The accurate quantification of fluid volumes is one of the most important tasks for determining the economic value of hydrocarbon reservoirs. Resistivity-based fluid saturation calculations have been established for many decades with known benefits and challenges. More recently, nuclear magnetic resonance (NMR) technology has developed as an alternative, robust method for direct fluid volume estimation. Resistivity logging data are used to calculate saturation of conductive (e.g., water) versus non-conductive (e.g., hydrocarbon) fluids in the formation. NMR logging data capture information about pore size in the formation and, thus, separate movable fluids, usually residing in large pores, from bound fluids. The bound fluids are further sub-divided into clay-bound water components and irreducible components held by capillary forces. The movable water and hydrocarbon volumes can be estimated using simple T2 cutoff approaches or more sophisticated 2D-NMR methods (the latter of which are generally only available for wireline NMR applications).
As today's reservoirs are becoming more challenging, conventional resistivity data evaluation involves increasing difficulties and ambiguities such as in complex lithology due to the presence of conductive minerals, low formation water salinity, fractures and vugs, or local variations in water resistivity. NMR data processing and interpretation are also not straightforward in complex carbonates and heavy oil reservoirs, as well as in case of wettability alteration and due to the presence of magnetic minerals. Ambiguities in either of the measurements can be efficaciously addressed by combining results from both approaches. In simple reservoirs, for instance, the combination of resistivity and NMR reduces uncertainties and can be used to identify and quantify effects such as invasion or water (-free) production.
We present a systematic compilation and discussion of main properties affecting resistivity and NMR fluid volume estimations such as Archie exponents and T2 cutoffs. This includes various reservoir types with various fluid types and measurement conditions. Guidelines will be provided on how to combine resistivity-based fluid volumetrics and NMR-based volumetrics most effective. An unpublished and four previously published log examples illustrate a wide range of reservoir scenarios. In addition to the log interpretation aspect, we also relate the results to their applications ranging from real-time drilling optimization through hydrocarbon-in-place estimates and reservoir modeling input to production and completion decisions.
