US20180171743A1

US20180171743A1 - Cathodically-protected plug assembly

Info

Publication number: US20180171743A1
Application number: US15/383,676
Authority: US
Inventors: Daniel C. Markel; Tauna Lea Leonardi
Original assignee: Schlumberger Technology Corp
Current assignee: Schlumberger Technology Corp
Priority date: 2016-12-19
Filing date: 2016-12-19
Publication date: 2018-06-21

Abstract

A technique includes deploying a plug assembly including a degradable material inside a tubing string of a well; engaging the plug assembly with the tubing string to anchor the plug assembly to the tubing string; and preventing a cathodic reaction from degrading the degradable material. Preventing the cathodic reaction includes electrically isolating the engaged plug assembly from the tubing string. During the prevention of the cathodic reaction, a fluid barrier is formed in the tubing string using the plug assembly.

Description

BACKGROUND

For purposes of preparing a well for the production of oil or gas, at least one perforating gun may be deployed into the well via a conveyance mechanism, such as a wireline, slickline or a coiled tubing string. The shaped charges of the perforating gun(s) are fired when the gun(s) are appropriately positioned to perforate a casing of the well and form perforating tunnels into the surrounding formation. Additional operations may be performed in the well to increase the well's permeability, such as well stimulation operations and operations that involve hydraulic fracturing. The above-described perforating and stimulation operations may be performed in multiple stages of the well.
The above-described operations may be performed by actuating one or more downhole tools (perforating guns, sleeve valves, and so forth) and by forming one or more fluid-diverting fluid barriers downhole in the well.

SUMMARY

The summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter.
In accordance with an example implementation, a technique includes deploying a plug assembly including a degradable material inside a tubing string of a well; engaging the plug assembly with the tubing string to anchor the plug assembly to the tubing string; and preventing a cathodic reaction from degrading the degradable material. Preventing the cathodic reaction includes electrically isolating the engaged plug assembly from the tubing string. During the prevention of the cathodic reaction, a fluid barrier is formed in the tubing string using the plug assembly.
In accordance with another example implementation, a technique includes running a plug assembly inside a tubing string of a well; and at a predetermined position in the tubing string, expanding a sealing element of the plug assembly and expanding an anchoring element of the plug assembly to cause the sealing element to engage a wall of the tubing string to form a fluid seal between a body of the plug assembly and the wall and to cause the anchoring element to engage the wall to secure the body to the wall. The technique includes using at least one material of the plug assembly to electrically isolate the plug assembly from the tubing string wall while the sealing element and the anchoring element are engaged with the tubing string wall to inhibit degradation of a degradable material of the plug assembly. The technique includes deploying an untethered object to land in the seat of the plug assembly to form a fluid barrier in the tubing string and performing a first downhole operation using the fluid barrier. The technique includes subsequently performing a second operation downhole that relies on removal of the plug assembly due to removal of the material(s) and a degradation of the material of the plug assembly.
In accordance with another example implementation, an apparatus includes a body, a sealing member and an anchoring member. The sealing member has a contracted state and an expanded state to form a fluid seal between the body and an outer tubing. The anchoring member has a contracted state and an expanded state in which the anchoring member secures the body to the outer tubing. The body has a through passageway and the sealing member and the anchoring member electrically isolate the body from the outer tubing.
In accordance with yet another example implementation, a system includes a tubing string and a plug assembly. The plug assembly includes a body, a sealing member and an anchoring member. The sealing member has a contracted state and an expanded state to form a fluid seal between the body and an outer tubing. The anchoring member has a contracted state and an expended state in which the anchoring member secures the body to the outer tubing. The body has a through passageway, and the sealing member and the anchoring member electrically isolate the body from the outer tubing.
Advantages and other features will become apparent from the following drawings, description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, 1C and 1D are schematic diagrams of a well illustrating the use of a plug assembly in connection with a well stimulation operation according to example implementations.

FIG. 2A is a cross-sectional view of a portion of the well illustrating a cathodically-protected plug assembly in an unset state and being mounted to a setting tool according to an example implementation.

FIG. 2B is a cross-sectional view of the portion of the well illustrating the cathodically-protected plug assembly in a set state and being mounted to the setting tool according to an example implementation.

