Multiple lateral hydraulic drilling apparatus and method

Hydraulic drilling apparatus and method in which a hole is formed with a series of drill heads and strings of successively smaller diameter. After each section of the hole is formed, the drill head is withdrawn back through the string, leaving the string in place in the hole to serve as a casing for the well. The next smaller size drill head and string are then introduced through the strings which have already been placed, and the process is repeated until the hole has reached the desired length. The course of the hole can be changed, e.g. from vertical to horizontal, without interruption of the drilling process by selective application of the drilling fluid to the nozzles in the drill head to steer the advancing string. Multiple laterals are formed by introducing a module having a plurality of extensible drilling tubes with drill heads at the distal ends thereof into the string and applying the pressurized drilling fluid to the module to advance the tubes from the string. The direction of the holes formed with the tubes is controlled by inclining the drill heads at oblique angles relative to the axes of the tubes. Once the laterals have been drilled, the module is withdrawn from the string, and the drilling tubes can either be withdrawn with the module or they can be cut off and left in the well. In one disclosed embodiment, the drill heads which form the laterals have a generally hemispherical nose with a plurality of vortex generators or nozzles distributed thereover. In another, the drill heads have a hemispherical button projecting from a forwardly facing planar surface, with forwardly directed nozzle openings toward the front of the button and laterally directed nozzle openings in the base of the button near the planar surface.

This invention pertains generally to the drilling of boreholes in the earth 
and, more particularly, to apparatus and a method of drilling by the use 
of hydraulic jets. 
For many years, oil and gas wells have been drilled by a rotary bit mounted 
on a tubular drill string which extends down the borehole from the surface 
of the earth. The drill string is rotated at the surface, and the rotary 
motion is transmitted by the string to the bit at the bottom of the hole. 
A liquid commonly known as drilling mud is introduced through the drill 
string to carry cuttings produced by the bit to the surface through the 
annular space between the drill string and the wall of the borehole. This 
method of drilling has certain limitations and disadvantages. The string 
must be relatively heavy in order to transmit torque to the bit at the 
bottom of the hole. In hard rock, the drilling rate is slow, and the bit 
tends to wear rapidly. When the bit must be replaced or changed, the 
entire string must be pulled out of the hole and broken down into tubing 
joints as it is removed. It is necessary to use heavy, powerful machinery 
to handle the relatively heavy drill string. It is also necessary to 
install a casing in the well as it is drilled. The string is relatively 
inflexible and difficult to negotiate around bends, and frictional contact 
between the string and the well casing or bore can produce wear as well as 
interfering with the rotation of the drill bit. Powerful equipment is also 
required in order to inject the drilling mud with sufficient pressure to 
remove cuttings from the bottom of the well. 
More recently, wells and other boreholes have been drilled with high 
velocity streams or jets of fluid directed against the material to be cut. 
Examples of this technique are found in U.S. Pat. Nos. 4,431,069, 
4,497,381, 4,501,337 and 4,527,639. In U.S. Pat. Nos. 4,431,069 and 
4,501,337, the cutting jets are discharged from the distal end of a hollow 
pipe positioned within an eversible tube having a rollover area which is 
driven forward by pressurized fluid. U.S. Pat. Nos. 4,497,381 and 
4,527,639 disclose hydraulic jet drill heads attached to drilling tubes 
which are driven forward by hydraulic pressure, with means for bending the 
tube to change the direction of drilling, e.g. from horizontal to 
vertical. 
U.S. Pat. Nos. 4,787,465, 4,790,394 and 4,852,668 disclose hydraulic 
drilling apparatus in which pressurized drilling fluid is discharged 
through a nozzle as a high velocity cutting jet in the form of a thin 
conical shell. The direction of the borehole is controlled by controlling 
the discharge of the drilling fluid, either in side jets directed radially 
from the distal end portion of the drill string which carries the drill 
head or in a plurality of forwardly facing cutting jets aimed ahead of the 
drill string so as to modify the geometry of the hole being cut. Other 
drill heads in which steering is effected by controlled discharge of the 
drilling fluid through jets of different orientations are disclosed in 
U.S. Pat. Nos. 4,930,586 and 4,991,667. 
U.S. Pat. Nos. 4,787,465, 4,790,394 and 4,852,668 also disclose a seal 
arrangement which permits a hydraulic drill head to be removed or 
withdrawn from a drill string without removing the string from the 
borehole, and U.S. Pat. No. 5,255,750 discloses a system and method for 
controlling the rate of advancement or penetration of a hydraulic drill 
head. 
