Pressure Transient and Decline Curve Behaviors in Naturally Fractured Vuggy Carbonate Reservoirs

Abstract

Abstract This study presents a new way to model high secondary porosity, mainly vuggy porosity, in naturally fractured reservoirs. New solutions are presented for two cases, where there is no primary flow through the vugs, which is an extension of the Warren and Root model, and where dissolution process of pore throats has created an interconnected system of vugs and caves. In both cases there is an interaction between matrix, vugs, and fracture systems. New insights are provided. Both pressure and production responses during transient and boundary dominated flow periods are explored. In transient well tests, for the case where there is no primary flow through the vugs, a change of slope could be present during the transition period. Thus, this study shows that slope ratios of 2:1 of early or late-time segment versus transition segment do not necessarily imply transient interaction between matrix and fractures. It is also shown that the presence of vugs and caves may have a definitive influence on decline curve and cumulative production behaviors; therefore it is necessary to incorporate vuggy porosity in the process of type curve match. Finally, the use of the methodology obtained in this work is illustrated with synthetic and field examples. Introduction Most of the world's giant fields produce from naturally fractured and vuggy carbonate reservoirs that have complex pore systems, mainly because carbonate rocks are particularly sensitive to post-depositional diagenesis, including dissolution, dolomitization and fracturing processes. Complete leaching of grains by meteoric pore fluids can lead to textural inversion which may enhance reservoir quality through dissolution or occlude reservoir quality through cementation1. Some works have classified carbonates based on fabric selective and non fabric selective pore types. The non-fabric selective are vugs and channels, caverns, and fractures1. For the purpose of this work no distintion is made on vugs, caverns and channels, and they will be denoted by the term vugs. Thus, vugs may vary in size from millimeters to meters in diameter. Vuggs are the result of carbonate and/or sulfate dissolution. From cores observations, the matrix porosity types adjacent to the vuggy zones are moldic, solution-enlarged microfractures, and solution-enlarged intercrystalline. Thus, it is possible to have a permeability enhancement adjacent to the vuggy zones. Three porosity types, matrix, fractures, and vugs, are usually present in naturally fractured, vuggy carbonate reservoirs. The determination of permeability and porosity in vuggy zones from core measurements are likely to be pessimistic because of sampling problems. In areas lacking cores, open-hole wireline logs may be used to help identify vuggy zones; however, vugs are not always recognized by conventional logs because of their limited vertical resolution2. Vuggy porosity is common in many carbonate reservoirs and its importance in the petrophysical and productive characteristics of a carbonate rock have been recognized by several works.