FIG. 2C is a cross-sectional view of the portion of the well illustrating the cathodically-protected plug assembly after removal of the setting tool according to an example implementation.

FIG. 3 is a cross-sectional view of a sealing member of the cathodically-protected plug assembly according to an example implementation.

FIGS. 4 and 5 are flow diagrams of techniques to use a and cathodically-protect a downhole plug assembly according to example implementations.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth but implementations may be practiced without these specific details. Well-known circuits, structures and techniques have not been shown in detail to avoid obscuring an understanding of this description. “An implementation,” “example implementation,” “various implementations” and the like indicate implementation(s) so described may include particular features, structures, or characteristics, but not every implementation necessarily includes the particular features, structures, or characteristics. Some implementations may have some, all, or none of the features described for other implementations. “First”, “second”, “third” and the like describe a common object and indicate different instances of like objects are being referred to. Such adjectives do not imply objects so described must be in a given sequence, either temporally, spatially, in ranking, or in any other manner. “Coupled”, “connected”, and their derivatives are not synonyms. “Connected” may indicate elements are in direct physical or electrical contact with each other and “coupled” may indicate elements co-operate or interact with each other, but they may or may not be in direct physical or electrical contact. Also, while similar or same numbers may be used to designate same or similar parts in different figures, doing so does not mean all figures including similar or same numbers constitute a single or same implementation. Although terms of directional or orientation, such as “up,” “down,” “upper,” “lower,” “uphole,” “downhole,” and the like, may be used herein for purposes of simplifying the discussion of certain implementations, it is understood that these orientations and directions may not be used in accordance with further example implementations.
In accordance with example implementations, a plug assembly may be run into a tubing string (a casing string, for example) of a well for purposes of forming a fluid barrier at a target downhole location. For example, the plug assembly may be run downhole inside the tubing string on a conveyance mechanism (a coiled tubing string or a wireline, as examples), and when the plug assembly is at the target location, a setting tool may be actuated for purposes for causing the plug assembly to radially expand to engage the wall of the tubing string to anchor the plug assembly in place. Moreover, in the setting of the plug assembly, a fluid seal may be formed between the plug assembly and the tubing string wall. The plug assembly may have a through passageway that may be blocked to form a fluid obstruction, or barrier, by deploying an untethered object inside the tubing string passageway such that the untethered object travels down through the tubing string passageway to land in a seat of the plug assembly. The fluid barrier may be used in connection with a well stimulation operation. For example, in accordance with some implementations, the fluid barrier may be used to divert fluid to the surrounding formation in a hydraulic fracturing operation.
In accordance with example implementations that are discussed herein, the untethered object may include one or multiple degradable, or dissolvable, materials to provide a temporary seal so that the segment, or zone above the plug assembly may be fractured over a relatively short window of time (a window of one to twelve hours, for example). After the ball dissolves, the through passageway of the plug assembly allows fluid flow from zones below the plug assembly.
In accordance with example implementations that are described herein, the plug assembly may include one or multiple materials, which degrade, or dissolve, after the fracturing operation has been completed. The degradable material(s) of the plug assembly may ideally degrade over a relatively longer time window (a time window of several days, weeks or months, as examples) as compared to the time window over which the untethered object degrades. Thus, a relatively fast dissolving untethered object, such as an activation ball, may be deployed to seal the through passageway of the plug assembly, thereby isolating the zone above the plug assembly from other zones below the plug assembly. After a well stimulation the relies on the fluid barrier is over, the untethered object dissolves at a relatively fast rate, and then the plug assembly may dissolve, at a relatively slower rate to completely remove the restriction created by the plug assembly.
