Downhole seismic energy source

A downhole seismic energy source for use in generating a seismic signal in a borehole including an elongated mass having a generally cylindrical surface of diameter less than the borehole, an actuator suspending the cylindrical mass in the borehole wherein the cylindrical surface thereof is in frictional contact with the borehole wall, the cylindrical mass being rotatable about its rotational axis to cause the mass to whirl within the borehole in the direction opposite the direction of the rotation of the mass, the mass cylindrical force serving to create a seismic energy signal.

BACKGROUND OF THE INVENTION 
The invention disclosed herein has the purpose of providing a powerful and 
controllable downhole seismic source that can function in either a cased 
or an open borehole. One of the problems with many previous downhole 
seismic energy sources has been their limited power. A seismic energy 
source with more power will allow fewer measurements to be taken to 
seismically characterize the subsurface, saving time and money. In 
circumstances where oil or gas well spacing is large, greater seismic 
energy power means greater range. The improved downhole seismic energy 
source of this disclosure will allow investigation of subterranean 
structure between wells that cannot be accomplished with presently 
available downhole seismic energy sources. 
It is important that seismic energy be generated at a known and 
controllable frequency. The most powerful seismic energy sources currently 
available are limited in use to only cased holes. This limitation can make 
it difficult and expensive to use well borehole to well borehole seismic 
surveys in open boreholes. For these and other reasons, a more powerful 
and yet controllable downhole seismic sources that can be used in either a 
cased or an open borehole is needed. 
For information relating to others who have provided methods and 
apparatuses for generating downhole seismic energy, reference may be had 
to the following previously issued U.S. Pat. Nos. 3,718,205; 4,252,210; 
4,671,379; 4,702,343; 4,722,417; 4,783,771; 4,796,723; 4,805,725; 
4,805,727; 4,815,557; 4,834,210 and 4,856,614. 
SUMMARY OF THE INVENTION 
Basically, the downhole seismic energy source of this disclosure is a 
cylindrical mass that is caused to backward whirl about a borehole. 
Backward whirling motion is identical to the motion of a pinion rotating 
in an internal gear or that of a "spiro-graph" child's toy. One salient 
feature of the motion is that it creates very large centrifugal 
accelerations that allow a relatively small mass to create a very large 
force. For example, a 4.5" diameter steel mass 10' long, inside 5" ID 
casing will create about 24,000 pounds of sinusoidally varying force at 
41.7 Hz when made to backward whirl by rotation at 250 RPM. 
In the seismic energy source of this disclosure the power necessary to 
drive the backward whirling motion is provided by either a mechanical 
linkage to the surface, i.e. a string of tubing or dill pipe, or by a 
downhole motor, either electric, hydraulic or pneumatic. An electric motor 
offers the advantage of being finely speed controllable from the surface, 
but its power is limited by size constraints dictated by the borehole 
size. A hydraulic motor, on the other hand, can deliver 5 to 10 times more 
power per unit volume than an electric motor. 
Systems required to maintain controllable rotational speed for electric or 
mechanical drive systems are well known in the art. A system to measure 
the frequency of the backward whirling seismic source used in combination 
with a downhole hydraulic motor is provided. To determine the position, 
velocity, or acceleration of the seismic source, an accelerometer can be 
placed on the motor near the cylindrical mass. This allows the drive 
frequency of the seismic source to be known through either electronic 
recording means downhole, or by transmission to the surface by mud 
telemetry, or via an electric conductor. This signal can then be used to 
control the speed of the driving motor, and thus the frequency of the 
seismic source. 
Control of a driving motor can be accomplished in a number of ways. The 
surface pumping rate of a positive displacement pump can be controlled to 
vary the pumping rate in response to the desired range of frequencies of 
the seismic source, which may be either constant or varying with time. A 
positive displacement pump can control the speed of a positive 
displacement downhole motor in proportion to the pump's speed. It is also 
possible to use a turbine or other type of downhole motor to drive the 
whirling cylindrical mass. 
