In oil and gas well drilling, the cost of equipment and labor is extremely high. In order to minimize the cost of this phase of oil and gas production, it is desir-. able to drill the holes through earthen formations, as rapidly as possible commensurate with good drilling prac¬ tices. In drilling earthen formation holes, particularly in harder formations (which are more difficult to drill) and as the depth of the hole increases, there are a number * of operating problems that tend to make the cost of such holes more expensive. Also, there are a number of trade¬ offs and drilling factors which must be considered in order to maximize the rate of penetration of the drill bit and minimize the cost.

The primary sources of drilling forces which affect the rate of penetration during drilling are: (1) the torque provided by the rotation of the drill bit as it bores its way through the earthen formation, (2) the weight, supplied by that portion of the drill assembly known as the drill collar, acting on the drill as it presses against the formation, and (3) the pressure of the drilling fluid which is delivered to the drill bit through the drill string.

As the depth of the well increases, the drilling forces available to the drill bit as a result of the rota-

tion of the drill bit and the pressure of the drilling mud are re ^' duced because of transmission losses between the drilling rig and the bit. Furthermore, as the hole gets deeper, the earthen formations become more difficult to drill. Therefore, the rate of penetration decreases.

To maintain the rate of penetration the weight acting on the drill bit and its torque can be increased. How¬ ever, where the weight on the drill bit is increased, the drill bit wears out much faster. It then becomes necessary to replace the drill bit more frequently. This is also a very undesirable trade-off since the entire drill string must be removed from the hole in order to replace the drill bit. For holes of 10,000 feet or more, replacing the bit often takes one or more days and is very costly.

Placing additional weight on the drill bit is also an undesirable trade-off because increased weight causes the bit to drill- in unwanted directions (directional insta¬ bility), which may cause expensive operating problems.

High pressure fluid jets provide a means of increasing the rate of penetration by increasing power levels at the bit without increasing directional control requirements. There are methods in current practice that use fluid jets to incre.se drilling rates. These methods involve increasing the fluid pressure of the conventional drilling mud stream from 2000 pounds per square inch (psi) to approximately 4000 psi. The added pressure is used to increase the velocity of the fluid leaving the nozzles. However, this is done only to assist in the removal of the cuttings, not to penetrate the rock. This method i ^' s commonly known as jet drilling and generally results in increased rates of penetration of about 30% to 50% over conventional approaches.

Experimental approaches have investigated higher pressure fluid jets as a means to assist the drilling process by actually cutting the rock with the jet. In one

program in which target pressures of 15,000 psi were attempted, the rates of penetration increased by factors of 2 to 4 but apparently the system held together only for a short time duration. The system was designed to increase the entire mud stream pressures from pump through jet nozzles. This required surface power sources of around 5000 horsepower (hp) at mud flow rates of about 400 gallons per minute (gp ) . Numerous operating problems ensued because of elevated pressures. Both pump and transmission systems (drilling assembly) failed in a rela¬ tively short time. The approach was proven to be tech¬ nically effective, but not practical..

Summary of the Invention

The present invention comprises a drilling method and apparatus which achieves the advantages of jet drilling without the attendant disadvantages. This invention com¬ prises a hybrid system which couples the advantageous effects of a fluid jet and a mechanical drill bit in a dual-fluid system. The resulting combined jet and mechan¬ ical drill provides a dramatic increase (up to five times) in the drilling rates available with conventional techniques, without increasing the weight acting on the drill bit or experiencing the related directional control problems.

The methodology of this invention, to solve the pro¬ blems encountered by the earlier experimenters, is to separate the power stream used for drilling from the end stream used for removing cuttings from the b ^' ore hole. The process takes a relatively small side stream from the total mud stream, raises it to much higher pressure, e.g., 20,000 psi or higher, transmits it via a dual conduit drilling assembly to a drill bit modified with a series of small nozzles, and reco bine the two fluid streams at the bottom of the hole to form a conventional drilling fluid that is circulated back to the surface to repeat the cycle. As a result, a low horsepower, (for example,

approximately 600 hp compared to 5000 hp when the entire mud stream is pressurized) highly effective jet power stream is provided at the bit that adds drilling capacity at the bottom of the hole and increases the rates of pene¬ tration by a factor of 5 or more over conventional systems.

