Patent Publication Number: US-2018045014-A1

Title: Wellbore plug structure and method for pressure testing a wellbore

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority benefit of U.S. Provisional Patent Application No. 62/375,203, filed on Aug. 15, 2016, the disclosure of which is incorporated herein by reference in its entirety. 
    
    
     FIELD 
     This disclosure relates to the field of wellbore plugs, such as wellbore plugs that are used for the construction and/or use of a wellbore for the extraction of natural resources from the earth, such as oil, gas, water and the like. 
     BACKGROUND 
     In the downhole drilling industry, e.g., for oil and gas extraction, it is common practice to drill a borehole and install a casing in the borehole to form a well. The casing is commonly cemented into the borehole, which may include pumping a wiper plug down the casing to force the cement through a port located at a distal end of a casing string. After completion of the cementing operation, it is often necessary to conduct an integrity test on the casing before fracturing operations to ensure that the casing can safely withstand operating pressures without failure. Commonly, such pressure tests are performed against a pressure-activated toe sleeve disposed at the bottom (e.g., the toe) of the casing. However, after the pressure test, a flow path must be formed through the toe sleeve by using a pressure in excess of the test pressure, thereby invalidating the initial pressure test. Also, the toe sleeve against which a pressurization test is conducted often has an atmospheric chamber. The applied pressure may allow fluid to move past the O-ring and into the atmospheric chamber, which would either prematurely open the sleeve or prevent the sleeve from opening after the pressure test. 
     SUMMARY OF THE INVENTION 
     There is a need for a wellbore plug structure that enables a full pressure test of the wellbore casing to be performed without the use of a pressure activated toe sleeve, and/or without requiring the application of a pressure in excess of the pressure test pressure to begin the fracturing process. 
     The present disclosure relates to a wellbore plug structure, such as a wiper plug, that advantageously allows a full pressure test of the casing to be performed against the plug structure, without the need for a toe sleeve. With the disclosed wellbore plug structure, the pressure test does not have to occur against an atmospheric chamber. 
     In one embodiment, a wellbore plug structure is disclosed. The wellbore plug structure includes a tubular member defining a longitudinally-extending fluid conduit having a proximal fluid inlet and a distal fluid outlet, a wiping member disposed around the tubular member, a piston member comprising a piston body that is at least partially disposed in the fluid conduit, and a temporary fluid stopper comprising a dissolvable fluid obstructing portion, where the dissolvable fluid obstructing portion is disposed in spaced-apart relation from the piston member toward the distal fluid outlet. The piston member and the dissolvable fluid obstructing portion define an interior chamber within the fluid conduit, the interior chamber being fluidly sealed from the proximal fluid inlet by the piston member and being fluidly sealed from the distal fluid outlet by the dissolvable fluid obstructing portion. 
     In one characterization, the piston member is configured to release and move toward the distal fluid outlet when the piston member is exposed to a pressure difference across the piston member that is equal to or greater than a piston release pressure. The release and movement of the piston member creates a fluid pathway from the proximal fluid inlet to the interior chamber of the fluid conduit. The piston member may include at least one shear element, wherein the at least one shear element operatively secures the piston member within the fluid conduit and is configured to shear when the piston member is exposed to the pressure difference across the piston member that is equal to or greater than the piston release pressure to release the piston body. At least a second shear element may be provided in the piston member. The shear element(s) may be a shear screw. 
     In certain characterizations, the piston member is fully disposed within the fluid conduit. The piston body may be fabricated from a metallic material, such as aluminum. The piston body may be substantially cylindrical, such as to operatively fit within a cylindrical fluid conduit. The piston member may include a sealing component disposed around an outer circumference of the piston body, such as one or more elastomeric O-rings. 
     In another characterization, the wiping member includes at least a first wiper blade extending radially from the tubular member, and the at least the first wiper blade may be fabricated from a flexible elastomeric material. 
     In another characterization, wherein the tubular member is fabricated from a metal, such as from aluminum, e.g., an aluminum alloy. 
     In yet another characterization, the dissolvable fluid obstructing portion is disposed across the fluid conduit. In another characterization, the dissolvable fluid obstructing portion is fabricated from a material that is dissolvable in an aqueous medium, e.g., in an aqueous chloride solution or in fresh water. The dissolvable fluid obstructing portion is fabricated from a metallic material, such as from a magnesium alloy. In another characterization, the dissolvable fluid obstructing portion is fabricated from a polymeric material. The dissolvable fluid obstructing portion may have a thickness measured along a central longitudinal axis of the fluid conduit that is at least about 0.5 mm, and that is not greater than about 300 mm, such as not greater than about 100 mm. 
     In another characterization, the dissolvable fluid obstructing portion abuts a distal material chamber on a side opposite the interior chamber. A cap member may abut the distal material chamber on a side opposite the dissolvable fluid obstructing portion, and the distal material chamber may be substantially filled with a hydrophobic material, such as with grease. 
     The wellbore plug structure may further include a landing arrangement disposed at a distal end of the wellbore plug structure that is configured to operatively engage a latch collar that is disposed in a wellbore, i.e., is disposed in the casing. 
     In one characterization of the temporary fluid stopper, the stopper includes a cup-like temporary fluid stopper body having a dissolvable fluid obstructing portion and a wall portion extending from a circumferential edge of the dissolvable fluid obstructing portion along an interior wall of the fluid conduit. A sealing component, such as an elastomeric O-ring, may be disposed around the temporary fluid stopper body, e.g., where the sealing component abuts against an interior wall of the fluid conduit. In this characterization, the temporary fluid stopper body and the dissolvable fluid obstructing portion comprise the dissolvable material, e.g., are fabricated from the dissolvable material. 
     In another characterization, the dissolvable fluid obstructing portion comprises a dissolvable disk body, e.g., a dissolvable disk body that is disposed across the longitudinally-extending fluid conduit. The dissolvable disk body may also be disposed across the distal fluid outlet, such as where the dissolvable disk body is secured to a landing arrangement disposed at a distal end of the wellbore plug structure, the landing arrangement being configured to operatively engage a latch collar disposed in a wellbore. The temporary fluid stopper may include a sealing component disposed around an outer edge of the dissolvable disk body, such as an elastomeric O-ring. 