It is important that the plug assembly retains its structural integrity and thus, does not dissolve before the well stimulation operation is over. If the dissolvable material(s) of the plug assembly cathodically reacts with the surrounding tubing string (a casing string, for example), then the galvanic-based corrosive reaction may immediately begin dissolving the dissolvable material(s) of the plug assembly; and as a result, the plug body may dissolve at a faster rate than desired. In accordance with example implementations that are described herein, the plug assembly has features that electrically insulate the assembly from the surrounding tubing string for purposes cathodically protecting the plug assembly.
As a more specific example, FIG. 1A depicts a well 100 in accordance with some implementations. The well 100 includes a laterally extending wellbore 120, which traverses one or more hydrocarbon-bearing formations. For the specific implementation depicted in FIG. 1A, the wellbore 120 is lined and supported by a tubing string 130. The tubing string 130 may be cemented to the wellbore 120 (i.e., the tubing string 130 may be a casing); or the tubing string 130 may be anchored or secured, to the surrounding formation(s) by one or multiple packers (i.e., the tubing string 130 may be installed in an “open hole wellbore”). For the specific example of FIG. 1A, the tubing string 130 is a casing that has been run into the wellbore 120, and a cementing operation has been performed to place cement 140 in the annular region between the exterior of the casing and the wall of the wellbore 120.
It is noted that although FIG. 1A depicts a laterally extending wellbore, the technique systems that are disclosed herein may likewise apply to vertically extending wellbores. Moreover, in accordance with example implementations, the well 100 may contain multiple wellbores, which contain tubing strings that are similar to the tubing string 130 of FIG. 1A. The well 100 may be a subsea well or may be a terrestrial well depending on the particular implementation. Additionally, the well 100 may be an injection well or may be a production well, depending on the particular implementation. Thus, many implementations are contemplated, which are within the scope of the appended claims.
As depicted in FIG. 1A, the tubing string 130 extends from a heel end 141 of a lateral segment 121 of the wellbore 120 to a toe end 143 of the segment 121. The lateral segment 121 may be associated with multiple stages, which may be isolated and stimulated separately.
For the specific example depicted in FIG. 1A, a plug assembly 150 has been set and thus, anchored, or secured, to the tubing string 130 at a target downhole location. For this example, the plug assembly 150 is located in a zone, or stage, of the well 100 to be fractured. In this manner, as shown in FIG. 1A, hydraulic communication with the surrounding formation may have been enhanced through, for example, a perforating operation that formed perforations 134 that extend through the surrounding tubing string wall and into the surrounding formation. Hydraulic communication may be enhanced using other techniques (abrasive jetting operations, for example). The plug assembly 150 may be set at the lower end of the zone to be fractured, as illustrated in FIG. 1A.
Referring to FIG. 1B, an untethered object, such as an activation ball 170, an untethered object (an activation sphere, or ball, as an example, not shown in FIG. 1A) may be deployed inside the central passageway of the tubing string 130 to land in a seat 154 of the plug assembly 150 for purposes of forming a fluid barrier inside the tubing string 130. In this regard, after the fluid barrier is formed, a well stimulation operation may be performed that relies on the fluid barrier. For example, a hydraulic fracturing operation may be performed in which a fracturing fluid is pumped into the tubing string 130, and the fluid barrier diverts the fluid into the surrounding formation.
In the context of this application, an “untethered object” refers to an object that is communicated downhole through the passage of a tubing string along at least part of its path without the use of a conveyance line (a slickline, a wireline, a coiled tubing string, and so forth). As examples, the untethered object may be a ball (or sphere), a dart or a bar. Regardless of its particular form, the untethered object travels through the passageway of the tubing string to land in the object catching seat of the plug assembly to form a corresponding fluid obstruction, or barrier.
Referring to FIG. 1C, the activation ball 170 is constructed to degrade at a relatively faster rate (within one to two hours, as an example), relative to the rate (a rate of days, weeks, or even months, as examples) that the plug assembly 150 dissolves. Therefore, after a certain time (one day, for example) after the well stimulation operation has been completed, the activation ball 170 dissolves to free the through passageway of the plug assembly 150, as depicted in FIG. 1C. As depicted in FIG. 1D, after a longer period of time, the plug assembly 150 dissolves to entirely remove the restriction created by the deployment and use of the plug assembly 150, as depicted in FIG. 1D.
