Acoustics – Geophysical or subsurface exploration – Well logging
Patent
1996-11-11
1998-08-25
Eldred, J. Woodrow
Acoustics
Geophysical or subsurface exploration
Well logging
181106, 181108, 181113, 7315203, 7315216, 175 50, G01V 140
Patent
active
057984883
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to drilling apparatus.
When drilling, and particularly when drilling for oil, it is desirable to know what strata will be drilled through at any time in order to allow appropriate drilling parameters to be employed, it is also desirable to have a detailed knowledge of the position of the drill bit relative to anticipated or known features of the rock strata and to allow geosteering. One particular concern in oil drilling is the identification of over pressure regions in the strata ahead of the drill in order to allow the fluid pressure within the borehole to be adjusted to the minimum safe level. Having the minimum safe fluid pressure is desirable because although too low a borehole fluid pressure, known as underbalancing, produces the danger that is a gas overpressure region is penetrated the resulting release of gas into the borehole, known as a blowback, can result in loss of the borehole and drill rig, too high a fluid pressure, known as overbalancing, can damage the borehole and reduce its oil production capability.
Thus in operation it is necessary to balance the risks of under or over balancing the borehole fluid pressure and to do this accurate data on the presence of overpressure regions is needed.
There have been two general approaches to this problem, the first has been the use of seismic surveying from the surface to map underground geological features combined with the use of inertial and magnetic sensors near to the drill bit and dead reckoning to allow the position of the drill bit relative to these features to be tracked. Unfortunately this approach suffers from the drawback that the range accuracy and spatial resolution of geological features deep underground obtained by using surface based surveying is very low due to the distances involved and the fact that the speed of soundwaves through underground strata is dependent on their makeup. The second approach has been to place acoustic and other surveying instruments inside the drill string in order to sense the rock strata around the drill during the drilling operation. This is generally termed "measurement while drilling" or "MWD" although in practice the actual drilling operation may be paused while measurements are being taken. This approach suffers from the drawback that although the rock strata around the drill can be sensed this information must then be used to deduce what geological formations are ahead of the drill bit.
A variation on this second technique is disclosed in WO 93/07514, in which the drill bit itself is used as a noise source whilst the drill bit is actually rotating.
As a result there is always considerable uncertainty in such systems regarding the makeup of the formations directly ahead of the drill bit since it is not always possible to accurately deduce or extrapolate what these formations will be.
It would of course also be possible to extract the entire drill string from the borehole and send down a wire line tool including an acoustic sensor which would acoustically survey through the bottom of the borehole to directly survey the rock formations beyond, but the time and cost penalties associated with extracting the drill string to do this are very high compared with a MWD survey conducted directly from the drill string, so this is an undesirable procedure.
This invention was intended to produce an acoustic sensor within the drill string overcoming these problems, at least in part.
The present invention provides drilling apparatus comprising a drill bit, an acoustic transducer for generating acoustic signals in rock about a borehole formed by the drill bit, and means for receiving acoustic signals returned from the rocks, characterised in that the acoustic transducer is connected to the drill bit such that the transducer generates acoustic signals in the rock ahead of the drill bit by vibrating the drill bit.
Preferrably a compliant device is provided for connecting a drill tool and the drill bit including an acoustic sensor to a drill collar and having a compl
REFERENCES:
patent: 4207619 (1980-06-01), Klaveness
patent: 4991685 (1991-02-01), Airhart
patent: 5248857 (1993-09-01), Ollivier
patent: 5309405 (1994-05-01), Brett et al.
Beresford John M.
Crowther Paul A.
Eldred J. Woodrow
GEC - Marconi Limited
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