     In one characterization of the fluid conduit, the conduit includes at least a first conduit portion and a second conduit portion, the second conduit portion having a larger diameter than the first conduit portion, wherein the first conduit portion is disposed near the proximal end of the fluid conduit and wherein the piston member is at least partially disposed within the first conduit portion. 
     The foregoing embodiments relate to a structure wherein the dissolvable material serves as the physical barrier to fluid flow through the wellbore plug structure. In another embodiment, a wellbore plug structure is disclosed that includes a tubular member defining a longitudinally-extending fluid conduit having a proximal fluid inlet and a distal fluid outlet, a wiping member disposed around the tubular member, a piston member comprising a piston body that is at least partially disposed in the fluid conduit, and a collet member at least partially disposed within the fluid conduit, the collet member comprising a collet member body, the collet member body comprising a plurality of collet fingers, and a dissolvable ring component disposed within the collet body to resist inward collapse of the collet fingers. The collet member and the piston member define an interior chamber within the fluid conduit, the interior chamber being fluidly sealed from the proximal fluid inlet by the piston member and being fluidly sealed from the distal fluid outlet by the collet member. 
     In one characterization of the piston member, the piston member is configured to release and move toward the distal fluid outlet when the piston member is exposed to a pressure difference across the piston member that is equal to or greater than a piston release pressure. The release and movement of the piston member may create a fluid pathway from the proximal fluid inlet to the interior chamber of the fluid conduit. In certain characterizations, the piston member includes at least one shear element (e.g., two or more shear elements), where the shear element(s) operatively secure the piston member within the fluid conduit and are configured to shear when the piston member is exposed to the pressure difference across the piston member that is equal to or greater than the piston release pressure to release the piston body. The shear elements may include, e.g., a shear screw. The piston member may be fully disposed within the fluid conduit, and the piston body may be fabricated from a metallic material such as aluminum. The piston body may be substantially cylindrical, and the piston member may include a sealing component disposed around an outer circumference of the piston body, such as an elastomeric O-ring. 
     In one characterization of the wiping member, the wiping member includes at least a first wiper blade extending radially from the tubular member. The first wiper blade may be fabricated from a flexible elastomeric material. 
     In one characterization of the tubular member, the tubular member is fabricated from a metallic material, such as from aluminum. 
     In one characterization of the collet member body, the collet member body is fabricated from a metallic material, such as from aluminum. The collet member may include a sealing component disposed around an outer circumference of the collet member body, such as one or more elastomeric O-rings. 
     In another embodiment of the present disclosure, a method of pressure testing a wellbore is disclosed. The method may include the use of any of the wellbore plug structures described herein. In one characterization, the method includes the steps of inserting a wellbore plug structure into a wellbore casing, the wellbore plug structure comprising a tubular member defining a longitudinally-extending fluid conduit having a proximal fluid inlet and a distal fluid outlet. The wellbore plug structure also includes a wiping member disposed around the tubular member, such as for wiping the casing during insertion of the wellbore plug structure into the wellbore. A piston member comprising a piston body is at least partially disposed in the fluid conduit, and a temporary fluid stopper comprising a dissolvable element is disposed in the fluid conduit, where the dissolvable element is configured to directly or indirectly form a seal with the fluid conduit. The wellbore casing is pressurized to a first wellbore pressure to advance the wellbore plug structure down the well bore, and is pressurized to a second wellbore pressure that is greater than the first wellbore pressure, such as by using a pressurization fluid, wherein the second wellbore pressure displaces the piston member into the fluid conduit and exposes the dissolvable element to the pressurizing fluid. A pressurization test (e.g., a casing integrity test) may then be performed at a third wellbore pressure before the pressurizing fluid completely dissolves the dissolvable element, thereby creating a fluid pathway through the wellbore pressure. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a cross-sectional view of a wellbore plug structure according to an embodiment of this disclosure. 
         FIG. 2  illustrates a cross-sectional view of a wellbore plug structure according to another embodiment of this disclosure. 
         FIG. 3  illustrates a cross-sectional view of a wellbore plug structure according to another embodiment of this disclosure. 
         FIG. 4  illustrates a cross-sectional view of a wellbore plug structure according to another embodiment of this disclosure. 
         FIG. 5  illustrates a cross-sectional view of a wellbore plug structure according to another embodiment of this disclosure. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     The present disclosure is directed to wellbore plug structures and methods of pressure testing a wellbore, e.g., methods utilizing wellbore plug structures having the relevant operational characteristics of the wellbore plug structures disclosed herein. 
     Generally, a wellbore plug structure according to certain embodiments of the present disclosure includes a tubular member, a wiping member disposed around the tubular member, and a piston member that is at least partially disposed in the tubular member. The piston member is configured to prevent a fluid from contacting a dissolvable component of the wellbore plug structure until the piston member is moved under the force of pressure to create a fluid pathway to the dissolvable component. In one embodiment, a temporary fluid stopper includes a dissolvable fluid obstructing portion, and the piston member releases (e.g., moves downward) to expose the dissolvable fluid obstructing portion to a fluid. In this regard, the tubular member defines a longitudinally-extending fluid conduit having a proximal fluid inlet and a distal fluid outlet. The piston member includes a piston body that is at least partially disposed in the fluid conduit. The dissolvable fluid obstructing portion is disposed in spaced-apart relation from the piston member and toward the distal fluid outlet. The piston member and the dissolvable fluid obstructing portion define an interior chamber within the fluid conduit, the interior chamber being fluidly sealed from the proximal fluid inlet by the piston member and being fluidly sealed from the distal fluid outlet by the dissolvable fluid obstructing portion. 
