Bond Log Theory and Interpretation. April 16th, Variable Density Log ( VDL). • Examples of good . Single CBL 3′ amplitude can not identify channeling. Properly run and interpreted, cement-bond logs (CBL) provide highly reliable estimates of Cement to Pipe Bond & Formation VDL. interpretation of this recently developed measurement and will detail case studies from Gulf of Mexico offshore wells comparing results from traditional. CBL /VDL.
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Cement bond logs –
The acoustic signal travels through borehole fluid, casing, cement, and the formation itself. Channeling is significant because it prevents a hydraulic seal. Conventional cement-bond logs CBLs comprise a pulsed transmitter and several receivers of acoustic energy positioned as a vertical array of transducers.
interpretatoon Track 1 contains the gamma ray for correlation and acoustic travel time for quality control. Acoustic logs do not measure cement quality directly, rather, this value is inferred from the degree of acoustic coupling cbll the cement to the casing and to the formation.
However, in foamed cements or when exotic additives are used, these calibrations can be inaccurate. The data relate to an interval that overlaps with the conventional CBL in Fig.
This has the effect of improving the signal strength of both the casing and cement-to-formation arrivals, respectively. Conventional CBL tools provide omnidirectional measurements, while the newer radial cement-evaluation tools provide azimuthally sensitive measurements for channel evaluation.
The height of the collar disturbance is a interpretatoon of measurement TR spacing amplitude, 3-ft interval; variable density, 5-ft interval courtesy of Baker Atlas.
These tools typically operate at higher frequencies than conventional openhole tools—between 20 and 30 kHz. The zigzag, wavy, or chevron pattern between these two arrivals is the formation signal indicating varying acoustic transit time.
Track 3 contains the CBL waveform, which indicates cement-to-casing bond as well as cement-to-formation bond. It deals with the problem of gas-contaminated cement, which has been a longstanding interpretation problem in the industry. Because the pads are in direct contact with the casing—in contrast to ultrasonic measurements—measurements are unaffected by:.
The most recent tools have a single rotating transducer that incorporates both the source and receiver of ultrasonic energy. The casing-evaluation presentation includes casing ovality, eccentricity, hole deviation, and gamma ray in Track 1. The resulting waveform is comprised of a casing signal and a formation signal; the casing signal arrives first, followed by the formation signal.
They operate on the principle that acoustic amplitude is rapidly attenuated in good cement bond but not in partial bond or free pipe. On the basis of empirical data, the log can be calibrated directly in terms of cement compressive strength. Transmitting elements and the firing sequence are controlled to direct steer and enhance the acoustic-energy output of both the pad transmitters and the VDL transmitter.
Fixed gates are set generally at the wellsite to open, remain open, interpretatino close at designated times; opening time for the gate is a function of the casing size and the borehole-fluid velocity.
If there is unset cement or water behind the casing, there will be a weak reflection. The pad spacing is such that the first arrival is the wave that has passed through the casing. Proper cement placement between the well casing and the formation is essential: The bonding between cement and formation is investigated through a CBL-type receiver array for wave-train presentation Fig.
Presentation of the full acoustic waveform assists in resolving bond ambiguities arising from use of an amplitude measurement alone and provides qualitative information about the cement-to-formation bond.
Cement bond logs
In an evolution similar to that of openhole acoustic logs, new designs were subsequently introduced that measured signal amplitude at a near receiver and a full waveform from a far receiver. In addition to specialized cement-bond devices, modern openhole array tools are designed to also provide conventional cement-evaluation measurements in cased hole.
A value of less than 1. One caveat regarding the use of the amplitude curve for bond evaluation: Measurements from these devices are influenced by the same factors as openhole televiewer devices. There is nearly free pipe above an apparent top of cement at a depth of approximately X80 depth units.
These tools use one or more azimuthally sensitive transducers to evaluate cement quality around the circumference of the casing. If the gate opening is too large, there may be interference between early and late-arriving signals.
There is poor cement condition between X and X depth units. Eventually, borehole-compensated devices using dual transmitters and dual receivers were introduced in the s, and today most commercial devices use multiple transmitters and receivers in a variety of arrangements to provide compensated measurements.
TR spacing typically ranges from 3 to 5 ft.
Pressuring the casing improves the acoustic coupling to the formation and the casing signal will decrease and the formation signal will become more obvious Fig. Variable density is a continuous-depth time display of full-waveform amplitude presented as shades of black and white.