Cased-Hole Hydrocarbon Detection in the Permian Basin Using the Full-Wave Sonic Tool

Abstract

The detection of hydrocarbons through casing in low porosity formations is extremely difficult using traditional log analysis techniques. Pulsed neutron logs are usually ineffective for saturation determination in this environment. At low porosities, the thermal neutron capture cross section, Σ, is dominated by the large volumetric effects of the matrix; therefore, the Σ-contrast between hydrocarbon- bearing and brine-bearing formations is generally small and virtually undetectable. Location of hydrocarbons through casing with dual-detector neutron tools is also difficult. Such tools respond primarily to formation hydrogen (which is found mainly in the pore fluids), and the effects of gas and low porosity on tool response are very similar. Sonic tools with conventional transmitter-to-receiver offsets are routinely run in cased holes in hard rock areas and can yield valid formation interval transit time (Δt) measurements when there is excellent cement bond. Under these conditions, cased hole Δt and neutron porosity measurements can be crossplotted for lithology identification and gas detection. The Full Wave Sonic tool has been used successfully to detect hydrocarbons behind casing. In cased hole applications, this tool is free of many of the problems that plague sonic logging tools with conventional offsets. In other than perfect bonding situations, its longer offsets allow ample time for the casing and formation signals to separate, thereby permitting accurate determination of Δt. This is verified by comparisons of open and cased hole compressive and shear Δt measurements, Δtc and Δts. Additional test well data from an ultra-long- offset Full Wave Sonic tool that has an 18- foot (5.49-m) transmitter-to-first-receiver offset are presented to show this effect. To further expand the application of cased hole Full Wave Sonic data analysis to other than perfect bonding conditions, enhanced processing techniques employing time windowing and frequency filtering are available to reduce the effects of the casing signal. This paper emphasizes field examples in which hydrocarbons were successfully detected using Full Wave Sonic data. The presence of a compressible pore fluid, such as gas or oil with dissolved gas, causes a reduction in an acoustic velocity ratio and an increase in the attenuation of the acoustic signal. The reduction in the velocity ratio is significantly greater than variations that may arise from slight lithology changes. Successful production of light hydrocarbons has resulted when zones were selected for completion based upon velocity ratio reductions and acoustic signal attenuation over intervals of sufficient length.