     In other embodiments, the wellbore plug structure includes a tubular member defining a longitudinally-extending fluid conduit having a proximal fluid inlet and a distal fluid outlet. The piston member including a piston body is at least partially disposed in the fluid conduit, and a collet member is at least partially disposed within the fluid conduit and includes a collet member body and a dissolvable ring component disposed around an interior surface of the collet body. When the piston moves it exposes the dissolvable ring component to the fluid, thereby dissolving the ring and enabling collet member fingers to move inwardly such that the collet body can be moved down the fluid conduit to create a fluid pathway through the wellbore plug structure. 
     As is noted above, one of the aspects of the wellbore plug structure is the presence of one or more components (e.g., the dissolvable fluid obstructing portion) that is fabricated from a dissolvable material. As used herein, the term dissolvable and its conjugates (e.g., dissolve, dissolved, etc.) refer broadly to any mechanism by which a unitary (e.g., intact) body or component will break down, rapidly corrode, disintegrate, etc., irrespective of the actual mechanism, e.g., irrespective of whether or not the material wholly or partially solubilizes in the fluid. Examples of dissolvable materials include, but are not limited to, magnesium alloys such as those that are sold under the tradename TervAlloy (Terves, Inc., Euclid, Ohio), and magnesium alloys sold under the tradename SoluMag (Magnesium Elektron North America, Madison, Ill.). Other useful dissolvable materials, including aluminum alloys, are disclosed in U.S. Pat. No. 8,770,261 by Marya and U.S. Pat. No. 8,211,247 by Marya et al., each of which is incorporated herein by reference in its entirety. Other useful dissolvable materials may include certain polymers, such as biodegradable thermoplastics, an example of which is polyglycolic acid (PGA). As is known to those skilled in the art, the dissolvable material may be selected for its rate of dissolution, e.g., a “slow” dissolution rate vs. a “fast” dissolution rate. The dissolvable material may comprise a composite of two or more materials, e.g., where one material phase is dispersed throughout another material phase or where the material is in the form of a multi-layer structure of different materials. Without limiting the present disclosure, such dissolvable materials are dissolvable in an aqueous medium, such as in freshwater and/or in a weak chloride solution (e.g., KCl, HCl, etc.). The dissolvable material may have a dissolution rate in the range of from about 30 mg/cm 2 ·hr to about 1000 mg/cm 2 ·hr in such aqueous mediums at about 200° F. 
     Referring now to  FIG. 1 , a wellbore plug structure  100  according to one embodiment of the present disclosure is illustrated in cross-section. The wellbore plug structure  100  is generally configured to be operatively disposed down a wellbore, e.g., during the formation of a bore in the earth&#39;s surface for the extraction of oil, natural gas, or other natural resources. In this regard, the wellbore plug structure  100  includes a tubular member  102 . The tubular member  102  comprises a substantially cylindrical shape and defines a longitudinally-extending fluid conduit  110  extending through a central portion of the tubular member  102 . The fluid conduit  110  includes a proximal fluid inlet  112  at a proximal end of the fluid conduit  110  and a distal fluid outlet  114  disposed at a distal end of the fluid conduit  110 . A wiping member  104  is disposed around the tubular member  102  and is configured to wipe the sidewall of the wellbore casing when the wellbore plug structure  100  is displaced (e.g., moved) down the wellbore casing. 
     A piston member  106  includes a piston body  116  that is disposed (e.g., at least partially disposed) in the longitudinally-extending fluid conduit  110 . Also disposed within the fluid conduit  110  is a temporary fluid stopper  108  that includes a temporary fluid stopper body  140  having a dissolvable fluid obstructing portion  118 , where the dissolvable fluid obstructing portion  118  is disposed in spaced-apart relation from the piston member  106  (e.g., in spaced-apart relation from the piston body  116 ) toward the distal fluid outlet  114 . The piston member  106  and the dissolvable fluid obstructing portion  118  define an interior chamber  120  within the fluid conduit  110 , e.g., that is bounded by the dissolvable fluid obstructing portion  118 , a wall portion  142  of the temporary fluid stopper  108 , and the piston member  106 . Thus, the interior chamber  120  is fluidly sealed from the proximal fluid inlet  112  by the piston member  106  and is fluidly sealed from the distal fluid outlet  114  by the dissolvable fluid obstructing portion  118 . 
     In this embodiment, the piston body  116  is configured to release and move toward the distal fluid outlet  114  when the piston member  106  is exposed to a pressure difference across the piston member  106  that is equal to or greater than a piston body release pressure, e.g., a predetermined piston body release pressure. The release and movement of the piston body  116  under this condition creates a fluid pathway from the proximal fluid inlet  112  to the interior chamber  120 . Typically, the pressure difference across the piston member  106  is created by a pressurized fluid (e.g., a pressurized liquid) that is applied to the proximal end of the wellbore plug structure  100  when the wellbore plug structure  100  is disposed in a wellbore casing and is sealed against the sidewall of the wellbore casing by the wiping member  104 . Thus, when the piston body  116  releases, the pressurized fluid will enter the longitudinally-extending fluid conduit  110  through the proximal fluid inlet  112  and come into contact with the temporary fluid stopper  108 . Typically, the interior chamber  120  will be at or very near ambient pressure, although the interior chamber  120  could be at a pressure less than ambient or more than ambient as may be desired, e.g., to facilitate movement of the piston member by the pressurized fluid. 
     In this regard, the piston body release pressure will typically be greater than ambient pressure. It will be appreciated that the wellbore plug structure, particularly the structure of the piston member, may be configured such that the piston body release pressure is well-controlled and may vary over a wide range of pressures. In certain characterizations, the piston body release pressure will be at least about 250 psi (pounds per square inch), such as at least about 4000 psi. 
