The acquisition of high-quality logging-while-drilling (LWD) cased-hole borehole sonic data in real-time (RT) and in memory (RM) dramatically improves drilling efficiency. RT sonic waveform amplitudes and slowness-time-coherency (STC) measurements enable qualitative cement evaluation either while running in hole (RIH) or when pulling-out (POOH). Operators can make appropriate decisions on cement quality to either continue drilling or perform remedial work. Data acquisition is transparent to drilling operations, removing wireline logging that can contribute to additional time and cost.
Furthermore, with RM data a quantitative cement evaluation can be performed, and when environment conditions are adequate, formation compressional slownesses through casing can be computed.
The advent of the latest LWD sonic acquisition technology and related processing techniques has significantly reduced the challenge of cased hole LWD sonic acquisition. More powerful sonic transmitters, improved receivers, altered transmitter-receiver spacings, and fundamental changes in tool design have meaningfully improved the acoustic signal-to-noise ratio. An improved understanding of LWD cased-hole borehole acoustic modes, the ability to transmit acoustic energies at more optimal frequencies, and the capability to simultaneously acquire cement evaluation information have all contributed to improved LWD cased-hole sonic logs.
Cementing is essential to well integrity, it supports casing and provides zonal isolation. Historically, quantitative cement evaluation has been acquired via wireline (WL) tools, the most common being the cement bond log (CBL), a principle based on the amplitude of casing arrivals. LWD sonics have historically been used for top of cement (TOC) logging, which shows cement presence or absence behind casing and is a quantitative cement evaluation. Contrary to WL CBL tools, LWD tools have a steel collar that permits acoustic propagation. As such with LWD tools it has been difficult to separate casing signals from those in the tool collar (Kinoshita, 2013). In poorly bonded conditions, the casing amplitude is much larger than the collar arrival. In well-bonded conditions, the casing signal is weak and can be less than the collar arrival.
From multiple cased wells in Deep Water Offshore West Africa, this paper demonstrates the application and results for the latest LWD sonic tool and processing techniques for the following;
RT TOC evaluation while RIH before drilling out the casing shoe
Quantitative cement evaluation from tool memory waveform data
Formation compressional slowness through casing
Approximately US$450,000 saving compared to wireline runs