Recent expansion of offshore drilling has created a shortage of offshore 
drilling rigs, with increasing costs and delays in the drilling of such 
wells. In the past two years, for example, the daily rate charges for 
offshore drilling ships have doubled or tripled, and the only way to 
reduce drilling costs significantly is to decrease the time required to 
drill the wells. 
It is in general an object of the invention to provide a new and improved 
hydraulic drilling apparatus and method. 
Another object of the invention is to provide a drilling apparatus and 
method of the above character which are particularly suitable for use in 
the drilling of offshore wells and other wells. 
Another object of the invention is to provide a drilling apparatus and 
method of the above character which reduce the time and cost of drilling 
offshore wells. 
These and other objects are achieved in accordance with the invention by 
providing a hydraulic drilling apparatus and method in which a hole is 
formed with a series of drill heads and strings of successively smaller 
diameter. After each section of the hole is formed, the drill head is 
withdrawn back through the string, leaving the string in place in the hole 
to serve as a casing for the well. The next smaller size drill head and 
string are then introduced through the strings which have already been 
placed, and the process is repeated until the hole has reached the desired 
length. The course of the hole can be changed, e.g. from vertical to 
horizontal, without interruption of the drilling process by selective 
application of the drilling fluid to the nozzles in the drill head to 
steer the advancing string. 
Multiple laterals are formed by introducing a module having a plurality of 
extensible drilling tubes with drill heads at the distal ends thereof into 
the string and applying the pressurized drilling fluid to the module to 
advance the tubes from the string. The direction of the holes formed with 
the tubes is controlled by inclining the drill heads at oblique angles 
relative to the axes of the tubes. Once the laterals have been drilled, 
the module is withdrawn from the string, and the drilling tubes can either 
be withdrawn with the module or they can be cut off and left in the well. 
In one disclosed embodiment, the drill heads which form the laterals have a 
generally hemispherical nose with a plurality of vortex generators or 
nozzles distributed thereover. In another, the drill heads have a 
hemispherical button projecting from a forwardly facing planar surface, 
with forwardly directed nozzle openings toward the front of the button and 
laterally directed nozzle openings in the base of the button near the 
planar surface.

As illustrated in FIGS. 1 and 2, the drilling apparatus includes an 
elongated tubular drill string 21 which is connected to a source of 
pressurized drilling fluid 22 and a launcher 23 which can be carried by a 
drilling ship (not shown) or otherwise located at the surface of the 
earth. A drilling and control module 24 having a drill head 26 at the 
distal end thereof is positioned within the distal end portion of the 
string. The drill head has a plurality of nozzles 27 through which the 
pressurized drilling fluid is discharged in the form of high velocity 
cutting jets, and ports 28 in the wall of the module permit the fluid to 
pass from the string to the drill head. The module contains 
instrumentation for sensing the orientation and position of the drill 
head, and valves for controlling the delivery of pressurized fluid to the 
nozzles to control the direction in which the hole is bored. Systems in 
which a drill string is steered in this manner are disclosed in greater 
detail in U.S. Pat. Nos. 4,787,465, 4,930,586 and 4,991,667. A wire line 
29 extends from the rear of the module for carrying electrical signals 
and/or power between the surface of the earth and the module. 
The drilling and control module abuts against a lip seal 31 at the distal 
end of the drill string and is urged into engagement with the seal by the 
pressurized drilling fluid. The module is of lesser diameter than the 
string, and can be withdrawn or retrieved through the string without 
removing the string from the well. This enables the string to be used as a 
casing for the well in addition to serving as a conduit for the 
pressurized fluid during the drilling process. 
The string consists of a plurality of sections 21a, 21b which are threaded 
together, with a nose section 21c at the distal end of the string. The 
joints between the sections are flush joints in which male threads 32 at 
one end of each section are offset inwardly from the side wall of the 
section and received in female threads 33 in the opposite end of the next 
section, with no external protrusion at the joints to impede the passage 
of the string into the hole. 
