15 So all the wells that his father’s servants had dug in the time of his father Abraham, the Philistines stopped up, filling them with earth.

Read Genesis 26:15 Using Other Translations

For all the wells which his father's servants had digged in the days of Abraham his father, the Philistines had stopped them, and filled them with earth.
(Now the Philistines had stopped and filled with earth all the wells that his father's servants had dug in the days of Abraham his father.)
So the Philistines filled up all of Isaac’s wells with dirt. These were the wells that had been dug by the servants of his father, Abraham.