Inconsistency between fracture and anisotropic systems in predicting horizontal-sink productivity

Modeling a reservoir containing unidirectional natural fractures by an equi valent anisotropic system is one of the most common methods in the realm of fractured reservoir engineering. The universal validity of such a macrosco pic treatment, however, has not been quantitatively confirmed. This study p rimarily examined whether an equivalent anisotropic system is consistent wi th a fracture system through horizontal-sink productivity problems. Numerical experiments for a horizontal sink completed parallel or perpendic ular to fractures with 1000 fracture realizations were performed to compare the productivity predictions based on naive fracture systems and equivalen t anisotropic systems. The results showed the anisotropic approach yields e rroneous productivities higher than the true values with mean relative erro rs of 11% and 13% for parallel and perpendicular completions, respectively. The conversion of fracture systems to equivalent anisotropic systems is no t always appropriate for flow in the vicinity of a horizontal sink and may mislead the productivity predictions. A pseudo-skin factor was introduced to correct the overestimation of the an isotropic systems. Mean pseudo-skin factors of 0.366 and 0.258 were require d for parallel and perpendicular completions, respectively. Versatility of the correction technique requires a simple means to evaluate the pseudo-ski n factor, thus a set of correlation equations with minimal information (hor izontal sink length, total fracture length, and geometric mean of fracture lengths) was established. The inconsistency between fracture and anisotropi c systems could be corrected by the systematic procedure proposed in this s tudy.