Hydrogen-oxygen acoustic logging device

A hydrogen-oxygen acoustic logging device is provided for generating acoustic waves for logging a subsurface formation when arranged in a borehole through the formation. The device is electrically driven for electrolyzing water to a gaseous hydrogen-oxygen mixture and for firing the gaseous mixture for generating the waves. The device comprises an inflatable compartment to be inflated by the gaseous hydrogen-oxygen mixture. The device further comprises a housing having an outer wall facing and closely fitting to a borehole wall portion, and provided with a recess. The recess contains the inflatable compartment and, when arranged in logging position against said borehole wall portion, defines a substantially closed wave generating chamber.

FIELD OF THE INVENTION 
The present invention relates to a hydrogen-oxygen acoustic logging device 
for generating acoustic waves in a borehole. 
In particular the invention relates to a hydrogen-oxygen acoustic logging 
device for generating acoustic waves for logging a subsurface formation 
when arranged in a borehole through the formation, the device being 
electrically driven for electrolyzing water to a gaseous hydrogen-oxygen 
mixture and for firing the gaseous mixture for generating the waves, and 
the device comprising at least an inflatable compartment to be inflated by 
the gaseous hydrogen-oxygen mixture. 
BACKGROUND OF THE INVENTION 
U.S. Pat. No. 3,587,775 discloses a seismic acoustic source that generates 
sound by electrolytically dissociating water into gaseous hydrogen and 
oxygen. The hydrogen and oxygen are then allowed to explosively recombine. 
The explosive recombination results in the sound waves. The source has 
been built in a tubular housing hung on a wireline cable within a well 
borehole. 
A portion of the outer wall of the housing is made from a flexible 
material, thus defining an annular compartment. Surface equipment provides 
both dissociating power and the firing power. The ends of the portion are 
clamped firmly to the outer wall of the housing whereas longitudinal slots 
in the tubular housing wall permit waves generated to travel outwardly 
through the borehole fluid and into the formation. 
In such a way a relatively compact device is designed. The apparatus is 
operable and firable repetitively at various levels in a borehole without 
requiring the addition of any material from the surface, although many 
problems remain. 
For example, although firing, the dissociated hydrogen and oxygen will 
result in inflating the flexible wall portion throughout the slots. Thus 
the source cannot be considered pointlike. Moreover waves will propagate 
in all directions thus dividing power both in the direction of the 
borehole wall, and upwardly and downwardly into the borehole fluid. 
It is also a disadvantage that the device can be employed only in a 
separate logging procedure instead of in combination with drilling 
operations. 
U.S. Pat. No. 2,679,205 discloses seismic sound source generating waves 
similar to that disclosed in U.S. Pat. No. 2,679,775. In particular U.S. 
Pat. No. 2,679,205 discloses a sound source comprising a compartment for a 
gaseous hydrogen-oxygen mixture which is flexible as a whole. Although a 
more pointlike source device results, unpredictable sound direction 
components and disadvantageous power dissipation remain. 
U.S. Pat. No. 4,207,619 discloses another seismic well logging system and 
method. In particular, it discloses a source device arranged at the lower 
end of the drill bit string and capable of producing a sonic pulse. The 
pulses may be produced, for example, by induced vibrations, implosions, 
explosions, or sudden injections of fluid against the bottom of the 
borehole. The source device is driven and controlled, for example, by a 
sequence timer whereas pulses are generated whenever the drilling 
operation is temporarily halted to add a new segment of drill pipe to the 
drill string. Thus measuring is carried out while drilling in a 
semi-continuous way. As a further feature pulses at a single level may be 
summed to enhance the distinction between the pulses and the noise 
generated by the engines associated with the drilling operations. The 
above logging method uses surface detector systems to detect the waves 
generated which have passed the subsurface formation, thereby having been 
refracted and/or reflected, resulting in formation mappings and drill 
steering information. 
Conditions for arranging and driving a source device near the bit are very 
severe. High rotation speeds, severe drilling and grinding conditions, and 
noise will occur as a result of the drill bit hammering onto the borehole 
bottom. 
Although an overall method and related device for transmitting, receiving, 
and interpreting acoustical signal data from start to finish are presented 
in the above document, difficulties met in arranging such a device near 
the drill bit are not addressed. 
Thus, it is an object of the invention to present a method and a logging 
device enabling a substantially undisturbed and immediate transfer of 
acoustic power from the device into the subsurface formation. 
It is a further object of the invention to provide such a method and device 
of creating acoustic pulses at any moment and at any borehole level 
desired as long as the drill operations continue. 
It is another object of the invention to provide a method and a logging 
device to create pulses from near or on the drill bit which are clearly 
distinguishable from further noise, such as bit noise. 
