flow within open fracture  16  to reach subterranean well  10 , thereby resulting in an increase in the fluid conductivity of open fracture  16 . There is no specific orientation required for the position of shape memory filter  18  within open fracture  16 . 
     In an example of operation, to enhance hydraulic fracturing productivity for recovery of a reservoir fluid from a reservoir formation  12 , open fracture  16  can first be created in reservoir formation  12  by traditional hydraulic fracturing methods. An austenitic alloy can be formed into shape memory filter  18  with an expanded filter shape. Looking at  FIG. 6 , shape memory filter  18  is at this time at zero stress, state A. A plurality of shape memory filters  18  can be contained in soluble containers  24 . This process, shown as process A-B in  FIG. 6 , applies a confining stress to shape memory filter  18 , causing shape memory filters  18  to undergo the austenite to martensite phase change and move to a contracted shape. After containing shape memory filter  18  in soluble container  24 , shape memory filter  18  is at state B. 
     A plurality of shape memory filters  18  and soluble containers  24  are then pumped into open fracture  16 , which can increase the temperature of shape memory filter  18 . This pumping process is shown as process B-C in  FIG. 6 . Note that although the temperature of shape memory filter  18  increases, at this time it remains contained in soluble container  24  as a martensitic alloy in a contracted shape at state D. Over time, soluble container  24  dissolves in open fracture  16 , thereby freeing shape memory filter  18 . This dissolving process, shown as process C-D in  FIG. 6 , relieves the confining stress on the shape memory filter  18 , thereby enabling the reverse phase transformation of shape memory filter  18  from martensite to austenite and allowing shape memory filter  18  to revert to an expanded filter shape. As can be seen in  FIG. 6 , the process C-D does not require a change in temperature. 
     At this stage, shape memory filter  18  will be at state D. In order for shape memory filter  18  to revert to an austenitic alloy, soluble container  24  need not dissolve completely, but merely needs to dissolve enough so that the confining stress is reduced and soluble container  24  can no longer contain shape memory filter  18  in a martensitic state. In this way, systems and methods as described herein will enhance the permeability and conductivity of fractures in reservoir formation  12 , improving the production of fluids from subterranean well  10 . 
     Looking at  FIG. 5 , after shape memory filters  18  have reverted to an expanded filter shape in place in open fracture  16 , a proppant with a plurality of proppant particles  22  are pumped in to open fracture  16  and are trapped by shape memory filters  18 , causing the buildup of proppant particles  22  to form agglomerations or columns  26  across open fracture  16 . These columns  26  can resist high closure stresses on open fracture  16 , and allow production fluids to flow in channels of fluid conductivity  20  between columns  26 . 
     Although the present invention has been described in detail, it should be understood that various changes, substitutions, and alterations can be made hereupon without departing from the principle and scope of the invention. Accordingly, the scope of the present invention should be determined by the following claims and their appropriate legal equivalents. 
     The singular forms “a,” “an,” and “the” include plural referents, unless the context clearly dictates otherwise. 
     “Optional” or “optionally” means that the subsequently described event or circumstances can or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur. 
     Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range. 
     Throughout this application, where patents or publications are referenced, the disclosures of these references in their entireties are intended to be incorporated by reference into this application, in order to more fully describe the state of the art to which the invention pertains, except when these references contradict the statements made herein. 
     As used herein and in the appended claims, the words “comprise,” “has,” and “include” and all grammatical variations thereof are each intended to have an open, non-limiting meaning that does not exclude additional elements or steps. 
     As used herein, terms such as “first” and “second” are arbitrarily assigned and are merely intended to differentiate between two or more components of an apparatus. It is to be understood that the words “first” and “second” serve no other purpose and are not part of the name or description of the component, nor do they necessarily define a relative location or position of the component. Furthermore, it is to be understood that that the mere use of the term “first” and “second” does not require that there be any “third” component, although that possibility is contemplated under the scope of the present invention.