In accordance with example implementations that are described herein, the plug assembly 150 is a cathodically-protected assembly for purposes of preventing premature degradation of the plug assembly before the downhole operation, which relies on the fluid barrier created by the plug assembly is over. As a more specific example, FIG. 2A depicts the plug assembly 150 in an unset state in accordance with an example implementation. In this state, the plug assembly 150 is mounted to a setting tool, which was a member 210 that extends inside the assembly's through passageway along a longitudinal axis 201. As shown in FIG. 2A, the plug assembly 150 may be initially secured to the setting tool member 210 by shear pins 220 and 221. In general, FIG. 2A depicts the plug assembly 150 extending from an uphole end 202 to a downhole end 203.
The plug assembly 150 has features to cathodically protect the assembly 150, i.e., electrically insulate the electrically conductive and degradable components of the plug assembly 150 from the surrounding metallic tubing string 130. In this manner, in accordance with example implementations, the plug assembly 150 has a sealing member 230, which, when the plug assembly 150 is set, forms an fluid annular seal between the plug assembly 150 and the interior surface of the tubing string 130. The sealing member 230 to electrically insulate the plug assembly 150 from the tubing string 130.
As an example, in accordance with some implementations, the sealing member 130 may be constructed from one or multiple dielectric materials, such as one or more degradable elastomer materials. Example degradable elastomeric materials are described in PCT/US2016/052577, entitled “DEGRADABLE ELASTOMERIC MATERIALS”, which was filed on Sep. 20, 2016, and which is incorporated herein by reference. In an example embodiment, a degradable elastomer may include a polymerizing blend of materials that includes a polymeric material and a degradable alloy material that is formed in to a degradable component from the polymerized blend materials. The degradable material may take any suitable form and in some embodiments may take the form of an aluminum alloy including alkali metals, alkaline earth metals, group 12 transition metals, and basic metals having an atomic number equal to or greater than 31. In some embodiments, the polymeric material may take the form of nitrile rubber, silicone, or any other suitable material.
As another example, in accordance with some implementations, the sealing member 230 may be a composite sealing element 300 (see FIG. 3), which has an electrically conductive base material 302 (see FIG. 3), such as a deformable metal, which is encased or surrounded, by a dielectric coating 304, that isolates the material 300 from the tubing string 130. As an example, the coating 304 may be one of the following: a Xylan impregnation coating, a Tellus 22 oil impregnation coating, a Tungsten Carbide coating, a Dykor coating, or a sol gel coating. Other coatings and other electrically insulative materials or compositions may be used for the sealing member of the plug assembly 150, in accordance with further example implementations. In accordance with some implementations, the sealing element may be a slotted metal sealing ring that is encapsulated by one of these coatings. For example, the slotted metal sealing ring may be similar to a slotted metal sealing ring described in U.S. patent application Ser. No. 15/153,085, entitled “METAL SEALING DEVICE,” which was filed on May 12, 2016, and is hereby incorporated by reference in its entirety.
In accordance with some implementations, the base material 302 may be a dissolvable or degradable alloy similar to or the same as one or more of the alloys that are discussed in the following patents and patent applications, which have an assignee in common with the present application: U.S. Pat. No. 7,775,279, entitled, “DEBRIS-FREE PERFORATING APPARATUS AND TECHNIQUE,” which issued on Aug. 17, 2010; U.S. Pat. No. 8,211,247, entitled, “DEGRADABLE COMPOSITIONS, APPARATUS COMPOSITIONS COMPRISING SAME, AND A METHOD OF USE,” which issued on Jul. 3, 2012; PCT Application Pub. No. WO 2016/085798, entitled, “SHAPING DEGRADABLE MATERIAL,” having a publication date of Jun. 2, 2016; PCT Application Pub. No. WO 2016/085804, entitled, “SEVERE PLASTIC DEFORMATION OF DEGRADABLE MATERIAL,” having a publication date of Jun. 2, 2016; PCT Application Pub. No. WO 2016/085806, entitled, “BLENDING OF WATER REACTIVE POWDERS,” having a publication date of Jun. 2, 2016; PCT Application Pub. No. WO 2015/184041, entitled, “DEGRADABLE POWDER BLEND,” having a publication date of Dec. 3, 2015; and PCT Application Pub. No. WO 2015/184043, entitled, “DEGRADABLE HEAT TREATABLE COMPONENTS,” having a publication date of Dec. 3, 2015.
Referring to FIG. 2A, in accordance with example implementations, the plug assembly 150 includes a slip, or gripping member 250, which is constructed to radially expand to anchor, or secure, the plug assembly 150 to the tubing string 130. The gripping member 250, in accordance with example implementations, is constructed from a dielectric material for purposes of cathodically protecting the plug assembly 150 from the tubing string 130. As depicted in FIG. 2A, the gripping member 250 has outer teeth 251 for purposes of biting, or gripping, into the tubing string wall. As examples, in accordance with some implementations, the gripping member and/or the teeth may be a ceramic material, a metal matrix composite, or the like.
