Percussion drilling is the manual drilling technique that was used in the first well drilled in North America. In this drilling technique, a hammering bit is attached to a long cable that is then lowered into a wide open hole. As such, it is also called cable drilling, wherein the driller uses a tripod to support the tools. By going back and forth with the bit, the action loosens the soil in the borehole, which is then extracted with the help of a bailer. At intervals, the bit is removed while the cuttings are suspended in water, which is then removed by pumping to the surface. The percussion or churn drill digs a vertical hole. It employs the principle of freely falling chisel bit hung on a cable to which percussive motion is imparted by one of the various types of power units. The power units are manual lift and drop, compressed air, and electrically driven winches. The tungsten carbide bit fitted in a hammer is lifted few meters and allowed to drop (Fig. 2.6) to hit the bottom of the hole. The process continues in succession. The churning motion of the bit crushes and scraps the ground, and so a hole is dug. The cutting of rocks thus produces mud or slurry by lowering water. The crushed material is removed from the bottom of the hole at a regular interval to make a sample. Churn drilling is suitable for soft and medium formation. In harder formation resharpening of cutting bit is required frequently resulting in lowering of progress. The capacity of the churn drill in its original form is limited to relatively short holes, under 40 m. Unless the formation is consolidated, a steel casing is necessary to prevent the collapse of the hole. Similarly, the casing may have to be cemented/isolated in order to protect the hole from contamination or prevent the hole from being a vehicle to bring various layers in communication (triggering environmental concerns). Only an uncemented casing can be used temporarily after permanent screen or casing is installed.
Figure 2.6. Schematic conceptual diagram of the percussion drilling procedure.
From Halder, 2013.
The percussion drilling itself is classified as top-hammer drilling (THD), down-the-hole (DTH) drilling, and rotary drilling (RD) rigs, depending on the operating method used (Song et al., 2016). Fig. 2.7 shows various methods. In general, THD is used mostly for mining and civil blasting works, for which the drilled hole is restricted to a length of 40 m at the most. DTH is used mainly for groundwater development and can create holes to a maximum depth of 4000 m. Although this depth is greater than many oil and gas wells, DTH is not applied to petroleum wells. Rotary drilling (RD) is most commonly used for petroleum production and geothermal development. In this technique, the drill bit is propelled by its own weight to reach depths of up to 10,000 m formations resulting in low progress at high labor cost. The capacity of the churn drill is limited to relatively short holes of 10–50 m.
Figure 2.7. Drilling mechanisms of two types (A and B) of percussive drilling system as compared to rotary drilling (C).
The principal mechanism involved in percussive drilling mechanism is the generation of percussive energy with repeated impact of the drifter (THD rigs) or the DTH hammer (DTH rigs). This energy is coupled with the feed force and rotation force that are transmitted to the drill bit through the drill rod. The energy generated from the repeated impacts is then converted into wave energy, which is transmitted to the rock via the drill bit. Finally, the drill bit, now with enough impact energy for drilling, cuts into and crushes the rock.
The rate at which the impact-generated energy in a percussive drilling system is transmitted is determined by complex effects such as drilling rod, coupling sleeve, the compressive strength of the rock, and interactions between the drill bit and the rock. The process has been studied for a simplified system. For instance, Li et al. (2000) used stress wave theory along with energy conservation law to analyze both DHT and DTH impacts and then related them to impact resistance index and rock hardness. With that analysis, it is found that certain drilling methods are highly efficient, with high rates of penetration, when drilling soft rock (uniaxial compressive strength, UCS, < 20 MPa) or medium hard rock (UCS 50–120 MPa), but the efficiency decreases when drilling very hard rock (UCS > 200 MPa).