It is yet a further object of the invention to drive and control the 
logging device continuously and reliably. 
SUMMARY OF THE INVENTION 
These and other objects are accomplished by a hydrogen-oxygen acoustic 
logging device for generating acoustic waves for logging a subsurface 
formation when arranged in a borehole through the formation, the device 
being capable of electrolyzing water to a gaseous hydrogen-oxygen mixture 
and firing the gaseous mixture for generating the acoustic waves, and the 
device comprising an inflatable compartment to be inflated by the gaseous 
hydrogen-oxygen mixture, wherein the device comprises a housing, having an 
outer wall facing and closely fitting to a borehole wall portion, and 
provided with a recess which contains the inflatable compartment and, when 
arranged in logging position against the borehole wall portion, defines a 
substantially closed wave generating chamber. 
In a first embodiment of the invention the housing is a blade of a drill 
string stabilizer unit. 
In a second embodiment of the invention the housing is a drill bit, 
preferably a junk slot of the drill bit. 
The recess advantageously comprises a bottom to which the inflatable 
compartment of the device is fastened. 
By employing the device as indicated above, in particular the wave 
generating chamber restricted to a well defined position within the 
borehole, a clear pointlike acoustic sound source for logging purposes is 
obtained. 
Furthermore, the sound source separated from the borehole fluid column 
present in the borehole advantageously results in a well defined power 
transfer to the subsurface formation.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1 a perspective view of a first embodiment of the logging 
device in accordance with the invention is shown. FIG. 1 presents a 
stabilizer unit 10 as part of a drill string assembly. As can be seen in 
the figure the stabilizer 10 is connected to a drill string 11 (only 
partly shown). The stabilizer unit 10 comprises a plurality of stabilizer 
blades 12, for example four blades as shown in this figure. Moreover the 
blades have outer walls which face the cylindrical borehole wall and which 
are closely fitting thereto. Consequently, indicated by its name, the 
blades are centralizing and stabilizing the drill string, in that the 
blades are closely contacting and sliding along a portion of the borehole 
wall, being substantially cylindrical. 
Advantageously a recess is arranged within such a blade 12. The blade 
functions as a housing for the recess, which is positioned per se adjacent 
to the cylindrical borehole wall, and which thus defines a substantially 
closed chamber. Within the chamber forming recess 13 an inflatable 
compartment 14 made of a flexible material is arranged. The compartment 
functions as a source for generating acoustic waves. 
As will be clear to those skilled in the art, although the recess mentioned 
above is arranged in a unit of a drill string assembly, a further 
embodiment of the present invention concerns a separate tool for logging a 
subsurface formation. The tool will also have the above explained outer 
wall/cylindrical borehole wall-geometry and can be positioned against the 
borehole wall in a suitable way. For example the tool can be clamped by 
means of a spring or a clamping device which comprises hinging arms, 
foldable outwardly and inwardly. 
Referring now to the FIGS. 2A to 2C details of the above embodiments are 
shown in more detail. 
In the cross-sections shown the stabilizer unit 10 is arranged in a 
borehole 21 which has been bored in subsurface formation 20. As can be 
seen in the FIGS. 2A to 2C the stabilizer blade 12 as shown is arranged 
against borehole wall 22 thus centralizing and stabilizing as mentioned 
before (for reason of clarity only one blade 12 has been shown; for the 
separate tool embodiment similar figures can be presented). 
When a drilling action is performed mud 23 will fill up the spacing left 
between the unit and the wall. 
In the cross-sections, the height of the logging device compartments 14 are 
shown empty in FIG. 2A, inflated in FIG. 2B and being fired in FIG. 2C. 
In FIGS. 2A to 2C the device defines a substantially closed wave generating 
chamber between the blade and the borehole wall. 
Compartments 14 contain an amount of water which is made electrically 
conductible by a small addition of an electrolyte. For example a certain 
amount of potassium hydroxide is added to make the water electrically 
conductible to a sufficient extent. A pair of electrodes (not shown) is 
positioned within the water in order to at least partly electrolyze the 
water to a gaseous hydrogen-oxygen mixture. In a further step the mixture 
is fired by means of a firing device also arranged within the compartment, 
resulting in an explosion. The explosion generates acoustic waves. After 
firing the hydrogen and oxygen will recombine to water and the wave 
generating cycle can be repeated. 
In FIGS. 2A to 2C conduits 24 guide electrical wires 25 to the compartment 
14. The wires 25 are connected to the above-mentioned electrodes and to 
the firing device. For example the electrical wires are coupled to the 
lower part of the compartment. 