FIG. 2A depicts the plug assembly 150 in its radially contracted, or unset, state, which allows the assembly 150 to freely pass through the tubing string 130 when being run downhole into position. For purposes of setting the plug assembly 150 at a target downhole location, the setting tool applies a downward force on an outer sleeve 219 of the assembly 150 for purposes of axially translating a seal retainer 214 toward the sealing member 230. In this manner, upon application of insufficient downward force, the shear pins 220 shear to free the outer sleeve 219 and allow the sleeve 219 to axially translate to force the seal retainer 214 into the sealing member 230 to radially expand the member 230. As shown in FIG. 2A, in accordance with some implementations, the seal retainer 214 may have outer ratchet teeth 215, which engage inner ratchet teeth (not shown) of the sealing member 230 for purposes of locking the seal retainer 230 in place. As also shown in FIG. 2A, the sealing member 230 has an inner, downwardly facing annular inclined surface 231, which engages an outer, upwardly facing annular inclined surface 217 of a body element 216 of the plug assembly 150 for purposes of forcing the sealing member 230 radially outwardly and causing the member 230 to exert an axial force for purposes of forming a seal between the plug assembly 150 and the tubing 130.
An outer, downwardly facing annular inclined surface 218 of the body member 216, in turn, contacts an inner, upwardly facing annular inclined surface 252 of the gripping member 250 for purposes of causing the gripping member 250 to radially expand due to axial movement of the outer sleeve 219. Thus, axial movement of the sleeve 219 causes the radial expansion of both the sealing member 230 and gripping member 250. As also depicted in FIG. 2A, a lower end piece 260 of the plug assembly 150 provides a lower stop to force the sealing member 230 and the gripping member 250 to radially expand. The end piece 260 may be attached to the setting tool member 210 by the shear pins 221. In this manner, the shear pins 221 may be constructed to shear after a sufficient force has been exerted to set the plug assembly 150 (the set state of the plug assembly 150 being depicted in FIG. 2B), to thereby free the setting tool from the plug assembly 150 and allow the setting tool to be retrieved from the well, as depicted in FIG. 2C.
Thus, referring to FIG. 4, in accordance with example implementations, a technique 400 includes deploying (block 404) a plug assembly that includes a degradable material inside a tubing string of a well and engaging (block 408) the plug assembly with the tubing string to anchor the plug assembly to the tubing string. The technique 400 includes preventing (block 412) cathodic reaction from degrading the degradable material, including electrically isolating the engaged plug assembly from the tubing string. During the prevention of the cathodic reaction, a fluid barrier is formed in the tubing string using the plug assembly, pursuant to block 416.
As a more specific example, FIG. 5 depicts a technique 500 that includes running (block 504) a plug assembly inside a tubing string of a well, where the plug assembly includes a central passageway, a degradable material(s) and an object catching seat. At a predetermined position in the tubing string, the sealing element of the plug assembly is expanded and an anchoring element of the plug assembly is expanded to cause the sealing element to engage the wall of the tubing string to form a fluid seal between a body of the plug assembly and the wall and to cause the anchoring element to engage the wall to secure the body to the wall, pursuant to block 508. One or multiple dielectric material(s) of the plug assembly may then be used (block 512) to electrically isolate the plug assembly from the tubing string wall while the sealing element and the anchoring element are engaged with the tubing string wall for purposes of inhibiting degradation of the degradable material(s) of the plug assembly. Pursuant to the technique 500, an untethered object may then be deployed (block 516) to land in the object catching seat of the plug assembly to form a fluid barrier in the tubing string such that a downhole operation may then be performed using the fluid barrier, pursuant to block 516. Subsequently, another downhole operation may be performed, which relies on removal of the plug assembly due to degradation of the dielectric material(s) and degradation of the degradable material(s) of the plug assembly, pursuant to block 520.
While the present techniques have been described with respect to a number of embodiments, it will be appreciated that numerous modifications and variations may be applicable therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the scope of the present techniques.