The drilling mud, containing the rock chips and debris, is filtered at the top of the well in the normal process. The side stream portion is filtered even further for use as a high pressure fluid. It may be necessary to clarify the side stream by decreasing suspended solids content and eliminating particle sizes larger than, for example, 300 microns. As an alternative to filtered or clarified drilling mud, any fluid that the high pressure pumps can handle without excessive wear and tear, and that won't plug the nozzle at the drill bit, can be utilized as the high pressure fluid.

Thus, the present method and system comprises a closed

« system in which the drilling fluid, including mud and high pressure fluid, continuously circulate. Because these two fluids mix at the drill bit, the drilling mud may be con¬ centrated so that the final solution contains the proper additives for those particular drilling conditions. This drilling mud concentration is accomplished at the surface where the additives are added to the mud before it is pumped down the drill pipe.

The volume of the high pressure fluid under the system of the present invention is much less than the total drilling mud volume as in prior art jet drilling systems. The volume of the high pressure fluid is ^' on the order of 25 - 75 gpm as opposed to 300 - 400 gpm for the total drilling mud stream. In addition, the high pressure of the jet fluid (for example, on the order of 15,000 - 25,000 psi or even higher) can be achieved at these lower volumes by means of only 200 - 900 hp. This compares with 3,000 - 6,000 hp under prior systems. Thus, there is a

-5- significant reduction in the horsepower requirements for jet cutting by the present drilling system. Because of this tremendous reduction in horsepower, even higher pressures, such as 40,000 - 50,000 psi, can be achieved in _ the jet fluid without uneconomical horsepower requirements when low flow rates are maintained.

There are several other associated advantages of the present drilling method. Because the high pressure fluid is filtered or clarified such that the abrasives and mud additives are reduced, there is minimal abrasion and wear

10 on the pumping equipment and the drill string conduits. Furthermore, because of the lower flow rates and the positioning of the jet nozzles with respect to the drilling bit, there is no overcut. This aids in main¬ taining good hole straightness. Moreover, the concentric

15 conduit, having the high pressure fluid within the outer, drilling mud fluid, minimizes any safety hazards associated with the high pressures of the fluid jet.

Because low volume - high pressure systems are safer than high volume - high pressure systems, and because the

20 use of a concentric dual conduit drilling assembly puts the high pressure - low volume power stream inside of a conduit which in turn is inside of regular drill pipe, further increasing safety, the jet drilling system herein described is able to meet the demand for a safe, econo¬

25 mical method that will increase rates of penetration at depth and in hard to drill earthen formations.

The present method and apparatus is also highly advan¬ tageous because it can be easily integrated into conven¬ tional drilling systems. Moreover, conventional drilling

30 can be continued without bit replacement should softer formations be encountered. Also, because the rate of penetration for the present method is so high, the delays and high expense associated with "fishing" may be eli¬ minated. Fishing occurs when an object is lost at the

35 bottom of the well and must be retrieved before drilling

can continue. With the higher drilling rates achieved under ^" the present system, the obstruction potentially can be drilled around or a new hole drilled with economic results.

Furthermore, under the present system, controlled directional drilling is faster. This is because the gra¬ vitational force component supplied to the conventional drill bit by the weight of the drill string decreases since gravity is no longer acting directly in line with the direction of the bit. Power levels to the bit are further reduced by the increased friction of the drill pipe and drill collar as it lays against the side of the hole. Because increases in power level are supplied, in the present invention, by high pressure fluid which is not affected by the "c ange in hole direction, the present invention produces faster directional hole drilling than conventional approaches. _ .

The present invention also contemplates an improved drill bit system. In prior jet drilling systems, the fluid jet acted on the rock independent of ^" the- mechanical- cutter. In the present invention,. however, the fluid jet acts in concert with the mechanical cutter. Two mechanisms are proposed.

