I need to remove a small section of polyurethane crack injection. What's the best way to go about this?

Basically you need to dig out the foam with a utility knife, razor blade scrapers. Sharp screw driver etc. ( Use safety glass) Are you sure this is a polyurethane injection and not and epoxy injection crack repair?

Epoxies are super hard its compressive strength could be as high as 12,000 psi or greater

Is the access hard to get at?

I managed to get it out using utility knives, sharp screwdrivers and a couple drill bits. I'm thinking now that it was an epoxy. Thanks for the help. Hopefully I will never have to do this again as it was very difficult.