The dimensions of the inflatable compartment are preferably chosen in such 
a way that the compartment is unstretched in the fired situation. Thus no 
work or energy is used for inflating the compartment against its own 
elasticity. Furthermore, because the wave generating chamber is 
substantially closed and the mud is incompressible, the impulse created by 
firing the gaseous hydrogen-oxygen mixture is transferred immediately into 
the formation. 
In FIG. 3 a second embodiment of the logging device in accordance with the 
invention is shown. In axial direction from the bottom side of the 
borehole 21 in the subsurface formation 20 a drill bit 30 at its outer end 
surface is presented. This is a bottom view of a drill bit containing a 
inflatable compartment. The drill bit 30 comprises bit blades 31. The bit 
blades, protrude from a hemispherical-like bit end surface. Cutter 
elements 32, mostly being elements of diamond, are arranged on the bit 
blades in order to bore the borehole by being rotated around its axis on 
the bit blades. Junk slots 33 between the bit blades 31 complete the outer 
end surface of the bit. 
The drill bit 30 continuously makes contact with the bottom of the borehole 
at one side for further cutting into the subsurface formation. The other 
side the circumference of the hemispherical-like end surface of the drill 
bit is closely fitted into the borehole against the borehole wall. In this 
embodiment a substantially closed wave generating chamber is defined 
between the surface of a junk slot 33, the respective bit blades 31 and 
the bottom part of the borehole. A recess 13 comprising an inflatable 
compartment is preferably arranged in a junk slot. Operation of such a 
logging device will correspond with that of the first embodiment. 
The inflatable compartment 14 is preferably fastened to the bottom of the 
recess 13. Thus, because the compartment is compressed by the hydrostatic 
pressure of the mud column within the borehole, damage of the compartment 
by scraping along the borehole wall is avoided. 
Because of the fact that the compartment has to be inflated against the 
hydrostatic compression forces it is made of a suitable flexible and 
elastic material such as rubber, a plastic material, or even a metal such 
as a sheet of a suitable metal, for example, steel. 
Dependent on the geometry of the recess an inflatable compartment formed as 
a bellows can be arranged. For example in the drill bit embodiment such a 
bellows can be fastened advantageously at the bottom of a hollow 
cylindrical recess. Furthermore, in an axially extending cylindrical 
recess or slot, having an upper and lower end and facing the cylindrical 
borehole wall, an elongated compartment such as an elastic tire section 
can be arranged and fastened with its end portions to said ends. 
In this field of application many ways of employing such waves are known. 
Two main applications are elaborated on as examples. 
Firstly, waves generated downhole can be detected by means of an array of 
detectors at the surface after the waves have passed through the 
subsurface formations. Wave characteristics so detected will result in 
mappings of the subsurface formation surrounding, and also beneath, the 
borehole concerned. These maps are obtained from direct travelling, 
reflecting and refracting waves. This technique is referred to as inverse 
vertical seismic profiling (IVSP). If this particular technique is used 
during a drilling operation it is called seismic while drilling (SWD). 
Secondly, waves generated downhole can also be detected downhole by means 
of one or more detectors relatively close to or even alongside the logging 
device of the invention. In that case information is obtained about the 
near source environment. For the situation that this technique is used 
during a drilling operation the data obtained could be useful for bit 
steering control. This technique is referred to as measuring while 
drilling (MWD), or logging while drilling (LWD). 
At least one receiver for detecting the above-mentioned acoustical waves is 
preferably included in the logging device of the invention. Additionally, 
in order to process the data obtained by the receiver a seismic data 
processing unit can be included. Such a data processing unit can further 
comprise or even can be made up of a memory unit. 
Particularly when the logging device is arranged in a unit of the drilling 
assembly, the device is preferably electrically driven by means of an 
in-situ arranged power generator. The power generator in turn can be 
driven by a mud motor. A mud motor driving a generator is preferred 
because this overcomes great problems presented when using wire lines 
during drilling due to the rotation of the drilling assembly. Because mud 
motors are widely available in many varieties effective matching and 
connecting to the device presented no major obstacles. 
The stabilizer device in accordance with the present invention is 
preferably used to permit more closely controlled steering of a drill bit 
and to prevent the possibility of generating tube waves (waves through the 
borehole column caused by reflections at the bottom of the borehole). 
Another preferred embodiment of the present invention includes a plurality 
of inflatable compartments arranged symmetrically around the housing. For 
example, two inflatable compartments may be arranged on opposite sides of 
the drill bit or stabilizer to permit symmetrical firing. Such a second 
device is electrically powered more economically. 
Widely known techniques such as mud pulse telemetry or electromagnetic 
pulsing can be used to transmit data to the device or from the device to 
the surface. Moreover, as soon as the drilling operations have been 
stopped, a wireline can be connected for further transmission of data, for 
example from the memory unit. A combination of mud motor driving and 
wireline transmission at the same time can also be applied.