Often, the region outside the well is either dimmer or brighter than inside the well, such that you can create a mask to mask out the exterior region. In your case here, it is about the same intensity in vs. out. So, you could possibly take a bright-filed image and do the procedure I mentioned above, or likely better and certainly easier, is to enhance the objects of interest and then the problem of the well becomes moot. That is assuming you only care about the cells here.
In the attached pipeline, I simply enhanced objects on the size scale of the cells, and thus suppressed the regions with no variation in intensity. Does this do well enough for you? If some of the cells are being suppressed too much, then make the EnhanceOrSuppressObjects scale bigger.
Hope that helps,
DL_enhance_pipeline.cppipe (3.53 KB)