Claims

What is claimed is:

1. A method comprising:

deploying a plug assembly comprising degradable material inside a tubing string of a well;

engaging the plug assembly with the tubing string to anchor the plug assembly to the tubing string;

preventing a cathodic reaction from degrading the degradable material, wherein preventing the cathodic reaction comprises electrically isolating the engaged plug assembly from the tubing string; and

during the preventing of the cathodic reaction, forming a fluid barrier in the tubing string using the plug assembly.

2. The method of claim 1, wherein deploying the plug assembly comprises deploying a tubular member having a though passageway and a seat to receive an untethered object to seal off the through passageway to form a fluid barrier inside the tubing string.

3. The method of claim 2, further comprising:

deploying the untethered object, wherein the untethered object comprises a degradable material having a relatively faster rate of degradation than the degradable material of the plug assembly.

4. The method of claim 1, wherein:

engaging the plug assembly with the tubing string comprises radially expanding a gripping member of the plug assembly; and

preventing the cathodic reaction comprises forming the gripping member from a dielectric material.

5. The method of claim 4, wherein forming the gripping member from a dielectric material comprises forming the gripping member from a ceramic material.

6. The method of claim 1, further comprising:

radially expanding a sealing member of the plug assembly to form a fluid seal between the plug assembly and the tubing string;

wherein preventing the cathodic reaction comprises forming the sealing member from a dielectric material.

7. The method of claim 6, wherein forming the sealing member from a dielectric material comprises forming the sealing member from an elastomer material.

8. The method of claim 6, wherein forming the sealing member from a dielectric material comprises forming the sealing member from an electrically conductive material and coating the electrically conductive material with a dielectric material.

9. A method comprising:

running a plug assembly inside a tubing string of a well;

at a predetermined position in the tubing string, expanding a sealing element of the plug assembly and expanding an anchoring element of the plug assembly to cause the sealing element to engage a wall of the tubing string to form a fluid seal between a body of the plug assembly and the wall and to cause the anchoring element to engage the wall to secure the body to the wall;

using at least one material of the plug assembly to electrically isolate the plug assembly from the tubing string wall while the sealing element and the anchoring element are engaged with the tubing string wall to inhibit degradation of a degradable material of the plug assembly;

deploying an untethered object to land in the seat of the plug assembly to form a fluid barrier in the tubing string and performing a first downhole operation using the fluid barrier; and

subsequently performing a second downhole operation that relies on removal of the plug assembly due to removal of the at least one material and degradation of the degradable material of the plug assembly.

10. The method of claim 9, wherein performing the first downhole operation comprises performing a hydraulic fracturing operation.

11. An apparatus comprising:

a body having a through passageway;

a sealing member having a contracted state and an expanded state to form a fluid seal between the body and an outer tubing;

an anchoring member having a contracted state and an expanded state in which the anchoring member secures the body to the outer tubing;

wherein the sealing member and the anchoring member electrically isolate the body from the outer tubing.

12. The apparatus of claim 11, wherein the body comprises a degradable material to degrade at a relatively faster rate than a material of the outer tubing.

13. The apparatus of claim 11, wherein the sealing member comprises an elastomer material.

14. The apparatus of claim 11, wherein the sealing member comprises:

an electrically conductive material; and

a coating to electrically isolate the conductive material from the body and the outer tubing.

15. The apparatus of claim 14, wherein the coating comprises a material selected from the set of materials consisting essentially of:

a Xylan impregnation coating, a Tellus 22 oil impregnation coating, a Tungsten Carbide coating, a Dykor coating, and a sol gel coating.

16. The apparatus of claim 11, wherein the anchoring member comprises a ceramic material.

17. The apparatus of claim 11, further comprising:

a seal retainer adapted to axially translate in response to a force exerted by a setting tool to transition the seal member from the contracted to the expanded state and transition the anchoring member from the contracted state to the expanded state.

18. The apparatus of claim 16, wherein the seal retainer comprises ratcheting teeth to lock the sealing member and the anchoring member in the contracted states.

19. A system comprising:

a tubing string; and

a plug assembly comprising:

a body having a through passageway;

20. The system of claim 19, wherein the tubing string comprises a casing.

21. The system of claim 19, further comprising:

an untethered object, wherein:

the plug assembly comprises a seat to receive the untethered object to form a fluid barrier inside the tubing string;

the untethered object comprises a first degradable material having a first degradation rate; and

the plug assembly comprises a second degradable material having a second degradation rate less than the first degradation rate.