     Thus, the piston member  106  is configured and placed relative to the longitudinally-extending fluid conduit  110  such that the piston member  106  maintains a fluid seal until such time as a pressure, greater than or equal to the piston body release pressure, is applied to the piston member  106 . In this regard, the piston body  116  may be precisely sized such that the outer circumference of the piston body  116  forms a fluid tight seal against the interior wall  126  of the fluid conduit  110 , or against a sleeve member  162  that is disposed between the piston body  116  and the interior wall  126  of the fluid conduit  110 , such that the piston body  116  frictionally resists movement until the piston body release pressure is reached. A portion of the piston body may also extend upwardly and over the upper circumference of the sleeve member (e.g., as a flange) to resist pressure until the flange collapses. In another characterization, and as is illustrated in  FIG. 1 , the piston member  106  may include at least one shear element  124   a , wherein the at least one shear element  124   a  operatively secures the piston body  116  within the fluid conduit  110 . The shear element  124   a  may comprise a shear pin, shear ring, or shear wire. In one particular characterization, the shear element comprises a shear screw. Further, the piston member  106  may include more than one shear element, such as shear elements  124   a  and  124   b . The shear element(s)  124   a / 124   b  are configured to shear when the piston member  106  is exposed to the pressure difference across the piston member  106  that is equal to or greater than the piston body release pressure, thereby releasing the piston body  116 . 
     As is illustrated in the embodiment of  FIG. 1 , the piston member  106  is fully disposed within the fluid conduit  110 . However, it is contemplated that the wellbore plug structure  100  may be configured such that the piston member  106  is only partially disposed within the fluid conduit  110 , e.g., where a proximal portion of the piston member  106  extends upwardly beyond the proximal fluid inlet  112 . 
     The piston body  116  may be fabricated from virtually any material. For example, it is contemplated that the entire piston body  116 , or at least a portion of the piston body  116 , may be fabricated from a dissolvable material, e.g., a material that is capable of dissolution in an aqueous and/or a saline aqueous solution as is discussed above. In certain characterizations, the piston body  116  is at least partially fabricated from a metallic material, particularly a millable metallic material such as aluminum or cast-iron. In this regard, for certain applications, it may be preferable to utilize a piston body that is at least partially fabricated from aluminum. As used herein, the term aluminum encompasses both pure aluminum and aluminum alloys, e.g., alloys that comprise at least about 50% aluminum. The piston body  116  may be substantially cylindrical, such as to operatively fit within a substantially cylindrical longitudinally-extending fluid conduit  110 , or, as illustrated in  FIG. 1 , within a sleeve member  162  that itself is disposed within the fluid conduit  110 . To ensure a substantially fluid tight seal between the piston body  116  the interior side wall  126  of the longitudinally-extending fluid conduit  110 , the piston body  116  may include a sealing component  130 , e.g., a sealing component that is disposed around an outer circumference of the piston body  116 . The sealing component  130  may comprise configurations such as a Chevron-type seal. In certain characterizations, the sealing component  130  comprises an elastomeric O-ring. Further, the piston member  106  may include more than one sealing component  130 , such as a plurality of elastomeric O-rings that are disposed around the piston body  116 . Although the embodiment illustrated in  FIG. 1  illustrates the use of an elastomeric O-ring as the sealing component  130 , the piston body  116  may be configured, e.g., precision machined, to a tight tolerance such that a metal-to-metal seal may be formed between the piston body  116  and the interior surface of the longitudinally-extending fluid conduit  110 . Further, although illustrated as comprising a sealing component that is operatively affixed to the piston body  116 , a sealing component may also be fixed to an interior of the longitudinally-extending fluid conduit  110  to form a seal between the piston body  116  and the interior wall  126  of the fluid conduit, or to the sleeve member  162 . 
     As can be seen from the wellbore plug structure  100  illustrated in  FIG. 1 , the piston member  106  fluidly seals the interior chamber  120  of the longitudinally-extending fluid conduit  110  from the proximal fluid inlet  112 . Thus, when a fluid (e.g., a liquid or slurry) is pressurized against the piston member  106  at a pressure that is less than the piston body release pressure, the piston member  106  will prevent fluid from entering the interior chamber  120  and from contacting the temporary fluid stopper  108 , particularly from contacting the dissolvable fluid obstructing portion  118  of the temporary fluid stopper  108 . 
     To ensure that the fluid pressure applied above the wellbore plug structure  100  is applied upon the piston member  106 , the wiping member  104  may include at least a first wiper blade  134   a  that extends radially from the tubular member  102  to form a tight seal against the casing when the wellbore plug structure is placed down the casing. The wiping member  104  may include additional wiper blades, such as wiper blade  134   b . As is illustrated in  FIG. 1 , the wiping member  104  comprises four wiper blades that extend radially from the tubular member  102 . The wiping member  104  may be integrally formed with the tubular member  102 , or may be a separate component that is attached to the tubular member  102 . The size of the wiper blades (e.g., outer diameter of the wiper blades) is configured to form a tight fluid seal when the wellbore plug structure  100  is placed down a wellbore casing. In this regard, the wiper blades may be fabricated from a flexible elastomeric material in order to form such a fluid-tight seal. During certain operations, the wiper blades may also force material that is loosely adhered to the interior wall of the wellbore casing down the wellbore casing, e.g., may force wet, flowable cement down the casing during a wellbore cementing operation. 
     The tubular member  120  may be fabricated from a variety of materials and in certain characterizations the tubular member is fabricated (e.g., machined) from a metallic material, such as aluminum. It will be appreciated that the fluid conduit  110  will have a diameter that is sufficiently large to accommodate tooling to be placed through the fluid conduit after removal of the piston member  108  and the fluid obstructing portion  118 . The wellbore plug structure  100  may also include a landing arrangement  136  disposed at a distal end of the wellbore plug structure  100 , e.g., at a distal end of the tubular member  102 . The landing arrangement  136  is configured to operatively engage with a latch collar (e.g., a sealing latch collar) or similar structure that is disposed near the bottom of the wellbore. Such landing arrangements are known to those of ordinary skill in the art. 
     The temporary fluid stopper  108  includes a dissolvable fluid obstructing portion  118 . The dissolvable fluid obstructing portion  118  is disposed in spaced-apart relation from the piston member  106  (e.g., from the piston body  116 ) toward the distal fluid outlet  114 . As illustrated in  FIG. 1 , the dissolvable fluid obstructing portion  118  is disposed across the fluid conduit  110 . The dissolvable fluid obstructing portion  118  is fabricated from a dissolvable material, e.g., that is dissolvable in the fluid that comes into contact with the dissolvable fluid obstructing portion  118  when the piston body  116  releases and creates a fluid pathway from the proximal fluid inlet  112  to the fluid obstructing portion  118 . 
