Tunable reflective elements based on graphene patches are proposed for reconfigurable-beam reflectarray antennas. The complex conductivity of graphene can be dynamically varied when an electric field is applied, allowing to control the phase of the reflected wave at each element of the reflectarray. The proposed patches support plasmonic slow wave propagation at THz frequencies, drastically reducing the physical dimensions of the cell. Here, an element which provides a dynamic phase range of more than 300 degrees is demonstrated with moderate losses in a wide band. The scattering matrix of the element has been obtained by full-wave electromagnetic simulations and also by a circuit approach accurately modeling the graphene cell response.