If desired, the string can be rotated slowly (e.g., 5-10 RPM) during the 
drilling process to reduce friction as the string advances through the 
earth. In the embodiment illustrated in FIG. 3, the drill head has three 
forwardly directed nozzles 36-38, each of which is inclined at a different 
angle relative to the axis of the string, with a separate control valve 
(not shown) for each nozzle. As the string rotates, the valves are 
actuated to turn the jets on and off to steer the drill head in the 
desired direction. For example, the most obliquely inclined of the jets 
can be turned on only during a portion of each revolution to deflect the 
string in a direction opposite to the direction of the jet. 
The hole which is bored is of greater diameter than the drill string, and 
spent fluid and cuttings flow to the surface through an annular region 39 
between the outer wall of the string and the wall of the borehole. If 
desired, the apparatus can be operated as a closed loop, or sealed, system 
in which the cuttings are separated from the fluid at the surface for 
disposal and the fluid is recirculated. 
Once the hole has been drilled to the desired depth and the drilling and 
control module has been withdrawn, the well can be sealed by pumping 
cement 41 down through the string to fill the annular region between the 
string and the earth. 
After drilling as far as possible with one size drill head, the hole can be 
extended by withdrawing that drill head and continuing on with a smaller 
one. Referring now to FIG. 4, after the first drill head is removed, 
cement is pumped down through the string to seal that string in place, 
then a smaller drill string 42 with a drilling and control module 43 at 
the distal end thereof is pumped down through the first string. Module 43 
is similar to module 24 except it is smaller in diameter, and has a 
smaller drill head 44 at its distal end. As drill string 42 advances, 
spent drilling fluid and cuttings flow up through the annular region 39 
outside it until they reach string 21, then pass flow up through that 
string to the surface. When string 42 reaches the desired depth, module 43 
is withdrawn, and cement is pumped down through string 42 to fill the 
region around it and seal it to the earth. 
If further extension of the hole is desired, a smaller drill string can be 
pumped down through drill string 42, and the process can be repeated with 
successively smaller strings until the desired depth is reached. 
As illustrated in FIG. 5, utilizing the invention it is possible to drill a 
vertical hole 46 to a desired depth, then steer the drill head around a 
90.degree. bend 47 and continue drilling in a horizontal direction 48 
without interruption of the drilling process. A rotating 41/2 inch 
diameter string, for example, will make a medium radius turn of about 400 
feet in going from the vertical direction to horizontal. The well can then 
extend in the horizontal direction for an extended distance, e.g. up to 
about 25,000 feet. 
FIG. 6 illustrates a module for drilling a plurality of laterals from the 
distal end of a borehole such as the horizontal extension in the 
embodiment of FIG. 5. This module includes a flexible cylindrical housing 
or canister 51 of slightly smaller diameter than the string through which 
it is introduced. It is propelled through the string by the pressurized 
drilling fluid and abuts against a lip seal 52 at the distal end of the 
string. 
A plurality of lateral drilling tubes 53 are mounted in longitudinally 
extending guide tubes 54 for extension beyond the distal end of the 
housing. The tubes are fabricated of a seamless, coiled tubing and have a 
length on the order of 30 to 100 feet, with drill heads 56 at the distal 
ends thereof. In the embodiment illustrated, four tubes are disposed in 
quadrature within the housing, but any suitable number of tubes can be 
used. With a housing sized for use in a string having a diameter of 41/2 
inches, the drilling tubes can a diameter on the order of 11/2 inches. 
With 5/8 inch drilling tubes, a housing of that size will accommodate up 
to eight tubes. In most common oil field formations, the 11/2 inch tubes 
will drill boreholes having a diameter on the order of 2-4 inches, and 5/8 
inch tubes will produce boreholes with a diameter on the order of one 
inch. 
The drilling tubes are extended from the housing by hydraulic pressure in 
much the same manner that the drill string is advanced through the 
formation. The pressurized drilling fluid is applied to the proximal ends 
of the tubes, one at a time, by a rotary valve 57 and is discharged 
through the drill heads 56 at the distal ends of the tubes. That valve 
can, for example, be of the type disclosed in U.S. Pat. No. 4,790,394 for 
controlling the delivery of fluid to a plurality of nozzles. Extending 
only one of the drilling tubes at a time requires substantially less 
pressurized fluid and pump capacity than would be required to extend all 
four of the tubes at once. However, if sufficient pumping capacity is 
provided, all of the laterals can be drilled at the same time. 
