On the technical side, there are a number of issues. Each site will have unique issues in terms of length of the return "duct", overall gradient and gradient changes along the "duct". Another one will be temperature along the returning "duct" and at the deposition site. The grind size and particle distribution will be another. Ensuring the plastic "emulsion" remains thoroughly mixed is another and getting the rheology of the plastic emulsion will require research.
Then there are issues with how the plastic solids would deposit in the disused well and whether they would simply block the pores near the well site preventing further deposition in the former reservoir. This also begs the question, what do you think the structure of an oil or gas reservoir is? It's not a large underground void that has been filled with oil or gas, it's the pore spaces between grains and sand and other minerals in a porous sedimentary rock which collected oil and/or gas. Getting ground plastic into such tiny pore spaces may be impossible to achieve, even if the ground has been fracted.
On the operations side of things, oil companies would resist being involved with such as system because they would prefer fewer commitments and not to be burdened with the cost of such a system despite their position on a social license to operate.
Additionally, the oil and gas industry hasn't had great success returning carbon dioxide from methane gas wells back into oil and gas reservoirs.