Objective
It is proposed to develop a set of equipment and software which can be installed on an operating oil well drilling rig, which will measure, record and interpret those drilling-induced vibrations which are transmitted to the surface.
Laboratory experiments have shown that drill-bit vibrations can be interpreted to give information about the degree of wear of the teeth of a three-cone bit, and to detect incipient or complete failure of the cone bearings. It has also been shown that the vibration signature is modified by the quality of cleaning of the hole bottom, and by the nature of the formations being penetrated.
It is the intention now to apply this knowledge to the development of field-worthy hardware and interpretation methods.
The major results are :
- quantification of roller cone teeth wear from spectral analysis of vibrations (ref 2)
- detection and localization of interactions between the drillstring and the wellbore (ref 3)
1) Numerical modelling of the bit :
A numerical model of a roller cone bit has been used to predict the vibration spectra produced when rotating the bit at a uniform speed.
2) Validation of bit model :
The bit model has been validated with laboratory experiments under atmospheric conditions.
3) Signals generated by bit in downhole environment :
Bit vibrations have been recorded in a large number of full scale drilling machine experiments covering a range of downhole pressure and lithology conditions at elevated temperature. Bit signal is modified by lithology variations. Field data suggests that downhole vibrations are dominated by drillstring effects.
4) Modelling of drillstring transfer function :
Numerical models of the drillstring for axial and torsional modes of vibration have been developed and extended to include the effects of propagation loss. Model based techniques for removing drillstring resonances from surface vibration data have been developed.
5) Validation of drillstring models:
Prediction from the numerical models have been compared to surface vibration data collected in field experiments. Major spectral features in the data agree well with the models if the correct downhole boundary conditions are applied.
6) Surface measurement equipment :
Surface instrumentation to record drilling vibrations has been developed and tested in a number of field experiments. Specifications for an integrated sensor package have been developed.
7) Development of signal processing techniques :
A deconvolution technique to separate drillstring transfer function and downhole vibrations has been developed. Signal processing has been applied to provide images of drillstring/wellbore interactions and to analyse bit generated vibrations.
8) Interpretation:D rillstring resonances have been interpreted to indicate rough drilling and cone locking.
It is believed that surface vibrations contain information about the state of the drilling equipment, and about the nature of the formations being penetrated. Specifically, laboratory experiments have shown that it is possible to detect the state of wear of the drill-bit teeth, and to detect incipient or complete failure of the cone bearings. If the same interpretations can be carried out in the field, it will on the one hand become possible to leave the drill-bit at work until it has reached a predetermined state of wear, and on the other, it will be possible to detect failure of the bit bearings before cone loss occurs.
In addition, it has been seen that the vibration signature of the bit is modified by the quality of cleaning of the hole bottom : if these observations can be developed into a valid diagnostic method, it will be possible to optimise the hydraulic parameters so as to avoid both inadequate and excessive flow.
Laboratory and field experiments have also shown that the drilling vibrations are modified by the nature of the rock being penetrated. Correct and timely detection of a change in lithology is important for the selection of points at which to set casing, to detect the penetration of the bit into reservoir rocks, and in the avoidance of the damaging effects of drilling into over-pressured zones without adequate precautions (kicks and blow-outs).
The detailed phases of development are as follows :
1. To model the bit behaviour, so as to have an understanding of the nature and meaning of the initial signal
2. To validate this model by simple tests under atmospheric conditions, but using a real bit drilling real rock.
3. To verify that similar signals are generated by the bit when it operates in a downhole environment, and to understand them.
4. To model and understand the transmission of these signals from the bit to the surface through the drill-string.
5. By comparing simultaneous downhole and surface vibration records, to validate the drill-string model
6.
To develop the necessary surface measuring and recording equipment.
7. To develop and apply the correct signal-processing techniques in order to extract meaningful information from the vibration signals.
8. To interpret these signals in terms of changes in bit status or operating conditions, or changes in lithology.
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Programme(s)
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Coordinator
CB3 0EL CAMBRIDGE
United Kingdom
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