The rate at which each of the drilling tubes is extended is controlled by 
releasing a limited amount of pressurized fluid from a chamber 58 which 
decreases in volume as the tube advances. This chamber is formed between 
annular seals 59, 61 which are affixed to the proximal ends of the 
drilling tubes and the distal ends of the guide tubes, with orifices 62 in 
the guide tubes through which a controlled amount of fluid entrapped in 
the chambers can escape. 
As illustrated in FIG. 8, drill heads 56 are inclined at oblique angles 
relative to the axes of the drilling tubes to control the direction in 
which the laterals are drilled. With the heads inclined in this manner, 
each of the tubes will tend to travel in a curved trajectory 63 in the 
plane of the angle with a radius of curvature and sense (clockwise or 
counter-clockwise) determined by the angle of the head. Angles on the 
order of 1.degree. to 3.degree. have been found to give radii of curvature 
on the order of 100 to 200 feet. 
In one presently preferred embodiment which is illustrated in FIG. 9, the 
heads on the four drilling tubes are arranged to provide an outwardly 
diverging pattern of laterals in a horizontal plane. For this purpose, the 
heads on two of the tubes 53a, 53b are inclined in opposite directions to 
provide radii of curvature of approximately 100 feet, and the heads on the 
other two tubes 53c, 53d are inclined in the opposite directions to 
provide radii curvature of approximately 200 feet. With these angles, the 
four laterals are formed generally in a horizontal plane, with the two 
innermost laterals curing away from each other with 200 foot radii of 
curvature and the two outermost laterals curving away from each other with 
100 foot radii of curvature. 
If desired, the heads can be oriented to provide other directions of 
curvature such as a combination of horizontal and vertical like the legs 
of a stool. 
Once the laterals have been drilled, the module can be withdrawn from the 
drill string using a wire line, a sucker rod or other suitable means. The 
drilling tubes can either be withdrawn with the module, or they can be cut 
off by suitable means such as electrochemical cutting and left in the 
formation. 
Further extension into the formation can be made by introducing another 
string of smaller diameter into the well and extending it out though the 
end of the horizontal section. Another set of multiple laterals can then 
be drilled through that string. Thus, as each local reservoir zone in a 
formation is depleted, that zone can be isolated and cased off, and 
another set of laterals can be extended. 
Drill heads 56 can be of any suitable design such as the conical jet drill 
head disclosed in U.S. Pat. No. 4,790,394. Such drill heads have an 
internal chamber in which the pressurized drilling fluid is turned into a 
whirling mass and a nozzle through which the whirling fluid is discharged 
as a high velocity cutting jet in the form of a thin conical shell. 
Another suitable drill head for use in drilling the laterals is illustrated 
in FIG. 10. This head has a rigid body 64 with a cylindrical side wall 65, 
a hemispherically curved end wall or nose 66, and an internal chamber 67 
which communicates directly with the interior of the drilling tube on 
which the head is mounted. A plurality of vortex generators are 
distributed over the nose in the form of nozzles which communicate with 
the chamber and discharge the pressurized fluid in the form of high 
velocity vortical cutting jets. The embodiment illustrated has three 
forwardly facing nozzles 69 which are inclined obliquely to the axis of 
the drill head, and a plurality of nozzles 71 which are inclined in a 
manner which produces the vortex action. 
In that regard, nozzles 71 are arranged in rings 72a-72e which are disposed 
concentrically about the axis of the drill head. The nozzles in each of 
the rings are directed along a conical surface, with the vertices of all 
of the cones being at the center of the hemisphere. Rather than extending 
perpendicular to the surface of the nose however, the nozzles are inclined 
at an angle on the order of 5.degree. to the perpendicular direction, with 
the nozzles in alternate rings being inclined in opposite directions. 
Thus, for example, in one embodiment, the nozzles in rings 72a, 72c and 
72e are inclined in a clockwise direction as viewed from the front of the 
drill head, and the nozzles in rings 72b and 72d are inclined in a 
counter-clockwise direction. Thus, alternate rings of nozzles produce 
conical masses of drilling fluid which whirl in opposite directions, 
producing a vortex action where they impact upon the formation. This drill 
head is particularly effective in cutting carbonates and non-crystalline, 
homogeneous, non-fractured rocks. It is also useful for cutting sandstone 
and granite formations. 
