The well and completion configuration of choice for development wells targeting unconventional gas and oil resources has become a horizontal well stimulated with multiple stages of hydraulic fracturing. The standard pressure transient model for this well and completion configuration assumes that each hydraulic stimulation stage creates one bi-wing, transverse fracture. Microseismic images of stimulation jobs in some wells support the choice of this model but many such images show an overprint of greater complexity in the geometry of the fracture system. Numerical simulator history matching of production does not definitively show that accounting for such fracture complexity is required to explain unconventional well performance data.
This paper is aimed at providing some insight into how pressure transient data might be used to identify the impact of fracture complexity overprint on well response. After reviewing the limited current literature addressing this issue, fine grid numerically simulated well responses for various overprints of fracture complexity on the base multiple transverse fracture model are presented. In particular, responses for cases impacted by the following are presented: 1) opening and propping nearby natural fractures, 2) creating an arbitrarily connected network of orthogonal fractures intersecting the dominant transverse fracture system, 3) creating independent transverse fracture strands at individual perforation sets in each stage and 4) creating initially independent fracture strands at individual perforation sets which then either bifurcate into multiple strands or amalgamate into fewer dominant strands away from the well perforation. Response sensitivity to lengths and conductivities across the spectrum of these complexity types are discussed.
Results are presented in the context of establishing when the pressure transient response including fracture complexity overprint can be definitively differentiated from that for the base model of a multiple transverse hydraulic fractured horizontal well. Analysis focuses on differences in derivative shapes to address this question. This work is especially relevant to analysis and understanding of shale formations but should also apply to other types of low permeability systems.