In the first mechanism, a jet assisted mechanical system, the fluid jet is configured with respect to each cutting tooth so that the jet is parallel and close to the cutting plane of the tooth and strikes the earthen forma¬ tion at the cutting surface/rock interface. Thus, the fluid jet serves the important function of cleaning the surface of the rock so that the cutting tooth can avoid crushing cut rock -and efficiently apply the cutting force. With conventional drilling methods, more than 75% of the cutting power is used up in crushing chips and rocks which have already been cut. This expenditure of power is wasteful and reduces the drilling rate. By the use of a fluid jet aimed at the cutter/rock interface,

previously cut rock is cleaned from the cutter, thus pro¬ viding ^" direct contact between the formation and the drill bit, thereby vastly increasing the drilling rate. An important advantage of the present fluid jet/mechanical drill bit is that the cutter forms cracks which may be propagated by the water jet. In particular, the fluid jet improves drilling in ductile failure condi¬ tions by encouraging the formations of cracks in the rock. This reduces pressure and horsepower requirements and improves bit life at the same time.

In accomplishing these advantages, the distance between the fluid jet and the substantially parallel cutting to the plane is approximately 0.5 - 3 millimeters and is located approximately 5 to 50 millimeters from the desired target. It should also be noted that a submerged jet is involved in this invention since the drilling mud surrounds the cutting environment. It has been found that any power loss in the fluid jet due to its submerged state is due more to the dispersion of the jet b ^* ^ the drilling mud than the interference of the mud itself. Thus, in the present invention, a long chained polymer of approximately 0.1 to 2% solution may be added to the high pressure fluid in order to maintain the cohesiveness of the fluid jet. This also reduces the friction and wear on the drill _ string conduit, pump valves, etc.

In the second mechanism, a mechanically assisted jet system, the fluid jets are located between the cutting teeth and actually form grooves in the rock. This facil¬ itates the formation of cracks and chips by the mechanical cutting teeth. In both systems the jet is 5-50 milli¬ meters from the cutting plane, commonly known as the stand-off distance.

In summary, the drilling method and apparatus of the present invention increases the drilling rate of conven¬ tional drill rigs by up to five times the usual amount, while at the same time reducing the horsepower require-

ments of previous drilling systems by an order of magni¬ tude.

Brief Description of the Drawings

Figure 1 is a schematic view illustrating the overall drilling method and apparatus of the present invention including the components at the well head and the drill string.

Figure 2 is a sectional view of a portion of the drill string illustrating the dual conduits thereof with the high pressure conduit .concentrically arranged within the lower pressure drilling mud conduit, and also illustrating the mixture of the two fluids rising in the annulus of the hole.

Figure 3 is a perspective view of a conventional drag bit which has been modified to receive jet nozzles at the cutting plane of each tooth.

Figure 4 is a close-up, cross-sectional view taken along line 4-4 of Figure 3 illustrating the position of the fluid jet with respect to the cutting plane at the cutter/rock interface.

Figure 5 is a close-up sectional view illustrating an alternate positioning of the fluid jet between cutting teeth.

Detailed Description of the Invention

Referring to Figure 1 , there is shown a conventional drilling rig with the additional components necessitated by the method and apparatus of the combined jet/mechanical drill of the present invention. The components of the conventional drilling system include the drill string 10 (both above ground and below ground portions) , the drill pipe handler 12 for attaching the individual sections of the drill pipe -14, the mud cleaning system 16 shown in the lower right hand portion of Figure 1 , and the separation system 20 located at the upper right hand portion of Figure 1. The mud cleaning system cycles the drilling mud back into the well through the swivel 18 located at the

top of the drill string 10. The separation system 20 further filters and clarifies the drilling mud so that it serves as the high pressure fluid for the jet drill.

The above ground portions of the drill string 10 include the swivel 18, as mentioned above, which permits the drill string to rotate while passing drilling fluid through the conduit of the drill pipe 14. In the present invention, a conventional swivel has been modified -to include a dual rotary hose system for the injection of both the lower pressure drilling mud through one hose 22 and the high pressure jet drill fluid through a second hose 24.

