Abstract: Hydrocarbon Detection behind Casing in the Wilmington Field, CA

MOOS, DANIEL, and SCOTT WALKER

As part of a project co-funded by the Department of Energy (DOE PON #PS22-94BC14972), we have logged a number of wells through casing to detect bypassed hydrocarbons in the Wilmington Field, Los Angeles, CA.

Two techniques have been employed to identify bypassed oil. In the first, monopole and dipole acoustic waveforms have been recorded to obtain compressional and shear wave velocities. Because oil in situ is more compliant and less dense than brine, the Vp/Vs of an oil-saturated sediment will be lower than that of a brine-saturated sediment with the same shearwave slowness dts. The second technique uses pulsed neutron logs to determine the chemical composition (specifically, the ratio C/O) in the formation. A high C/O indicates the presence of hydrocarbons; a low ratio indicates their absence. A benefit of the C/O technique is the direct detection of the chemical signature of oil, but it is sensitive to near-wellbore effects. Although the acoustic technique has a significantly greater depth of investigation, it relies on a more indirect assessment of hydrocarbon saturation.

Both techniques showed promise. Although acquisition of the acoustic data was difficult due to the characteristics of the formation and of the wells, the results agreed qualitatively in one well with open-hole logs, and clearly differentiated in two wells between water-flooded and non-waterflooded sands. C/O log saturations agreed qualitatively with open-hole data and previous station measurements in an adjacent well. In a short section in which both logs were obtained, the results were in qualitative agreement.

Search and Discovery Article #90945©1997 AAPG Pacific Section Meeting, Bakersfield, California