There are numerous previous studies regarding drill bit, rock drilling, the transmission of impact energy, and drilling efficiency. Hustrulid and Fairhurst (1971a,b; 1972a,b)Hustrulid and Fairhurst (1971a)Hustrulid and Fairhurst (1971b)Hustrulid and Fairhurst (1972a)Hustrulid and Fairhurst (1972b) investigated energy transmission between the drill steel and rock and measured the specific energy resulting from the impact force. All design works follow modeling of a simplified model of the actual drilling process. The primary mechanism is the creation of crack within the rock body. The crack is initiated by the tensile stress associated with the expansion of the crushed zone during the loading process. In the crushed zone, the mechanism of side crack is mixed tensile and shear failure, but outside the crushed zone, the dominant mechanism of side crack is tensile failure. A comprehensive model is absent for this analysis but numerous semiempirical and semitheoretical relationships among the side crack length, the drilled rock property, and the drilling force are formulated to approximately predict the side crack length. In the simultaneous loading, the interaction and coalescence of side cracks induced by the neighboring button-bits with an optimum line spacing enable formation of largest rock chips, control of the direction of subsurface cracks, and a minimum total specific energy consumption. Based on this depiction, a formula was derived by Liu et al. (2008) to determine the optimum line spacing on the basis of the drilled rock properties, the diameter and shape of the button-bit, and the drilling conditions. In the rock fragmentation by multiple button-bits, most of the rock between the neighboring button-bits is chipped as a result of the coalescence of side cracks. In the remaining rock, the intensely crushed zones and significant extensional cracks are observed adjacent to the sidewall and the inside of the borehole. Fragment side distribution shows more than 80% of the fragments are fines in the crushed zones as well as the cracked zones, large fragments are indeed observed, which are the big chips caused by the coalescence of side cracks.
Although not commonly well known, percussive drilling opens up opportunities for sustainable drilling practices. Consider some of US patents issued on the topic. Mishkin et al. (1973) invented a percussive drilling machine in which an air-operated striking rearhead connected to a hammer piston was used. The hammer piston reciprocates under the action of compressed air so as to deliver blows at a drill steel located at the front end of the machine. At the same time, a reversible rotary impulse fronthead has a body accommodating two rotatable and axially movable annular pistons provided with impact projections and indentations formed between the projections. While the original patent had an embedded high-frequency reciprocatory angular oscillations system, today, we have the technology for remote sensing that can make this process dynamic. Depending on the nature of the rock and in anticipation of the rock that lies ahead, a system can be optimized. Similarly, the rotary impulse fronthead that has a ratchet mechanism, which ensures the rotation of one of the annular pistons and the drill steel only in one direction, can be optimized dynamically depending on the drilled rock information.
One significant advancement of this technology was in its application in directional drilling (Johns et al., 1993). In this invention, an air-operated hammer drill is used to onset and follow-up directional drilling. Similar to the 1973 invention, this one has a piston that reciprocates while simultaneously rotating within its housing. A hammer drill bit slidably keyed to the bottom of the piston transfers the impact energy to the formation and rotates during operation independent of an attached drillstring, making it ideally suited to directional drilling activities. Because the hammer impacts while simultaneously rotating the bit, maximum penetration of the bit is assured. Although in percussive drilling system, drill bit rotational parameters, e.g., torque and rpm, are not relevant from a rock formation breaking point of view, they become relevant in a directional drilling case. Typically, industry experience has proven that the bit optimum rotational speed is approximately 20 rpm for an impact frequency of 1600 bpm (beats per minute). This rotational speed translates to an angular displacement of approximately 4–5 degrees per impact of the bit against the rock formation. Another way to express this rotation is the cutters positioned on the outer row of the hammer bit move at the approximate rate of one half the cutter diameter per stroke of the hammer.
Other patents in percussive drilling involve various forms of incremental improvement of the original concept. For instance, Guimaraes and Cruz (2009) invented a drill bit that has a central longitudinal axis and is operable by applying repetitive axial percussive impacts on the drill bit in a direction having a component along the axis and by applying rotary motion about the axis relative to the earth formation. The principal mechanism involves introducing one or more axial cutters for predominantly axially cutting the formation triggered by the axial percussive impacts and one or more shear cutters for predominantly shear cutting the subterranean earth formation in response to the rotary motion. Table 2.3 shows a list of patents with their relevant information. The main principle of all these patents is the improvement of the transfer of energy from percussive to shear form. Fig. 2.8 shows the general trend of percussive force on the displacement of the bit. The chaotic nature of the graphs is indicative of the fact that the relationship is not linear and there are numerous other factors that play a role.
Table 2.3. Patents in percussive drilling patent citations (43).