The drill string 10 also includes a normal kelly section 26 for imparting rotation to the drill string 10 ^■ and a series of inter-connected drill pipe 14. The. drill pipe of the present invention has been modified, as will be explained in more detail in connection with Figure 2, to comprise a dual conduit system. Individual sections of the drill pipe are interconnected at tool joints 64, only one of which is illustrated in Figures 1 and 2. As with conventional drilling rigs, the below ground portion of the drill string 10 includes a weighted drill collar 28 which provides gravitational weight acting on the drill bit and a MWD (measurement while drilling) collar (not shown) which gathers vital information at the bottom of the well and transmits it up through the hole to a monitor 30. Finally, at the b.ottom of the hole, the drill bit 32 is attached to the end. of the drill string 10. The drill bit of the present invention is described in more detail in connection with Figures 3-5.

A conventional mud cleaning system 16 as shown in Figure 1 includes a solids/fluid separator 34 (sometimes referred to as a "shale shaker") located above a tank 36. The drilling mud is pumped from the well through a conduit 38 to the shaker 34. The cuttings and other major solids 40 which are contained in the drilling mud as it

emerges from the hole are dumped into a cuttings pit 42. ^• The drilling mud may or may not receive a secondary treat¬ ment 44 before being pumped back into the well. The amount and types of mud treatment depend on the individual drilling operation, geographic location, type of drilling mud, and a number of other factors. Typically, however, the mud may be passed through mechanical or vacuum de¬ gasing equipment and then through a series of hydro- cyclones which remove successively finer solid components from the mud stream. Such de-sanding and de-silting cyclones can remove virtually all material greater than 40 microns and about 50% of the material greater than 15-20 microns. De^-silting cyclones frequently remove the b?rite in weighted drilling muds and other additives which then must be replenished before the mud is ready to be pumped back into the well. Furthermore, sufficient additives must be added to the mud so that -it is slightly concen¬ trated as it goes back down the well.

At this point, the drilling mud is pumped by means of pumps 46 to the--normal pressure of 3,000-5,000 psi through a conduit 48 back to the low-pressure rotary hose 22 of the swivel 18. The drilling mud is slightly concentrated as it re-enters the well so that when it mixes with the high pressure fluid at the bottom of the well it will have the proper concentration to perform its usual work o_: bit cooling, cuttings removal, and hole maintenance.

Still referring to Figure 1 , the separation system 20 of the present invention provides the high pressure .fluid for the jet assisted drilling. Unlike drilling mud, the high pressure fluid may need to be of higher clarity than the mud since suspended solids could cause serious erosion and damage to pumps and other exposed equipment. In this system a portion of the drilling mud stream, about 10-25%, is drawn through a conduit 50 to a decanting centrifuge 52 which removes fine colloidal material from the drilling mud. Such centrifuges can remove material in the 3-5

micron range to provide a clarified fluid for pressuri- zation purposes. Further clarification involves the use of gravity sedimentation techniques (not shown) , including the use of thickeners, clarifiers and flocculating agents to neutralize the surface charges on the colloidal par¬ ticles. The clarified liquid may be distilled, if necessary, utilizing waste heat from the drilling rig power source and then passed through ultra filtration devices or used directly as the fluid source for the high pressure pumps. The appropriateness of further treatment would be considered separately for each type of mud system and would depend on the adequacy of the solids control system available in the mud cleaning system.

Preferably, the high pressure liquid would contain particles only .015 inches or less and would be not any larger than one half the diameter of the jet nozzles (shown in Figures 4 and ^' 5). The clarified fluid is then conducted through a conduit 54 to a conventional inten- sifier 56 which pressurizes the fluid to at least 20,000 psi at a flow rate of 25-75 gallons per minute. At these levels, only about 200-900 hp is required in the inten- sifier 56 or the pumping system. The pressurized fluid is then passed through a high pressure conduit 58 to the high pressure rotary hose 24 of the swivel 18.

Thus, it can be seen that the method and apparatus of the present invention contemplates a closed system in which two fluids are continuously circulated, being sepa¬ rated, mixed and separated again.

