A grand jury shall serve until discharged by the court which convened it, but no grand jury may serve more than eighteen months. The tenure and powers of a grand jury are not affected by the beginning or expiration of a term of court. At any time for cause shown the court may excuse a juror either temporarily or permanently, and in the latter event the court may impanel another person in place of the excused juror in the same manner as the original juror was impaneled.
SDCL 23A-5-20