A new method of evaluation of the closeness of relationships between well outputs is proposed. It is based on a modification of the method of group consideration of arguments, viz. the method of directed orthogonalization. The essential distiguishing feature of this method is the possibility of evaluating the closeness of the nonlinear relation by identifying its structure using the method of group consideration of arguments (MGCA). A correlation based on the use of the MGCA criterion is introduced as the index of closeness of such a relation.