Rate decline analysis for naturally fractured reservoirs.

摘要

In this work, transient rate analysis for constant pressure production in a naturally fractured reservoir is presented. The solution for the dimensionless flowrate is based on a model which treats interporosity flow as a function of a continuous matrix block size distribution. Several distributions of matrix block size are considered. This approach is similar to that of Ref. 1, which examined the pressure response. The flowrate response is investigated for both pseudo-steady state (PSS) and unsteady state (USS) interporosity models, which include slab, cylindrical, and spherical matrix block geometries. It was found that the flowrate decline becomes smooth, specially for the unsteady state model, and approaches the decline behavior of a nonfractured reservoir when matrix block size variability is large, i.e., when fracturing is extremely nonuniform. The difference in flowrate for various geometric models of blocks is not significant, with the spherical geometry yielding the highest and the slab yielding the lowest flowrate. This work suggests why certain naturally fractured reservoirs do not exhibit a sudden rate decline followed by a period of constant flowrate as predicted by classical double porosity models. Also, the results indicate that reservoir producibility is directly proportional to fracture intensity and inversely proportional to the degree of fracture nonuniformity. Hence, the Warren and Root model which assumes fracturing is perfectly uniform, provides an upper bound of reservoir producibility and cumulative production.