Add a Frame to the style formatting for the initial heading for the chapter.
See Space Before Heading at beginning of page - Using Frames.


(Up until Word 2013, simple Space-Before formatting would work. The new document rendering engine in Word 2013 strips space-before formatting at the top of the page except at the beginning of a Section. You do not want a Section break just for this. When to use Section breaks in Microsoft Word (and when to not use Section Breaks) )