Type Curves for Analyzing Naturally Fractured Reservoirs

Abstract

ABSTRACT A solution representing flow at the wellbore in a naturally fractured reservoir is presented in this paper. The model used to develop this solution assumes unsteady state interaction between the fractures and the matrix. Wellbore storage and skin are considered in determining the boundary conditions for this model. A new set of type curves for analyzing pressure transient data from naturally fractured reservoirs is presented. Each type curve within the set is for a different value of CDe2S. Each type curve presents one radial flow solution with the associated family of curves approximating behavior at intermediate flow times. Each curve within a family is for a different ω′λ′e-2S product, where ω′ and λ′ depend upon fracture/matrix storativity and permeability ratio, respectively. Ambiguity is eliminated when a simultaneous match of data against the dimensionless pressure curves and the differential dimensionless pressure curves is performed. Reservoir and well parameters including permeability, skin, dimensionless wellbore storage, ω′, and λ′ can be calculated from this match. Truer matches against actual natural fracture pressure transient data are obtained by using these curves instead of existing type curves. The principal advantages of using these curves are discussed. The first advantage is that when sufficient transition period data is measured, Horner data does not have to be available for an analysis to be performed. Actual pressure buildup data is used to illustrate this. Secondly, after reviewing many actual well tests, the upper practical limit on values for permeability ratio has been expanded. Values for λ′ > 10-3 can now be calculated. Use of these curves is explained in detail. Applicability of the curves is demonstrated by successfully matching actual pressure transient data against the curves.