RWY INCURSION DURING TAXI.

Narrative

WE ARRIVED AT MSN WITHOUT INCIDENT; LNDG ON RWY 21. AFTER A SHORT STAY ON GND WE CONTACTED CLRNC DELIVERY AND WERE APPROVED TO SHOOT PHOTOS OVER THE SE CORNER OF THE ARPT. WE ASKED FOR AND RECEIVED A PROGRESSIVE TAXI FROM THE S RAMP TO RWY 21 FOR DEP. AFTER ABOUT AN HR OF FLYING OVERHEAD; WE LANDED ON RWY 21 AND RETURNED TO THE RAMP FOR DEBRIEFING. AT ABOUT XA30 WE WERE PLANNING TO RETURN TO ZZZ. WE CONTACTED CLRNC DELIVERY AND WERE GIVEN TAXI INSTRUCTIONS TO WHAT I NOW BELIEVE TO HAVE BEEN TO RWY 31. I READ BACK THE INSTRUCTIONS; AND TAXIED TO RWY 21; XING RWY 31 AT ONE POINT IN THE TAXI. WHEN I CALLED TO TWR TO RPT READY FOR TKOF HE CLRED ME FOR DEP ON RWY 31. I CALLED BACK AND NOTIFIED HIM THAT WE WERE ON THE DEP END OF RWY 21. HE ASKED IF RWY 21 WOULD WORK FOR MY DEP; I SAID IT WOULD AND WE TOOK OFF. AS I THINK ABOUT THE ENTIRE SIT; I CAN IDENT A COUPLE OF FACTORS THAT LED TO THIS ERROR: 1) I WAS UNFAMILIAR WITH THE ARPT. 2) HAVING USED RWY 21 3 TIMES ALREADY THAT DAY; I ASSUMED WE WOULD BE USING IT AGAIN. THE SIMILAR SOUND OF '21/31' MADE MY ASSUMPTION SEEM A REALITY. 3) HAVING ASKED FOR A PROGRESSIVE TAXI EARLIER IN THE DAY; I WAS RELUCTANT TO ASK AGAIN. THIS WAS AN INCREDIBLY INEXCUSABLE LAPSE IN PAYING ATTN ON MY BEHALF. IN THE FUTURE I WILL HAVE AN ARPT DIAGRAM HANDY AT ANY UNFAMILIAR ARPT; AND NEVER HESITATE TO ASK FOR INSTRUCTION TO THE PROPER DEST.