Even in cases where a positive displacement downhole motor is used as the 
drive source, it is possible that compressibility of the circulating 
fluid, uncertainties in the actual diameter of the borehole, leakage in 
the motor or pump, and pressure expansion effects in the tubing may 
prevent adequate control. In that case, a portion of the circulating 
medium can be shunted past the motor to provide a way to control the 
excitation frequency of the whirling cylindrical mass. The pump at the 
surface is controlled to provide a greater flow than necessary to achieve 
the desired excitation frequency, and this flow may change with time. The 
measured excitation frequency is then compared with a desired value. The 
measured frequency can either be transmitted from the surface or produced 
by downhole electronic systems, as is well known in the art. The downhole 
shunt valve is then operated to port fluid away from the positive 
displacement motor so as to maintain the measured excitation rate at the 
desired level. The downhole shunt valve may throttle fluid at the desired 
level. The downhole shunt valve may throttle fluid (i.e. act as a 
proportional control valve) or it may act in a fully opened or fully 
closed manner and to control the amount of fluid shunted past the motor by 
duty cycle modulation. 
The outer surface of the whirling cylindrical mass can be controlled to 
create a high friction between the mass and either the borehole wall or 
the inside of a cased hole to promote development of the whirling motion. 
In a cased hole a high friction surface can be rubber with a tread that 
reduces any slipping tendencies due to fluid in the well. In an open hole 
operation the surface may have steel ribs, studs or such, in a rubber 
matrix to promote high friction between the side of the borehole and the 
whirling cylindrical mass. 
The motor may be hung in the borehole with "slip" type elements so as to 
isolate the accelerations experienced by the whirling mass from the tubing 
string, in which case the use of a U-joint or flexible coupling is 
necessary. Such a hanging system can also be used in conjunction with the 
driver power being transmitted from the surface by the tubing string. In 
this case the hanging system can have a gearing system to change the 
rotational speed of the drive string to a speed that is optimum to drive 
the whirling cylindrical mass. 
A "starter spring" system can be used to ensure that the whirling 
cylindrical mass is in contact with the borehole or the sidewall of a 
casing. A starter system ensures that a self-regenerative whirling motion 
begins in every circumstance. Basically, the starter system biases the 
position of the cylindrical mass to one side of the borehole or casing so 
that contact is ensured to allow an initial force to be created between 
the mass and the borehole wall. After rotation is initiated the 
regenerative nature of backward whirling motion ensures that frictional 
contact is continued. The starter system can be constructed to be 
retracted when whirling motion is started. 
In one embodiment of the concept, the whirling cylindrical mass is never 
actually in contact with the borehole or casing. In this embodiment, the 
whirling mass acts as a pinion and the "contact" gear is an internal gear 
that is anchored in the open borehole or casing by means of "slips" or 
other systems. This embodiment has the advantages of eliminating the need 
to create friction between the whirling cylindrical mass and the borehole 
wall, ensures a known diameter of whirl, and also provides greater contact 
area so that less stress is placed on the borehole. 
The downhole seismic energy source of this disclosure is different than an 
eccentric mass in several important ways. First, an eccentric mass must be 
rotated at its excitation frequency. This means that high speed motors are 
required to achieve practical excitation frequencies. With the whirling 
cylindrical mass of this disclosure, however, the physics of whirling 
itself magnifies the frequency of excitation. This means that lower speed 
downhole motors can be used as a power source. Second, an excitation force 
of an eccentric mass must be transmitted through some kind of drive shaft. 
This means that the forces possible are limited due to physical stress 
limits in practical sized drive shafts. With a whirling cylindrical mass, 
however, the excitation force is supported by the borehole itself. This 
means that much greater excitation forces are possible. 
A better understanding of the invention may be obtained with reference to 
the following description and drawings, taken in conjunction with the 
attached views.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, the basic concept of this invention is illustrated 
diagrammatically and is illustrated for use in an open borehole. The 
concept of FIG. 1 can also be used in a cased borehole. A borehole 10 
extends from the earth's surface, as drilled in the usual manner for 
drilling oil or gas wells. The object of the present invention is to 
generate a seismic energy signal, that is, to generate pressure wave 
signals that travel in the earth and that can be detected by geophones 
placed on the earth or in adjacent boreholes. Through the science of 
seismology the detected seismic waves can be analyzed to provide 
geologists with important information concerning the structure through 
which borehole 10 traverses and the structure surrounding the borehole. 
