Dissolvable ballast for untethered downhole tools

A method comprising:

• mixing metallic particles and a liquefied polymer to form a mixture;
• placing the mixture within a mold;
• placing a magnet in the vicinity of the mixture within the mold, thereby causing the metallic particles to position themselves in a self-assembly formation within the mixture in response to a magnetic field generated by the magnet;
• and
• solidifying the liquefied polymer, such that a polymer matrix is formed, wherein the metallic particles are distributed and secured in the self-assembly formation throughout the polymer matrix, thereby forming a ballast for an untethered downhole tool;
• coupling the ballast to the untethered downhole tool;
• and
• lowering the untethered downhole tool with the ballast into a well formed in a subterranean formation, wherein the polymer matrix is configured to dissolve in response to being exposed to downhole fluid within the well at specified downhole conditions.