def update_well_positions(self):
        """Set ``well_attrib['FieldXStartCoordinate']`` and
        ``well_attrib['FieldYStartCoordinate']`` to FieldXCoordinate and
        FieldYCoordinate of first field in well.
        """
        for well in self.wells:
            well_x = well.attrib['WellX']
            well_y = well.attrib['WellY']

            first_field = self.well_fields(well_x, well_y)[0]

            x_start = first_field.FieldXCoordinate.text
            y_start = first_field.FieldYCoordinate.text

            well.attrib['FieldXStartCoordinate'] = x_start
            well.attrib['FieldYStartCoordinate'] = y_start