Positioned within the borehole is an elongated cylindrical mass 12 having 
an external cylindrical or nearly cylindrical surface 14. The external 
surface 14 could be rough or have ribs or other non-cylindrical features. 
The mass is rotated by a rotational means 16 in a manner to be described 
subsequently. Cylindrical surface 14 is in contact with borehole wall 10. 
FIG. 2, a cross-sectional view of FIG. 1, illustrates the concept of this 
disclosure. Assuming cylindrical mass 12 is rotated clockwise in the 
direction indicated by arrow 18, the mass, due to frictional contact with 
the borehole wall, will migrate or "backward whirl" in a counterclockwise 
direction, that is, the direction opposite of that of arrow 18. After an 
incremental time the mass will take the position as indicated by 14A. The 
mass will continue to rotate in a counterclockwise direction, whirling 
around the interior of borehole 10. 
Assuming no slip at the contact point 20 between external surface 14 of the 
cylindrical mass and borehole wall 10, the whirling mass will contact each 
point on the borehole wall, such as at point 20 at a frequency rate 
according to the formula: 
##EQU1## 
Where "f" is the excitation frequency in Hz; Where "Dw" is the diameter of 
borehole 10; 
Where "RPM" is rotations per minute; 
Where "P" is the difference in the diameter of the borehole 10 and the 
diameter of the cylindrical mass 12 (P=Dw-D); 
Where "D" is the diameter of the cylindrical mass 12. 
As an example, if cylindrical mass 12 has a diameter of 4.5 inches and 
borehole 10 has a diameter of 5 inches or, instead, if the borehole is the 
interior of a casing, a sinusoidal varying force at 37.5 Hz will be 
created by the backward whirl of the cylindrical mass when it is rotated 
at 250 RPM. 
The contact force of cylindrical mass 12 against each point of contact with 
borehole wall 10 is expressed according to the formula: 
##EQU2## 
Where "Fc" is the force of contact; Where "W" is the weight of the 
cylindrical mass. 
The centrifugal acceleration in "g's" of the whirling mass is determined by 
the formula: 
##EQU3## 
If in the example given above wherein cylindrical mass 12 is 4.5 inches in 
diameter and is formed of steel and is 10 feet long, and with a diameter 
of borehole 10 equaling 5 inches, at 250 RPM the whirling mass would 
create about 19,440 pounds of sinusoidal varying force at 37.5 Hz. 
FIG. 3 shows more details of the typical application of the concepts of 
FIGS. 1 and 2. Positioned within borehole 10 is a tubing string 22 
extending from the earth's surface 24 and suspended by a surface system 26 
of the type typically employed for drilling or working over an oil or gas 
well. The surface system 26 may include a source of rotary energy 28 of 
the type such as used for rotating a drill string during drilling 
operations which may be adapted to be rotated at higher speeds. By 
rotating tubing 22 cylindrical mass 12 can be rotated to generate a 
seismic energy signal in the method as previously described. 
Instead of rotating tubing string 22 a rotary power source 30 may be 
suspended by the tubing string and connected to cylindrical mass 12, such 
as by means of a coupling member 32. The rotary power source 30 may be an 
electric power, either DC or AC, or a downhole hydraulic motor. When the 
rotary power source is a downhole hydraulic motor, a source of hydraulic 
power 34 is provided at the earth's surface and connected to tubing string 
22 by which fluid under pressure is forced downwardly through the tubing 
to the rotary power source 30. The fluid passes out of the rotary power 
source and into the well annular area 36 and back to the earth'surface. 
FIG. 4 shows one means of providing a rotary power source 30 of the 
hydraulic type. In this type, the rotary power source is a positive 
displacement downhole motor 38 affixed to tubing string 22. The motor 38 
has a shaft 40 connected to coupling member 32 that may be in the form of 
a flexible coupling or universal joint by which the rotary energy from 
positive displacement motor 38 is coupled to cylindrical mass 12. 