     As illustrated in  FIG. 1 , the temporary fluid stopper  108  is cup-like (e.g., cup-shaped) and includes a dissolvable fluid obstructing portion  118  and a wall portion  142  extending from a circumferential edge of the dissolvable fluid obstructing portion  118  toward the proximal inlet  112 . Although illustrated in  FIG. 1  as having a cup-like configuration, it will be appreciated that the temporary fluid stopper may have any configuration (e.g., shape) to fit within the fluid conduit  110  and to temporarily obstruct fluid flow upon release of the piston body  116 . 
     In the embodiment of  FIG. 1 , substantially the entire temporary fluid stopper  108  (e.g., the wall portion  142  and the fluid obstructing portion  118 ) is fabricated from a dissolvable material. Thus, in this embodiment, when the piston body  116  is displaced toward the distal end of the fluid conduit  110 , the wall portion  142  is exposed to the fluid and begins to dissolve. In addition, because the outer circumference of the piston body  116  is less than an inner diameter of the wall portion  142 , a fluid pathway will also form around the piston body  116  such that the fluid obstructing portion  118  will also begin to dissolve. Over a period of time, the fluid will dissolve and “eat through” the wall portion  142  and the fluid obstructing portion  118 . The thickness of the wall portion  142  and/or of the fluid obstructing portion  118  may be selected to maintain a degree of control over the time that is needed to dissolve through the fluid stopper body  140 . 
     As is illustrated in the embodiment of  FIG. 1 , the temporary fluid stopper  108  also includes a sealing component  150  that is disposed around the wall portion  142  of the temporary fluid stopper body  140 . In this regard, the sealing component  150  abuts and seals against an inner surface of the tubular member  102 , i.e., against an inner surface of the longitudinally-extending fluid conduit  110 . As with the sealing component  130 , the sealing component  150  may be of any configuration, and in one characterization is an elastomeric O-ring. Further, a distal material chamber  154  is disposed between the fluid obstructing portion  118  and a distal fluid outlet  114 , and includes a hydrophobic material  158  (e.g., grease) to prevent liquids from prematurely contacting the dissolvable material that constitutes the fluid obstructing portion  118 . 
     Thus, when the piston body  116  releases it drops into the interior chamber  120  and exposes the dissolvable material (e.g., the temporary fluid stopper  108 ) to the fluid. 
       FIG. 2  illustrates a cross-sectional view of a further embodiment of a wellbore plug structure  200  according to the present disclosure. Those components of the wellbore plug structure  200  that are illustrated but not described in detail are similar to the components of the wellbore plug structure  100  described above with respect to  FIG. 1 , and the components may be constructed from similar materials and in similar fashion as those components described with respect to  FIG. 1 . 
     The wellbore plug structure  200  is also configured to be operatively disposed down a wellbore. The wellbore plug structure  200  includes a tubular member  202  having a substantially cylindrical shape and defining a longitudinally-extending fluid conduit  210  extending through a central portion of the tubular member  202 . The fluid conduit  210  includes a proximal fluid inlet  212  at a proximal end of the fluid conduit  210  and a distal fluid outlet  214  disposed at a distal end of the fluid conduit  210 . A wiping member  204  is disposed around the tubular member  202  and is configured to wipe the sidewall of the wellbore casing when the wellbore plug structure  200  is displaced down the wellbore casing. 
     A piston member  206  includes a piston body  216  that is disposed (e.g., at least partially disposed) in the longitudinally-extending fluid conduit  210 . Also disposed within the fluid conduit  210  is a temporary fluid stopper  208  that comprises a dissolvable disk body  240 , i.e., that is fabricated from a dissolvable material. The dissolvable disk body  240  is disposed in spaced-apart relation from the piston member  206  (e.g., in spaced-apart relation from the piston body  216 ) toward the distal fluid outlet  214 . The piston member  206  and the dissolvable disk body  240  define an interior chamber  220  within the fluid conduit  210 , e.g., that is bounded by the interior wall  226  of the fluid conduit  210 , by the dissolvable disk body  240 , and by the piston member  206 . As a result, the interior chamber  220  is fluidly sealed from the proximal fluid inlet  212  by the piston member  206  and is fluidly sealed from the distal fluid outlet  214  by the dissolvable disk body  240 . The dissolvable disk body  240  is protected from moisture by a hydrophobic material  258  that is retained by a disk  260  (e.g., a plastic disk) at a distal end of the landing arrangement  236 . 
     As with the embodiment described above with respect to  FIG. 1 , the piston body  216  is configured to release and move toward the distal fluid outlet  214  when the piston member  206  is exposed to a pressure difference across the piston member  206  that is equal to or greater than the piston body release pressure. The release and movement of the piston body  216  creates a fluid pathway from the proximal fluid inlet  212  to the interior chamber  220 . In this regard, the longitudinally-extending fluid conduit  210  includes a first conduit portion  210   a  and a second conduit portion  210   b  where the second portion  210   b  has a diameter that is greater than the diameter of the piston body  216  and is greater than the diameter of the first portion  210   a . Thus, when the piston body  216  is released and drops into the second portion  210   b  of the fluid conduit  210 , a fluid pathway is created, e.g., around the piston body  216 . 
     As illustrated in  FIG. 2 , the temporary fluid stopper  208  includes a dissolvable disk body  240  that extends across the fluid conduit  210 . A sealing component  250  (e.g., an elastomeric O-ring) is provided to form a tight seal against an interior surface of the landing arrangement  236 . The thickness of the dissolvable disk body  240  (e.g., along a longitudinal axis of the fluid conduit  210 ) may be selected to achieve a desired dissolution time. As illustrated in  FIG. 2 , an outer periphery of the disk body  240  has a greater thickness to reduce the possibility of the disk body  240  failing (e.g., fracturing) prematurely such as due to the impact of the piston body  216 . 
