UNTETHERED DOWNHOLE TOOLS

1 . A wellhead attachment for deploying an untethered downhole tool into a well, the wellhead attachment comprising:

a housing configured to house the untethered downhole tool;

a relief valve in fluid communication with the housing and located at a first end of the wellhead attachment;

a mounting flange located at a second end of the wellhead attachment, wherein the second end is opposite the first end, wherein the mounting flange is configured to couple to a tree of a well formed in a subterranean formation; and

a gate valve coupled to the housing and the mounting flange, wherein the gate valve is positioned between the housing and the mounting flange, wherein the housing is positioned between the relief valve and the gate valve.

2 . The wellhead attachment of claim 1 , comprising a first pressure seal positioned between the relief valve and the housing, wherein the first pressure seal is configured to prevent leakage of fluid from between the relief valve and the housing.

3 . The wellhead attachment of claim 2 , comprising a second pressure seal positioned between the housing and the gate valve, wherein the second pressure seal is configured to prevent leakage of fluid from between the housing and the gate valve.

4 . The wellhead attachment of claim 1 , comprising a vent valve coupled to the housing and positioned between the relief valve and the housing.

5 . The wellhead attachment of claim 4 , comprising a first pressure seal positioned between the relief valve and the vent valve, wherein the first pressure seal is configured to prevent leakage of fluid from between the relief valve and the vent valve.

6 . The wellhead attachment of claim 5 , comprising a second pressure seal positioned between the vent valve and the housing, wherein the second pressure seal is configured to prevent leakage of fluid from between the vent valve and the housing.

7 . The wellhead attachment of claim 6 , comprising a third pressure seal positioned between the housing and the gate valve, wherein the third pressure seal is configured to prevent leakage of fluid from between the housing and the gate valve.

8 . A method for deploying an untethered downhole tool into a well, the method comprising:

placing the untethered downhole tool within a housing of a wellhead attachment, wherein the wellhead attachment a gate valve;

after placing the untethered downhole tool within the housing, closing the gate valve;

closing a master valve, a wing valve, and a swab valve of a tree of the well;

after closing the master valve, the wing valve, and the swab valve, venting gas from a tree cap of the tree;

after venting gas from the tree cap, removing the tree cap from the tree;

installing the wellhead attachment to the tree, thereby replacing the removed tree cap;

after installing the wellhead attachment to the tree, opening the swab valve;

after opening the swab valve, opening the gate valve, thereby allowing the untethered downhole tool to descend from the housing into the tree below the swab valve;

after the untethered downhole tool has descended into the tree, closing the gate valve and the swab valve; and

after closing the gate valve and the swab valve, opening the master valve, thereby allowing the untethered downhole tool to descend from the tree into the well.

9 . The method of claim 8 , comprising, after opening the master valve, opening the wing valve to allow fluid flow into the well.

10 . The method of claim 9 , comprising, after installing the wellhead attachment to the tree and before opening the swab valve, pressure testing the wellhead attachment to verify absence of fluid leakage from between the tree and the wellhead attachment.

11 . The method of claim 10 , wherein installing the wellhead attachment to the tree comprises lifting the wellhead attachment, with the untethered downhole tool positioned within the housing of the wellhead attachment, above the tree.

12 . The method of claim 11 , wherein installing the wellhead attachment to the tree comprises installing a gasket on the tree where the tree cap was removed.

13 . The method of claim 12 , wherein the wellhead attachment comprises a mounting flange, and installing the wellhead attachment to the tree comprises coupling the mounting flange of the wellhead attachment to the tree after installing the gasket.

14 . A method for retrieving an untethered downhole tool from a well formed in a subterranean formation, the method comprising:

opening a wing valve of a tree of the well, thereby allowing flow of fluid out of the well through the open wing valve and directing the untethered downhole tool into the tree;

closing a master valve of the tree, thereby trapping the untethered downhole tool within the tree and stopping flow of fluid out of the well;

after closing the master valve, closing the wing valve;

flowing a fluid that is denser than the untethered downhole tool into the tree, thereby causing the untethered downhole tool to float out of the tree and into a wellhead attachment attached to the top of the tree, wherein the wellhead attachment comprises a gate valve;

closing the gate valve of the wellhead attachment, thereby trapping the untethered downhole tool within the wellhead attachment; and

after closing the gate valve, removing the wellhead attachment, with the untethered downhole tool positioned within the wellhead attachment, from the tree.

15 . The method of claim 14 , comprising, after closing the wing valve and before flowing the fluid that is denser than the untethered downhole tool into the tree, venting gas from the tree.

16 . The method of claim 15 , wherein the fluid that is denser than the untethered downhole tool and flowed into the tree comprises water.

17 . The method of claim 15 , wherein the fluid that is denser than the untethered downhole tool and flowed into the tree comprises a hydrocarbon.

18 . The method of claim 15 , comprising, after removing the wellhead attachment from the tree, installing a tree cap onto the tree where the wellhead attachment was removed.