Extracting More Value From Basic Cased Hole Services – Deriving Confident Formation Slowness Measurements From Simple Cement Evaluation Services

Abstract

Measuring the changes in the formation acoustic slowness over the life of a well can give valuable insight into changes in formation properties with time. However, once a well has been cased and completed, measuring formation slowness with conventional open hole acoustic tools can be impractical due to rig and completion factors preventing their deployment and access to the formations of interest. Acoustic Variable Density Log (VDL) cement bond (CBL) data are commonly recorded in wells over their lifetimes to monitor cement integrity, often as a component of more advanced sectored services. These tools are usually more compact than open hole acoustic logging tools and often not subject to the deployment challenges associated with open hole tools. CBL tools record full waveform data from two closely spaced receivers (or transmitters), traditionally located 3ft and 5ft from the transmitter (or receiver), cement bond being estimated by measuring the amplitude of the received waveform at the expected casing signal arrival time compared to a known unbonded interval of casing. The recorded wavefield can contain both casing and formation signals. However, the formation signals, not being the signal of interest, are often disregarded. With an appropriate methodology, these can be extracted and quantified to provide a formation slowness. First the casing arrival is minimised and formation arrival enhanced by application of specialist filtering including frequency band pass and depth based pseudo-FK: The formation compressional arrivals can then be isolated in the receiver wavefields. With knowledge of the tool and borehole geometry, the through casing formation compressional can then be estimated. We demonstrate that in cased wellbores acoustic slowness can still be derived where CBL data is available. The methodology provides a confident estimation of formation compressional slowness, showing close agreement to open hole slownesses. Where differences are observed these can often be attributed to changes in the formation in the time since the original acoustic data was logged; comparison to other cased hole measurements supporting these observations. We present a case study where a radial cement bond and two receiver CBL VDL log was recorded in a North Sea monitoring well. During evaluation of the well, an acoustic slowness measurement was desired for quantifying changes in the properties and fluids of the formations of interest. The goal for this logging was calibrating 4D seismic observation to well observation. However, because of rig limitations, logging a conventional acoustic tool was determined to be impossible or extremely costly. By application of this methodology to the CBL data a confident formation slowness could be derived for further interpretation such as baseline for time lapse acoustic measurement for use with yearly acquired seismic data.