     Compared to the embodiment illustrated in  FIG. 1 , the temporary fluid stopper  208  utilizes less dissolvable material than the temporary fluid stopper  108 . Further, the inner diameter of the fluid conduit  210  may be larger than the fluid conduit  110 , enabling better fluid flow. 
       FIG. 3  illustrates another embodiment of a wellbore plug structure according to the present disclosure. Those components of the wellbore plug structure  300  that are illustrated but not described in detail are similar to the components of the wellbore plug structures  100  and  200  described above with respect to  FIG. 1  and  FIG. 2 , and the components may be constructed from similar materials and in similar fashion as those components described with respect to  FIG. 1  and  FIG. 2 . 
     Broadly characterized, the wellbore plug structure  300  includes a tubular member  302  defining a longitudinally-extending fluid conduit  310  having a proximal fluid inlet  312  and a distal fluid outlet  314 . A wiping member  304  is disposed around the tubular member  302 . A piston member  306  including a piston body  316  is disposed in the fluid conduit  310 . A temporary fluid stopper  308  includes a dissolvable fluid obstructing portion. A piston member  306  is disposed within a fluid conduit  310  having a proximal fluid inlet  312  and a distal fluid outlet  314 . 
     As compared to the embodiment illustrated in  FIG. 2 , the temporary fluid stopper  308  includes a dissolvable body  340  that extends across the fluid conduit  310 , where the dissolvable body  340  has a well  344  formed through a central portion of the body  340 , e.g., along a longitudinal axis of the fluid conduit  310 . In this manner, the dissolution time can be controlled by adjusting the depth of the well  344 , i.e., by adjusting the thickness of the dissolvable material below the well  344 . 
     Another feature illustrated by the wellbore plug structure  300  of  FIG. 3  is that the interior wall  326  of the fluid conduit  310  includes an inward flange  328  that is disposed below the piston body  316  and above the temporary fluid stopper  308 . The purpose of the inward flange  328  is to reduce the velocity of the piston body  316  when the piston release pressure forces the piston body  316  downwardly toward the temporary fluid stopper  308 . Reducing the velocity of the piston body  316  will reduce the likelihood that the temporary fluid stopper  308  will become fractured by the piston body  316 . 
     In any of the foregoing embodiments, the thickness of the dissolvable fluid obstructing portion measured along a central longitudinal axis of the fluid conduit (e.g., through the center of the obstructing portion) may be selected to control the dissolution time needed to dissolve through the dissolvable material. While not limited to any particular thickness, in certain characterizations the thickness of the dissolvable fluid obstructing portion measured along the longitudinal axis of the fluid conduit will typically be at least about 0.5 mm, such as at least about 1 mm or even at least about 5 mm. In another characterization, this thickness will typically be not greater than about 300 mm, such as not greater than about 200 mm, or not greater than about 100 mm. 
       FIG. 4  illustrates another embodiment of a wellbore plug structure according to the present disclosure. In the embodiments illustrated in  FIGS. 1 to 3 , a dissolvable material is utilized to directly block the fluid flow through the longitudinally-extending fluid conduit, e.g., by being placed directly across the fluid conduit. In the embodiment illustrated in  FIG. 4 , the dissolvable material is utilized in combination with a non-dissolvable component, such as a collet member, such that once the material dissolves, the collet member collapses (e.g., the collet fingers collapse inwardly), under the pressure of the fluid thereby opening up a fluid pathway through the fluid conduit from the proximal end to the distal end. 
     Referring to  FIG. 4 , the wellbore plug structure  400  includes a tubular member  402  defining a longitudinally-extending fluid conduit  410  having a proximal fluid inlet  412  and a distal fluid outlet  414 . Disposed near the proximal end of the tubular member  402  is a piston member  406  that includes a piston body  416  that is at least partially disposed in the fluid conduit  410 . 
     A collet member  470  is disposed within the fluid conduit  410 . The collet member  470  includes a collet member body  472  having a distal fluid obstructing portion  480  and a plurality of fingers  478  extending therefrom, i.e., extending toward the proximal end. The proximal end of the collet member body  472  includes an internally notched portion  482  having a larger internal diameter than the portion disposed below the notched portion  482 . A dissolvable ring  474  is sized and configured to be placed within the notched portion  482  and includes an aperture  484  that is sized to permit the piston body  416  to pass through the aperture  484  when subjected to a sufficiently high pressure. The dissolvable ring  474 , when placed within the notched portion  482  of the collet body  472 , restricts inward movement of the fingers  478  and thereby inhibits movement of the collet body in a downward direction, i.e., toward a distal end of the tubular member  402 . 
     The collet member  470  and the piston member  406  define an interior chamber  420  within the fluid conduit  410 , the interior chamber  420  being fluidly sealed from the proximal fluid inlet  412  by the piston member  406  and being fluidly sealed from the distal fluid outlet  414  by the collet member  470 , i.e., by the fluid obstructing portion  480  of the collet member body  472 . 
     As with the embodiments illustrated above in  FIGS. 1 and 2 , the piston member  406  is configured to release and move toward the distal fluid outlet  414  when the piston member  406  is exposed to a pressure difference across the piston member  406  that is equal to or greater than a piston release pressure. This release and movement of the piston member  406  creates a fluid pathway from the proximal fluid inlet  412  to the interior chamber  420  of the fluid conduit. 
     To facilitate the release and movement of the piston member  406  when exposed to the piston release pressure, the piston member  406  includes at least one shear element  424   a  that operatively secures the piston body  416  within the fluid conduit  410  and is configured to shear when the piston member  406  is exposed to the requisite pressure difference across the piston member  406 . It will be appreciated that the piston member  406  may include a plurality of shear elements, and as illustrated in  FIG. 4 , the piston member comprises at least a second shear element  424   b  that is also configured to shear when the piston member is exposed to the pressure difference across the piston member that is equal to or greater than the piston release pressure. The shear elements  424   a / 424   b  may comprise a shear pin, shear ring, or shear wire, and in certain characterizations the shear elements  424   a / 424   b  are shear screws. 