Referring to Figure 2 there is shown a section of the drill pipe 14 located within the hole and just below the surface of the ground. As discussed above, the drill pipe is concentric, with the high pressure conduit 60 con¬ taining the jet drill fluid located within the outer conduit 62 which conducts the concentrated drilling mud to the bottom of the hole. This configuration promotes the safety of the present invention by locating the high pres-

sure conduit 60 within the drill pipe 14. Two sections of the dr-ill pipe 14a and 14b are joined at a tool joint 64 - by a threaded connection. At this location, the joined portions 60a and 60b of the high pressure conduit are connected by a stab joint 66 connection and high pressure seals 68.

The arrows within the drill pipe 14 indicate that the flow of fluid therein is downward. Also shown in Figure 2, as indicated by the arrows, is the drilling mud of normal concentration rising in the annulus 70 of the hole after the concentrated drilling mud in conduit 62 has mixed with the high pressure fluid in conduit 60 at the bottom. The mud is pumped to the mud cleaning and separa¬ tion systems 16 and 20, respectively, (shown in Figure 1) through a ^" conduit 38 at the surface.

Figure 3 illustrates a conventional drag bit 32 which has been modified to receive-fluid jet nozzles to provide a jet assisted mechanical drill; although the principles βpf the present invention can also, be utilized with other types of drilling bits. The bit 32 is located at the end of the drill string, as shown in Figure 1. The cutting surface 76 of the drag bit 32 contains a number of stra¬ tegically located cutting teeth 78, each having a coated, inclined cutting surface 80 manufactured from a very hard material, such as polycrystalline diamond compact (PDC) . As the bit 32 rotates, these teeth 78 bite and cut into the formation. The fluid jet nozzles 82 are located immediately adjacent the cutting plane 80 of the teeth 78, shown in more detail in Figure 4, to provide a jet assisted mechanical drill. Also, in the cutting surface 76 of the drag bit 32 is found a number of large holes 90 where the concentrated drilling mud exits. The high pressure conduit 60 is manifolded at the bit 32 to the openings 82 which form-the fluid jets. Likewise, the low pressure conduit 62 is manifolded to the holes 90. Because of the rotary action of the drill bit 32 and the

respective pressures of the high pressure fluid and the drilling mud itself, the fluid and the mud mix instan¬ taneously at the bottom of the well in order to provide a drilling mud of normal concentration. Figure 4 illustrates the interaction of the high pressure fluid jet stream and a cutting tooth 78 on the drag bit 32 of the jet assisted mechanical drill. The tooth 78 is shown having a standard mounting in a recess of the cutting surface 76 of the drag bit 32 and is pro¬ vided with a PDC cutting plane 80. The jet nozzle 82 is located so that the fluid jet (indicated by arrow 94) is parallel to the cutting surface 76 and aimed at the cutter/rock interface 96. In this embodiment, the cutter 78 opens a crack or deformation in the formation which is then propagated by the fluid jet 94. Cuttings and splash¬ back of the jet 94 are away from the cutter surface 76 to minimize erosion and wear on the cutter surface 80. The jet 94 may be located anywhere from ^' 0.5-3 millimeters in front of and parallel with the cutting plane 80 and approximately -5 to 50 millimeters from the target which is' the cutter/rock interface 96. In addition to cleaning the cuttings and dirt from the interface area, the fluid jet 94 also cools the cutting plane 80 and the tooth 78 in order to vastly increase the drilling rate and life of the bit 32. Preferably, the fluid contains a long chain polymer in order to maintain the integrity of the jet 94 in its submerged conditions.

Figure 5 illustrates an alternate arrangement for the fluid jets 94 which are between a pair of cutting teeth 78. In this configuration, a mechanically assisted jet drill, the jets 94 actually form grooves 98 in the rock which facilitate the formation of cracks and chips by the mechanical cutting teeth 78. In this embodiment, as in that of Figure 4, the jets preferably are about 5 to 50 millimeters from the target. Although the jet locations shown in Figures 4 and 5 are preferred, other locations

can also accomplish the advantages of the present inven¬ tion, namely, increased drilling rate and extended bit life.

In conclusion, it can be seen that the present inven¬ tion dramatically improves the typical drilling rates by providing a high pressure fluid jet stream acting in com¬ bination with a conventional mechanical cutter. Further¬ more, the fluid jet is economically provided by diverting and clarifying only a small portion of the total drilling mud stream and then combining the two fluids. t the drill bit.