Remaining Oil Saturation from NMR in a Mixed-Wet, Three-Phase Carbonate Reservoir

Accurate determination of remaining oil saturation is critical for the successful implementation of enhanced recovery programs planned for the Yates field. Determination of oil saturation in this strongly heterogeneous, mixed-wet, and vuggy carbonate reservoir, however, is difficult. Conventional methods generally fail to produce robust results because of the variations in the Archie exponents m, and n, both in space and time. The saturation variations originate from the double displacement processes employed for improved recovery, which have generated a sizable gas cap and a moving oil/water contact.

Because of its direct reservoir fluid characterization capabilities, NMR logging was earlier identified as a potential alternative for the direct determination of remaining oil saturation at the Yates Field. A series of MRIL logs was run in 1998, where the main objective of the logging program was to exploit the diffusion contrast between oil and water. Results from this logging program were unsatisfactory because the apparent T₂ contrast between the oil and water phases was not sufficiently large to allow an accurate determination of the various fluid fractions.

An extensive modeling study was then initiated to study the feasibility of NMR logging for oil saturation measurements. Existing MRIL logs and limited laboratory data were used as input into the models. Modeling showed that interecho times in excess of 6 ms. could increase the apparent T₂ contrast between the oil and water phases. However, modeling also showed that such an increase in T₂ contrast would be accompanied by severely lowered T₂ resolution, which would in turn increase the uncertainty in computed saturations. Further modeling showed that the combination of higher magnetic field gradients and long interecho times could yield sufficient T₂ contrast without compromising T₂ resolution. It was then decided to modify an existing MRIL-C tool to achieve these objectives.

An observation well that was included in the 1998 program was relogged in late 1999 with the specially configured MRIL-C tool. Interpretation of the data acquired with this special tool and the remaining oil saturation determined from an NMR only technique agreed very well with the existing field model and petrophysical data. Expansion of the pilot study to a multiple-well evaluation program is currently being planned.