     As illustrated in  FIG. 4 , the piston member  406  is fully disposed within the fluid conduit  410 , although it is contemplated that the wellbore plug structure  400  could be configured such that the piston member  406  is partially disposed within the fluid conduit  410 , e.g., where the piston member  406  is partially disposed outside of the fluid conduit  410 . 
     The piston body  416  may be fabricated from virtually any material. For example, it is contemplated that the entire piston body  416 , or a portion of the piston body  416 , may be fabricated from a dissolvable material. In certain characterizations, the piston body  416  is at least partially fabricated from a metallic material, particularly a millable metallic material such as aluminum or cast-iron. In this regard, for certain applications, it may be preferable to utilize a piston body that is at least partially fabricated from aluminum. The piston body  416  may be substantially cylindrical, such as to operatively fit within a substantially cylindrical longitudinally-extending fluid conduit  410 , or, as illustrated in  FIG. 4 , within a sleeve member  462  that itself is disposed within the fluid conduit  410 . To ensure a substantially fluid tight seal between the piston body  416  the interior side wall of the longitudinally-extending fluid conduit  410 , the piston member  406  may include a sealing component  430 , e.g., a sealing component that is disposed around an outer circumference of the piston body  416 . The sealing component  430  may comprise configurations such as a Chevron-type seal. In certain characterizations, the sealing component  430  includes an elastomeric O-ring. Further, the piston member  406  may include more than one sealing component  430 , such as a plurality of elastomeric O-rings that are disposed around the piston body  416 . Although the embodiment illustrated in  FIG. 4  illustrates the use of an elastomeric O-ring as the sealing element, the piston body  416  may be configured, e.g., precision machined, to a tight tolerance such that a metal-to-metal seal may be formed between the piston body  416  and the interior surface of the longitudinally-extending fluid conduit  410  or the sleeve  462 . Further, although illustrated as comprising a sealing component that is operatively affixed to the piston body  416 , a sealing component may also be fixed to an interior of the longitudinally-extending fluid conduit  410  or to form a seal between the piston body  416  and the interior wall of the fluid conduit, or the sleeve member  462 . 
     As with the embodiments illustrated in  FIGS. 1 and 2 , a wiping member  404  is disposed around the tubular member  402 . The wiping member  404  includes at least a first wiper blade  434   a  extending radially from the tubular member  402 . The wiping member  404  of  FIG. 4  comprises a second wiper blade  434   b  and includes five wiper blades in total that extend radially from the tubular member  402 . The wiping member  404  may be integrally formed with the tubular member  402 , or may be a separate component that is attached to the tubular member  402 . The size of the wiper blades (e.g., outer diameter of the wiper blades) is configured to form a tight fluid seal when the wellbore plug structure  400  is placed down a wellbore casing. The wiper blades may be fabricated from a flexible elastomeric material in order to form such a fluid-tight seal. During certain operations, the wiper blades may also force material that is loosely adhered to the interior wall of the wellbore casing down the wellbore casing, e.g., cement during a wellbore cementing operation. 
     The tubular member  420  may be fabricated from a variety of materials and in certain characterizations the tubular member is fabricated (e.g., machined) from a metallic material, such as aluminum. The wellbore plug structure  400  may also include a landing arrangement  436  disposed at a distal end of the wellbore plug structure  400 , e.g., at a distal end of the tubular member  402 . The landing arrangement  436  is configured to operatively engage with a latch collar (e.g., a sealing latch collar) or similar structure that is disposed near the bottom of the wellbore. 
     In the embodiment illustrated in  FIG. 4 , the collet member  470  comprises a sealing component  476  disposed around an outer circumference of the collet member body  472 . The sealing component  476  may comprise, for example, an elastomeric O-ring. As illustrated in  FIG. 4 , the collet member  470  includes two elastomeric O-rings. 
     In the embodiment illustrated in  FIG. 4 , when the piston body  416  releases, it moves through the aperture  484  and into the collet member body  472 . The movement of the piston exposes the dissolvable ring  474  to the fluid. Upon dissolution of the dissolvable ring  474 , the fingers  478  are no longer restricted from moving inwardly. Therefore, when pressure is applied to the collet member body  472 , the fingers  478  are forced inwardly and the entire collet member will be displaced downwardly and remove through the fluid conduit  410  and out of the distal fluid outlet  414 . As a result, a fluid pathway through the entire fluid conduit  410  will be formed. 
       FIG. 5  illustrates yet another embodiment of a wellbore plug structure according to the present disclosure. The embodiment illustrated in  FIG. 5  is similar to the embodiment illustrated in  FIG. 4  in that the dissolvable material is utilized in combination with a collet member such that once the material dissolves, the collet member is exposed to the fluid pressure and collapses (e.g., the collet fingers collapse inwardly), thereby opening up a fluid pathway through the fluid conduit from the proximal end to the distal end. 
     Referring to  FIG. 5 , the wellbore plug structure  500  includes a tubular member  502  defining a longitudinally-extending fluid conduit  510  having a proximal fluid inlet  512  and a distal fluid outlet  514 . Disposed within the tubular member  502  is a piston member  506  that includes a piston body  516  that is at least partially disposed in the fluid conduit  510 . 
     A collet member  570  is disposed within the fluid conduit  510 , between the piston body  516  and the distal fluid outlet  514 . The collet member  570  includes a collet member body  572  having a distal fluid obstructing portion  580  and a plurality of fingers  578  extending upwardly therefrom, i.e., extending toward the proximal end. The proximal end of the collet member body  572  includes an internally notched portion  382  having a larger internal diameter than the portion disposed below the notched portion  582 . A dissolvable ring  574  is sized and configured to be placed within the notched portion  582  and includes an aperture  584  that is sized to permit the piston body  516  to pass through the aperture  584  when subjected to a sufficiently high pressure. The dissolvable ring  574 , when placed within the notched portion  582  of the collet body  572 , restricts inward movement of the fingers  578  and thereby inhibits movement of the collet body in a downward direction, i.e., toward a distal end of the tubular member  502 . 