As fluid is forced down the interior of tubing string 22, it passes through 
the positive displacement motor 38, causing shaft 40 to rotate, the fluid 
returning in the annular area 36 back to the earth'surface. In some 
applications it may be important to control the RPM of the whirling 
cylindrical mass 12 so as to control the frequency of the seismic energy 
signal being generated. This can be accomplished in a variety of ways. In 
one way as illustrated in FIG. 4, a shunt bypass valve 42 is positioned in 
communication with the interior of tubing string 22 above the rotary power 
source 30. By means of a conductor 44 extending to the earth's surface, 
electrical signals may be applied to shunt bypass valve 42 to control the 
opening and closing of the valve. When valve 42 is opened, or partially 
opened, some of the hydraulic fluid flowing downwardly through tubing 
string 22 is diverted directly into the well annulus 36 and therefore does 
not flow through positive displacement motor 38. Therefore, it can be seen 
that by bypassing a portion of the fluid flow the rotary force generated 
by positive displacement motor 38 can be altered, to thereby alter the RPM 
of rotation of cylindrical mass 12. 
Another way of controlling the RPM of cylindrical mass 12 when rotated by a 
positive displacement motor, as illustrated in FIG. 4, is by controlling 
at the earth's surface the rate of fluid output of the hydraulic power 
source 34, as shown in FIG. 3. Thus, in summary, the rate of rotation of 
cylindrical mass 12 and therefore the frequency of the seismic energy 
signal can, when the power source is hydraulically actuated, be controlled 
by means at the earth's surface or downhole to achieve the desired 
frequency of the seismic energy signal. 
To determine the frequency of the seismic signal generated by the whirling 
cylindrical mass 12 an accelerometer 46, or other frequency sensing 
device, may be affixed to the lower end of the tubing string or on the 
whirling mass. By a conductor 48 extending to the earth's surface a signal 
can be delivered to indicate to operators at the earth's surface the 
frequency of the signal so as to be able to control the frequency to 
obtain that which is desired. 
Referring back to FIG. 3, as previously stated, rotary power source 30 may 
be an electric motor supplied by electric energy over cable 50 extending 
to the earth's surface and connected to a source of electrical power 52. 
As referenced in FIG. 4, the measured frequency of the seismic signal 
generated by whirling cylindrical mass 12 can be detected by accelerometer 
46. FIG. 5 shows a means of using such detected measurement to attain the 
desired frequency of the seismic energy signal. The measured signal 
appears on cable 48 as previously described. This signal is fed to control 
electronics 54 having a frequency selector 56 input by which the desired 
frequency of the required seismic signal is selected. The selected 
frequencies could change with time. The control electronics 54 compares 
the desired frequencies selected at 56 with the detected frequency 
appearing on conductor 48 and generates an output signal at 58 that can be 
connected, such as to conductor 44, to control shunt valve 42. 
Alternatively, output signal 48 can be used to control the source of 
hydraulic power 34 as shown in FIG. 3. Where rotary power source 30 is 
electrically operated, control signal 58 may be used to control either the 
voltage, if the rotary power source is a DC motor, or the frequency of the 
power signal if the rotary power source is a AC motor. 
With tubing string 22 suspended in a borehole or a casing as shown in FIG. 
3, and with a rotary power source 30 at the lower end of tubing string it 
can be seen that the tubing string would be subject to substantial 
vibration as power is applied to rotate cylindrical mass 12. An alternate 
arrangement is illustrated in FIG. 6 which shows a lower end portion of 
borehole 10 with tubing string 22 extending from the earth's surface. 
Positioned at the lower end of tubing string 22 is a borehole anchor means 
60. This may be in the form of slips or a hydraulic mechanism as is 
commonly employed in the oil and gas well drilling industry. The rotary 
power source, such as positive displacement motor 38, is supported below 
borehole anchor means 60. Flexible coupling member 32 extends from the 
positive displacement motor shaft 40 to connect to the whirling 
cylindrical mass 12. Thus, the borehole anchor means serves to anchor the 
lower end of tubing string 22 to prevent undue vibration of the tubing 
string as the whirling cylindrical mass 12 is rotated. It is also possible 
to have motor 38 positioned above anchor means 60 with shaft 40 extending 
through the anchor means to connect to flexible coupling member 32. 
In order for the whirling cylindrical mass 12 to rotate within the borehole 
or casing, cylindrical surface 14 must be in frictional contact with 
borehole wall 10, or the wall of the casing if operated within a casing. 
