Acoustic Logging through Casing to Detect Hydrocarbons and Determine Porosity in the Wilmington Field, CA
Daniel Moos, Andrew Hooks, and Scott Walker
The Wilmington Field, located in the Los Angeles Basin, CA, is composed of relatively unconsolidated turbiditic sands waterflooded for more than 40 years. As is common in this and other oil fields in California and elsewhere, considerable bypassed oil remains in place. The water-oil ratio from one well selectively completed in high-oil- saturation sands is significantly lower than the water-oil ratio in adjacent wells.
We have begun a 2-year test program to identify sands with high remaining oil saturations by logging old cased wells using a high power low frequency acoustic logging tool as part of a project co-funded by the Department of Energy (DOE PON #PS22-94BC14972). Formation compressional-wave velocity is obtained from monopole data. Formation shear-wave velocity is obtained by analyzing dipole wave modes. In some wells, however, problems associated with poor cement-casing cement-formation bond, casing eccentricity in the well, and tool eccentricity in the casing make it difficult to detect the dipole mode.
Where good data has been obtained, compressional-wave velocities determined in the same cased hole from logs recorded by two different companies agree quite well, as do open- and cased-hole compressional and shear-wave velocities. Porosities determined through casing using shear-wave velocities match conventional open-hole log values. Saturations determined from the velocities and their ratio are similar to those calculated using Archie's Law. Relationships between porosities, saturations, and velocities are consistent with theoretical expectations.
The results indicate that it is possible to determine porosity and saturation through casing using acoustic methods even in relatively unconsolidated sands such as those found in the Wilmington Field, provided sufficiently good monopole and dipole waveforms can be obtained.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California