Various embodiments disclosed relate to a composite mandrel. In various embodiments, the present invention provides a method of forming a composite mandrel for use in a subterranean formation. The method can include securing an elongated mold to at least a portion of an exterior surface of a rigid tubular to form a cavity between the mold and the exterior surface of the rigid tubular. The elongated mold can include a recessed portion. The cavity can extend longitudinally along the tubular. The method can include filling the cavity with a curable resin composition. The method can also include curing the resin to provide a cured resin including a utility recess corresponding to the recessed portion. Curing the curable resin composition can form the composite mandrel including the tubular and the cured resin.