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The Behavior of Naturally Fractured Reservoirs
JO I% WARREN P, J, RQOT MEMBERS AIME GULF RESEARCH d DEVELOPMENT PITTSBURGH, PA, CO.

ABSTRACT
An idealized model has been developed for the purpose of studying the characteristic behavioroja permeable medium which contains regions which contribute sigizificantly to tbe pore volume O! the system but contribute negligibly to the flowcapacity; e.g., a naturally fractured or vugular reservoir, Vnsteady-state flow in this model reservoir has been analytically. The pressure buiid-up investigated performance has been examined insomedetait; and, a technique foranalyzing tbebuild.up data to evaluate the desired parameters has been suggested. The use of this ap$roacb in the interpretation of field data has been discussed. As a result ofthis study, the following general conclusions can be drawn: 1. Two parameters are sufficient to characterize the deviation of the behavior of a medium with “double porosity ”from that of a homogeneously porous medium. 2. These Parameters can be evaluated by the buildup data ob~ained proper analy~is of pressure from adequately designed tests. 3. Since the build-up curve associated with this issimilar to that obtained type of porous system from a stratified reservoir, an unambiguous interpretation is not possible without additional information. 4, Dif@rencing methods which utilize pressure data from the /inal stages of a buik-kp test should be used with extreme caution.

INTRODUCTION In order to plan a sound exploitation program or a successful secondary-recovery pro ject, sufficientreliable information concerning the nature of the reservoir-fluid system must be available. Sincef it is evident chat an adequate description of the reservoir rock is necessary if this condition is to be fulfilled, the present investigation was undertaken for the purpose of improving the fluid-flow characterization, based on normally available data, ofs particular porous medium. DISCUSSION OF THEPROBLEM For many years it was widely assumed that, for the . . purpose of making engineering studies, two psram-. .
Origlml manuscriptreceived fn eociaty of Petroleum Ertatneere offiae AUS. 17, 1962.Revieed manuscriptreceived.March21, 1963. P eper pr+$eented at the Fetl Meeting of the %ciot Y of. Petreleum Enginsera In Lo= Ar@Ies on Oct. 7-10, 1962. ‘ .

eters were sufficient to desckibe thesingle-phase flow properties of a prodttcing formation, i.e., the absolute permeability and the effective porosity. It later became evident that the concept of directional permeability was of more thin academic interest; consequently, the de$ee of permeability anisotropy and the orientation of the principal axes of permeability were accepted as basic parameters governing reservoir performance. 1,2 Morerecently, 3“6 it was recognized that at least one additional parameter was required to depict the behavior of a porous system containing region,s which contributed significantly to the pore volume but contributed negligibly to the flow capacity. Microscopically, these regions could be “dead-end” or “storage” pores or, microscopically, they could be discrete volumes of lowpermeability inatrix rockcombined with natural fissures in a reservoir. It is obvious thst some provision for the ;.ncIusion of all the indicated parameters, as weIl as their spatial vstiations$ must be made if a truly useful, conceptual model of a reaetvoir is to be developed. A dichotomy Qf the internaI voids of reservoir rocks has been suggested, r~s These two classes of porosity can be described as follows: a.Primary porosity is intergranular and controlled by deposition and Iithification. It ie highly intercoririected arid “usually can be correlated with permeability since it is largely dependent on the geometry, size distribution and spatial distribution of the grains. The void systems of sands, sandstones and oolitic limestones are typical of this type. b. Secondary porosity is foramenular and is...