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.