Requirements clustering (RC) provides a different approach to system decomposition, by enabling a system to be partitioned into user-recognizable components, where each component can be used, almost independently to satisfy part of the user's needs. Requirements clustering is essential for a software development approach called incremental delivery (ID). A successful clustering of system requirements produces a set of useful, usable, and semi-independent clusters that can be developed and delivered to the customers in increments. This paper presents a requirements clustering process based on ER modeling, scenarios, and the formal specification notation Z.