FIG. 11 illustrates another drill head which will cut the same formations 
as the embodiment of FIG. 10, but with a substantially lesser number of 
jets. This head has a cylindrical body 73 with a forwardly facing annular 
planar wall 74 and a hemispherical button 75 which protects in a forward 
direction from the planar wall. It has three forwardly facing nozzles 76 
toward the tip of the button inclined obliquely to the axis of the nozzle 
and 10 laterally extending nozzles 77 in the base portion of the button 
near the planar surface. The button has a diameter on the order of 
one-third the diameter of the cylindrical body, and in a 11/2 inch drill 
head, the button has a diameter of about 3/8 inch. Each of the nozzles has 
a diameter on the order of 0.012-0.020 inch. It is believed that because 
of the Coanda effect, the jets produced by the laterally extending nozzles 
tend to follow the contour of the planar surface and are directed 
outwardly in a plane perpendicular to the axis of the head. 
With the drill head of FIG. 11, the forwardly directed jets first cut into 
the formation in front of the drill, then the laterally directed jets cut 
laterally, forming a borehole having a diameter on the order of twice the 
diameter of the drill head, with an impression of the button at the end of 
the hole. With only 13 nozzles, it requires only about one-third the flow 
required for the embodiment of FIG. 10 to cut at approximately the same 
rate in the same formation. 
If desired, an abrasive can be added to the drilling fluid to enhance the 
drilling rate. Suitable abrasives include silica (SiO.sub.2); iron oxides 
such as hemitite (Fe.sub.2 O.sub.3), magnetite (Fe.sub.3 O.sub.4) and 
limonite (FeO.OH.NHO.sub.2); alumina (Al.sub.2 O.sub.3); garnet (A.sub.3 
B.sub.2 (SiO.sub.4).sub.3, iron slag, copper slag and steel balls, either 
stainless or carbon steel. The particles of abrasive should preferably be 
of a size no greater than about 1/6 to 1/5 of the diameter of the openings 
in the nozzles to prevent bridging of the particles across the openings. 
By using abrasives, drilling fluid pressure can be reduced from frac pump 
pressure (10,000-20,000 psi) to mud pump pressure (2500-10,000 psi). 
FIG. 12 illustrates a well drilled in accordance with the invention in 
which a vertical bore is drilled to a depth of about 8000 feet, using a 9 
inch drill string 81. The drill head is then removed from that string, and 
a 7 inch string 82 is introduced through it and steered around a 
90.degree. bend to change the direction of the well from vertical to 
horizontal. The 7 inch drill head is then removed, and a 4 inch string 83 
is introduced to extend the well horizontally up to about 25,000 feet. The 
4 inch drill head is then removed, and a module containing a plurality of 
lateral drilling tubes 84 is pumped down to the distal end of the 4 inch 
string. The tubes are then extended one at a time to form a pattern of 
laterals which is determined by the angles of the drill heads at the ends 
of the tubes. 
The invention has a number of important features and advantages. It 
significantly reduces the time and cost to drill oil and gas wells, 
particularly offshore wells. Using the drill string as a casing eliminates 
the need to install a separate casing, and that in itself can result in a 
saving of up to about 30 percent of on-station time, i.e. the time a drill 
ship must remain at a site where a well is being drilled. Drilling is done 
without a mud motor, and drilling control can be implemented while the 
drilling is being done. Thus, drilling around corners can be done 
seamlessly and continuously without interruption of the drilling process. 
The drill heads can be withdrawn and reinserted without tripping the drill 
string or drilling tubes, and the system can be operated in a closed loop 
mode with environmentally non-damaging fluids. The rate of penetration in 
oil reservoir rocks is comparable to or faster than with conventional 
rotary drilling. The system operates quietly and does not vibrate or 
damage electronic or mechanical devices within the drilling and control 
module at the distal end of the string. 
In oil wells with differential permeability, extending multiple laterals in 
different directions at a given level assures penetration of those areas 
around the well bore with the highest productivity, i.e. the zones of 
highest permeability. The angled drill heads produce sufficiently 
predictable and reproducible paths for the laterals that the need for a 
guidance and control system for each lateral is eliminated. 
By drilling a well with a series of strings of successively smaller 
diameter, it is possible to extend the well farther than might otherwise 
be possible. 
It is apparent from the foregoing that a new and improved drilling 
apparatus and method have been provided. While only certain presently 
preferred embodiments have been described in detail, as will be apparent 
to those familiar with the art, certain changes and modifications can be 
made without departing from the scope of the invention as defined by the 
following claims.