Carbonate reservoirs provide a unique set of challenges in attempting to determine critical properties. The exact determination of porosity, permeability, and fluid type from logs is difficult in tight-reservoir conditions, which is evident in unaltered carbonate systems. Prolific carbonate production can also be achieved when secondary porosity enhancements, such as fractures, vugs, and karsts are present.
Log characterization of these various systems has been rigorous and difficult. Log analysts sometimes arrive at complex conclusions that have little relationship to the reservoir properties, the rates that can be achieved from the system, or the fluid types that may be expected in production. Extensive, as well as expensive, log evaluation techniques and rock characterization studies in complicated laboratory analyses have been used in an attempt to capture these elusive qualities.
Magnetic resonance (NMR) logs were added to logging suites in an attempt to more accurately delineate these important values. The NMR log measures physical properties related to the fluid types and their ability to be manipulated in wellbore conditions. These measurements can then be allocated to establish fluid types and permeability.
This paper examines many types of carbonate reservoirs and summarizes the NMR response to those conditions. Reservoirs will include tight carbonates, with 0–2% porosity, systems that are heavily altered by extensive natural fracturing, and those that have a significant component of vug development. In each of these, permeability will be calculated directly from the NMR relaxation, or T2, measurement, and the fluid type will be indicated by using crossplots of polarization, or T1, with T2. The conclusions will be compared to production rates achieved in each of these reservoirs and to image and core data where available. A positive, generalized description of reservoir properties is achieved in every case, emphasizing the value of NMR as a complex carbonate reservoir characterization device.