Objective
The aim of the project was to develop a variable frequency electrodynamic downhole seismic energy source capable of generating signals up to and above the frequencies conventionally achieved with surface based technologies.
A pair of suitable concepts have been proposed, an electro-magnetic tool and as an alternative, a servo-hydraulic source. The former is the preferred option. A preliminary design has been prepared and this is still being refined to improve its performance. A 10 cm diameter device is proposed, this having the widest range of applications. Increasing the diameter rapidly increases the force which may be generated. The design constraints are those of producing sufficient magnetic flux in the confined space, solving the thermal environment and source thermal load problems, getting sufficient primary electrical power downhole using existing industrial hardware and providing a sufficiently stiff clamping device for the borehole coupling.
The source requires, typically, about twice as much primary power downhole than has been tried before. It is believed that we have obtained solutions to these problems.
A commercial seismographic processing package has been adapted for use in the cross-hole and walk-away configurations and agreements with the owner obtained to develop the system as required. A synthetic seismogram software package has been written and implemented on an IBM-PC.
The prototype unit was successfully used under laboratory conditions to frequencies in excess of 600 Hz. While a number of improvements can be made to improve the efficiency of the device, it is proposed that the development moves on to shallow field testing where the seismic performance of the device can be investigated. The work and hardware necessary to proceed to downhole testing has been identified and operators and contractors are being consulted in order to establish the industries requirements and the best way to obtain the performance of the source in the field. When the device has been demonstrated in the field the power loss and clamping arrangements will be further developed.
The prototype unit was successfully tested under laboratory conditions to frequencies in excess of 600Hz. While a number of improvements could be made to improve the efficiency of the device it was considered that the project was ready to move onto Stage 4, where the seismic performance of the device could be evaluated. Hardware and effort necessary to proceed to downhole testing was identified and operators and contractors consulted on the best way to proceed. While the response was not encouraging due to the power consumed and the use of non-standard cable, there does seem to be a requirement for such a device within the industry, however, the impetus for any further industry sponsored development would have to await improved market conditions. Field trials were not conducted because seismsic activity in the UK at the moment is at an all time low, and due to the difficulty of finding a suitable bore hole test site.
Main problems remaining seem to be temperature and energy loss due to the generation of tube waves up to hole. Ways should also be found to reduce the input power requirements.
The work is split into stages A to F as defined in the work programme. Briefly, these are split into 4 groups :
1. A preliminary study phase which includes a review of existing techniques, defining the desirable specification for such a device and proposing some technical solutions. This first stage also includes identifying the software needed to support such a source and adapting such software to the type of surveys for which such a machine would be used. Stage 1 comprises phases A and D of the work programme.
2. Technical design phase, in which the detailed engineering design of the downhole source and the sensor investigation is undertaken, phases B and E of the work programme. Phase B includes a shallow depth field test.
3. Construction and laboratory testing of a prototype source and its associated control equipment.
4. Field trial and operational evaluation of the source.
Each phase will produce a report.
PERFORMANCE ACHIEVED
Based on Prototype Laboratory Tests
Dimensionsoutside diameter 0.01 m
length (source alone) 0.25 m
Mass (Approx) 16 kg
Drive Amplifier rating 15 kVA
max. current 35 Amp
voltage 428 Volts
Cable outside diameter 5.1 to 6.8 mm
(multi-strand power
conductor pair 6 to 10 mm2)
Temperature rate 0.5 C/sec
maximum 100 to 150 C (magnet limit)
Cooling passive
Maximum Operating Depth 2000 m (initially)
Force Amplitude 500 to 1000N
Frequency Range 50 to 600 Hz
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Coordinator
G2 8JE GLASGOW
United Kingdom
The total costs incurred by this organisation to participate in the project, including direct and indirect costs. This amount is a subset of the overall project budget.