Abstract
Abstract
The most prominent aspects of carbonate reservoirs are their heterogeneity, variable wettability and dual pore network, which collectively contribute to complex fluid flow and uncertainty in reservoir performance and recovery efficiency predictions. A review of ultimate recovery efficiency in 250 mature carbonate fields from around the world provides constraints for quantifying uncertainty in recovery efficiency predictions. Key determinants of ultimate recovery are fluid type, pore network, reservoir heterogeneity, drive mechanism and wettability. Development strategies and reservoir management techniques play crucial roles in maximizing expected ultimate recoveries for given reservoir/fluid parameters. Five main fluid type/carbonate reservoir classes, with characteristic ultimate recovery distributions and controls are:heavy and viscous oil reservoirs, in which recovery factor (RF) is controlled by fluid viscosity variations, field size and application of horizontal drilling,karstic/fractured carbonate oil reservoirs, in which RF is controlled by fracture intensity, matrix permeability, wettability and drive mechanism,conventional carbonate oil reservoirs, in which RF is controlled by reservoir connectivity, permeability and mobility ratio,organic buildup oil reservoirs, in which RF is controlled by nature and size of organic buildups and diagenetic modifications, andgas/condensate reservoirs, in which RF is controlled by aquifer encroachment and condensate drop-out.
Examination of actual case histories reduces uncertainty in predevelopment recovery efficiency predictions and shows what is possible in new or old fields.
Introduction
Carbonate reservoirs are notorious for their generally low and variable recovery factors. This, together with unpredictability in reservoir geometry, continuity and quality, often leads to great uncertainty in evaluating the economic potential of carbonate prospects. Lack of consistent criteria and reliable data for the definition and calculation of recovery factors creates further problems.
Historically, a large portion of the original geologic and reservoir engineering investigations in the petroleum industry dealt with physical principles involved in the production of oil and gas in clastic reservoirs with intergranular porosity. Most of this knowledge can also be applied to conventional carbonate reservoirs with interparticle or intercrystalline porosity. However, for carbonate reservoirs with a dual pore system, much of the theory and many of the practices developed for clastic reservoirs do not apply because fluid flow through cavernous, vugular, and/or fractured carbonates is fundamentally different from flow through clastic reservoirs with homogeneous pore systems.
In order to understand the key geologic and engineering factors that control reservoir performance and recovery efficiency in various types of carbonate reservoirs, we have undertaken a systematic study of more than 250 carbonate reservoirs throughout the world. This paper examines 196 of these reservoirs in the fractured/karstic, conventional carbonate and organic buildup categories. Only reservoirs for which a comprehensive spectrum of parameters was available, were chosen for study. The effect on recovery of both inherent parameters and reservoir management techniques were examined, in order to achieve a thorough understanding of the relative importance of each variable.
Recovery Efficiency in Carbonate Reservoirs
C&C Reservoirs' DIGITAL ANALOGS System [1] currently contains nearly one thousand producing reservoirs worldwide. There are more than 250 carbonate reservoirs that can be analyzed and compared based on their depositional facies, diagenetic modifications, rock properties, fluid types, drive mechanisms, development strategies, EOR techniques and production histories. The advanced search engine provided in the DIGITAL ANALOGS System allowed us to easily find all of the carbonate reservoirs in the relational database and to group them into the five reservoir types defined in the abstract. Using this data, we were able to systematically evaluate genetically related reservoirs and identify the common factors that control reservoir performance and recovery efficiency in each group.