It can be seen that if the cylindrical mass is merely rotated within a 
borehole, no frictional contact is established between the rotating mass 
cylindrical surface and the interior of the borehole, that is, the mass 
will not whirl within the borehole to create a seismic source. For this 
reason, as illustrated in FIG. 7, a biasing force, exemplified by a bias 
spring 62, may be employed. The function of bias spring 62 is merely to 
hold cylindrical mass 12 so that cylindrical surface 14 is in frictional 
contact with borehole wall 10. In this matter, when rotary energy is 
applied to cylindrical mass 12 it will migrate in a direction opposite of 
its direction of rotation around borehole wall 10. Once the whirling 
migration is initiated the centrifugal force applied by the rotating 
cylindrical mass is such as to sustain the frictional engagement of the 
cylindrical mass with the wall. The only time the biasing force, as 
exemplified by biasing spring 62, is required is at the initiation of the 
rotation of the cylindrical mass to make sure that it is in frictional 
contact with borehole 10 and once the rotation is initiated, biasing 
spring 62 is no longer required. 
In order to increase the frictional contact of the whirling mass 12 with 
borehole 10, the whirling mass cylindrical surface 14A may be roughed, 
ribbed or otherwise provided with a pattern, such as protruding diamond 
shaped projections as shown in FIG. 7. This arrangement reduces slippage 
between the surface of whirling mass and the borehole so that the backward 
whirl of the mass occurs at a more predictable rotational rate. 
Another means to increase the frictional engagement of rotating cylindrical 
mass 12 with the interior of the borehole and thereby provide a more 
certain whirling diameter is illustrated in FIG. 8 which shows the 
employment of a tubular base member 64. The external cylindrical surface 
66 of the tubular base member fits in close contact with borehole 10. 
Tubular base member 64 may include a wall anchor system that may be 
hydraulically or mechanically actuated, such as to anchor the tubular base 
member to borehole 10. The tubular base member 64 may be made of hard 
rubber, polyurethane or other material that provides an interior 
cylindrical surface 68 having a high co-efficient of friction surface. 
With tubular base member 64 properly positioned within borehole 10, 
cylindrical mass 12 will whirl with less slippage as it is rotated. 
While tubular base member 64 may be a smooth high friction interior surface 
68 an alternate arrangement, as illustrated, includes the provision 
wherein the tubular base member includes a tubular contact gear member 70 
having teeth 72 on the interior cylindrical surface. The exterior 
cylindrical surface 14 of cylindrical mass 12 is likewise provided with 
teeth 74 that mesh with the teeth 72 on the tubular contact gear member. 
In the embodiment of FIG. 9, the rotation of cylindrical mass 12 will take 
place as teeth 74 mesh with teeth 72 on the tubular contact gear member 70 
so that no slippage of the cylindrical mass relative to the borehole can 
occur. 
Tubular base member 64 may include a wall anchor system that may be 
hydraulically actuated, such as to anchor the tubular base member to 
borehole wall 10. 
Thus, the system of this invention provides a relatively inexpensive means 
of achieving a high intensity seismic energy signal in an open borehole. 
The high energy is achieved since the borehole itself, or if operated 
within a casing the casing itself, forms an integral part of the energy 
system. That is, the whirling cylindrical mass transfers energy into the 
earth via the tubular base 64 so that maximum energy transfer of the 
seismic signal is obtained without the possibility of damaging the 
integrity of the well borehole. 
The claims and the specification described the invention presented and the 
terms that are employed in the claims draw their meaning from the use of 
such terms in the specification. The same terms employed in the prior art 
may be broader in meaning than specifically employed herein. Whenever 
there is a question between the broader definition of such terms used in 
the prior art and the more specific use of the terms herein, the more 
specific meansing is meant. 
While the invention has been described with a certain degree of 
particularity, it is manifest that many changes may be made in the details 
of construction and the arrangement of components without departing from 
the spirit and scope of this disclosure. It is understood that the 
invention is not limited to the embodiments set forth herein for purposes 
of exemplification, but is to be limited only by the scope of the attached 
claim or claims, including the full range of equivalency to which each 
element thereof is entitled.