Toolstring and method for inner casing perforating, shattering annulus cement, and washing the first annulus in a second casing

A method of sealing a partly or entirely annulus-cemented casing portion of a first casing in a second casing in a petroleum well, includes running a drill pipe string-conveyed perforating gun with a cement shattering device into said well; positioning said perforating gun at said cemented casing portion and activating first, shallow-perforating charges to make first, shallow perforations through said casing portion into thus making a perforated, cement-filled annulus portion and leaving said second casing intact; activating said cement shattering device to disintegrate all or part of said cement within said first perforated annulus portion; running a washing tool along said perforated casing portion, washing out said shattered cement from said perforated annulus portion. The washed-out casing portion may then be cut using a cutter tool and pulled out of hole, or cemented using a cementing tool, or a combination thereof.