Would this work for you?
SELECT
well.wellide AS Prospect,
well.wellidc AS Pad,
well.wellname AS Well,
job.idrec AS JobID,
rig.contractor + ISNULL(' '+ rig.rigno, '') AS Rig,
SUM(CASE WHEN rig.dttmstart < '01/01/2014' THEN
CAST(CAST(DATEDIFF(MI, '01/01/2014', rig.dttmend) AS DECIMAL(12,2))/1440 AS DECIMAL(12,2))
WHEN rig.dttmend > '01/01/2015' THEN
CAST(CAST(DATEDIFF(MI, rig.dttmstart, '01/01/2015') AS DECIMAL(12,2))/1440 AS DECIMAL(12,2))
ELSE CAST(CAST(DATEDIFF(MI, rig.dttmstart, rig.dttmend) AS DECIMAL(12,2))/1440 AS DECIMAL(12,2))
END) AS DaysDuration
FROM
wv90.dbo.wvwellheader AS well
INNER JOIN wv90.dbo.wvjob AS job
ON well.idwell = job.idwell
LEFT JOIN wv90.dbo.wvjobrig AS rig
ON job.idrec = rig.idrecparent
GROUP BY
well.wellide,
well.wellidc,
well.wellname,
job.idrec,
rig.contractor + ISNULL(' '+ rig.rigno, '')
I changed something else just for fun. I hope that you like that alternative.