     The collet member  570  and the piston member  506  define an interior chamber  520  within the fluid conduit  510 , the interior chamber  520  being fluidly sealed from the proximal fluid inlet  512  by the piston member  506  and being fluidly sealed from the distal fluid outlet  514  by the collet member  570 , i.e., by the fluid obstructing portion  580  of the collet member body  572 . 
     As with the embodiment illustrated above in  FIG. 4 , the piston member  506  is configured to release and move toward the distal fluid outlet  514  when the piston member  506  is exposed to a pressure difference across the piston member  506  that is equal to or greater than a piston release pressure. This release and movement of the piston member  506  creates a fluid pathway from the proximal fluid inlet  512  to the interior chamber  520  of the fluid conduit. 
     To facilitate the release and movement of the piston member  506  when exposed to the piston release pressure, the piston member  506  includes at least one shear element, e.g., shear elements  524   a  and  524   b  that operatively secure the piston body  516  within the fluid conduit  510  and is configured to shear when the piston member  506  is exposed to the requisite pressure difference across the piston member  506 . The shear elements  524   a / 524   b  may comprise a shear pin, shear ring, or shear wire, and in certain characterizations the shear elements  524   a / 524   b  are shear screws. 
     The piston body  516  may be fabricated from virtually any material as is discussed above, e.g., with respect to  FIG. 4 . The piston body  516  may be substantially cylindrical, such as to operatively fit within a substantially cylindrical longitudinally-extending fluid conduit  510 . To ensure a substantially fluid tight seal between the piston body  516  the interior side wall of the longitudinally-extending fluid conduit  510 , the piston member  506  may include a sealing component  530 , e.g., a sealing component that is disposed around an outer circumference of the piston body  516 . The sealing component  530  may comprise configurations such as a Chevron-type seal. In certain characterizations, the sealing component  530  includes an elastomeric O-ring, and the sealing component may include more than one element, e.g., more than one elastomeric O-ring. Although the embodiment illustrated in  FIG. 5  illustrates the use of an elastomeric O-ring as the sealing element, the piston body  516  may be configured, e.g., precision machined, to a tight tolerance such that a metal-to-metal seal may be formed between the piston body  516  and the interior surface of the longitudinally-extending fluid conduit  510 . The sealing component may also be fixed to an interior of the longitudinally-extending fluid conduit  510  such as to form a seal between the piston body  316  and the interior wall of the fluid conduit. 
     The tubular member  520  may be fabricated from a variety of materials as is discussed above with respect to  FIG. 3 . The wellbore plug structure  500  may also include a landing arrangement  536  disposed at a distal end of the wellbore plug structure  500 , e.g., at a distal end of the tubular member  502 . The landing arrangement  536  is configured to operatively engage with a latch collar (e.g., a sealing latch collar) or similar structure that is disposed near the bottom of the wellbore. 
     In the embodiment illustrated in  FIG. 5 , when the piston body  516  releases, it moves through the aperture  584  and into the collet member body  572 , e.g., into the interior chamber  520 . The movement of the piston body  516  exposes the dissolvable ring  574  to the fluid. Upon dissolution of the dissolvable ring  574  by the fluid, the fingers  578  are no longer restricted from moving inwardly. Therefore, when pressure is applied to the collet member body  572  (e.g., directly against the fluid obstructing portion  580 ), the fingers  578  are forced inwardly and the entire collet member will be displaced downwardly, moving through the fluid conduit  510  and out of the distal fluid outlet  514 . As a result, a fluid pathway through the entire fluid conduit  510  will advantageously be formed. 
     The wellbore plug structures illustrated in  FIGS. 1 to 5  are particularly useful for the pressure testing of a completed wellbore, e.g., integrity testing of the casing. In one embodiment, a method of pressure testing a wellbore is provided, where the method includes inserting the wellbore plug structure into a wellbore casing where the wellbore plug structure includes a tubular member defining a longitudinally-extending fluid conduit having a proximal fluid inlet and a distal fluid outlet, a wiping member disposed around the tubular member, a piston member comprising a piston body that is at least partially disposed in the fluid conduit, and a dissolvable element disposed in the fluid conduit. For example, the dissolvable element may include a portion that is disposed completely across the fluid conduit to block fluid flow ( FIGS. 1 to 3 ), or may be an element that holds another component in the fluid conduit (e.g.,  FIGS. 4 and 5 ) The wellbore is pressurized to a first wellbore pressure to advance the wellbore plug structure down the wellbore, i.e., down the casing. Thereafter, at any time after placement of the structure in the wellbore, the wellbore is pressurized with a pressurizing fluid to a second wellbore pressure that is greater than the first wellbore pressure. The second wellbore pressure (e.g., the piston release pressure) displaces the piston member downwardly into the fluid conduit and exposes the dissolvable element to the pressurizing fluid. Pressure testing of the wellbore (casing integrity test) may then be completed at a third wellbore pressure before the pressurizing fluid completely dissolves the dissolvable element. Once the fluid dissolves the dissolvable element, a fluid passageway is opened through the wellbore plug structure, and tooling may be passed through the wellbore plug structure. 
     In one characterization, the step of pressurizing the wellbore casing to a first wellbore pressure to advance the wellbore plug structure down the well bore includes landing a distal end of the wellbore plug structure onto a stop collar that disposed in the wellbore, e.g., is disposed around a circumference of the casing. In another characterization, the wellbore plug structure is utilized during a cementing operation. Thus, during the step of pressurizing the wellbore casing to a first wellbore pressure to advance the wellbore plug structure down the wellbore, the wellbore plug structure forces a cementing composition down the wellbore. 
     While various embodiments of a wellbore plug structure and a method for pressure testing a wellbore have been described in detail, it is apparent that modifications and adaptations of those embodiments will occur to those skilled in the art. However, is to be expressly understood that such modifications and adaptations are within the spirit and scope of the present disclosure.