Two employees were observing a crew who was in the process of reconfiguring a well head. Trapped pressure was released within the well and blew shrapnel (nuts, bolt, plastic parts of the inside of the valves that are part of the well head tree) at the two employees. One employee received an amputation injury to his right thumb. The other employee received shrapnel injuries to his body requiring him to be hospitalized and undergo surgery to remove the shrapnel.