Publication numberPriority datePublication dateAssigneeTitleUS2998085A1960-06-141961-08-29Richard O DulaneyRotary hammer drill bitUS3140748A1963-05-161964-07-14Kennametal IncEarth boring drill bitUS3258077A1963-12-301966-06-28Phipps OrvillePiercing point hammer drill bitUS3269470A1965-11-151966-08-30Hughes Tool CoRotary-percussion drill bit with antiwedging gage structureUS3388756A1965-03-291968-06-18Varel Mfg CompanyPercussion bitUS3709308A1970-12-021973-01-09Christensen Diamond Prod CoDiamond drill bitsUS3788409A1972-05-081974-01-29Baker Oil Tools Inc.Percussion bitsUS3955635A1975-02-031976-05-11Skidmore Sam CPercussion drill bitUS4051912A1976-02-031977-10-04Western Rock Bit Company LimitedPercussion drill bitUS4296825A1977-11-251981-10-27Sandvik AktiebolagRock drillUS4558753A1983-02-221985-12-17Nl Industries, Inc.Drag bit and cuttersUS4607712A1983-12-191986-08-26Santrade LimitedRock drill bitUS4676324A1982-11-221987-06-30Nl Industries, Inc.Drill bit and cutter thereforUS4716976A1986-10-281988-01-05Kennametal Inc.Rotary percussion drill bitUS4823892A1984-07-191989-04-25Nl Petroleum Products LimitedRotary drill bitsUS4991670A1984-07-191991-02-12Reed Tool Company, Ltd.Rotary drill bit for use in drilling holes in subsurface earth formationsUS5004056A1988-05-231991-04-02Goikhman Yakov APercussion-rotary drilling toolUS5025875A1990-05-071991-06-25Ingersoll-Rand CompanyRock bit for a down-the-hole drillDE4200580A11991-09-131993-03-18Hausherr & Soehne RudolfRock bitUS5244039A1991-10-311993-09-14Camco Drilling Group Ltd.Rotary drill bitsEP0563561A11992-04-021993-10-06Boart HWF GmbH & Co. KG HartmetallwerkzeugfabrikSuperimposing drilling bitUS5460233A1993-03-301995-10-24Baker Hughes IncorporatedDiamond cutting structure for drilling hard subterranean formationsUS5595252A1994-07-281997-01-21Flowdril CorporationFixed-cutter drill bit assembly and methodUS5601477A1994-03-161997-02-11U.S. Synthetic CorporationPolycrystalline abrasive compact with honed edgeUS5890551A1996-03-141999-04-06Sandvik AbRock drilling tool including a drill bit having a recess in a front surface thereofUS5992547A1995-10-101999-11-30Camco International (UK) LimitedRotary drill bitsUS6202770B11996-02-152001-03-20Baker Hughes IncorporatedSuperabrasive cutting element with enhanced durability and increased wear life and apparatus so equippedWO2001033031A11999-11-032001-05-10Relton CorporationMultiple cutter rotary hammer bitUS6253864B11998-08-102001-07-03David R. HallPercussive shearing drill bitUS6290002B11999-02-032001-09-18Halliburton Energy Services, Inc.Pneumatic hammer drilling assembly for use in directional drillingUS20020066601A12000-12-062002-06-06Meiners Matthew J.Rotary drill bits exhibiting sequences of substantially continuously variable cutter backrake anglesWO2002099242A12001-06-052002-12-12Andergauge LimitedDrilling apparatusWO2003004249A12001-07-032003-01-16Boston Scientific LimitedMedical device with extruded member having helical orientationUS6527065B12000-08-302003-03-04Baker Hughes IncorporatedSuperabrasive cutting elements for rotary drag bits configured for scooping a formationWO2003031763A12001-10-032003-04-17Shell Internationale Research Maatschappij B.V.System for rotary-percussion drilling in an earth formationWO2003042492A12001-11-132003-05-22Sds Digger Tools Pty LtdAn improved transmission sleeveUS6672406B21997-09-082004-01-06Baker Hughes IncorporatedMulti-aggressiveness cutting face on PDC cutters and method of drilling subterranean formationsWO2004104363A12003-05-262004-12-02Shell Internationale Research Maatschappij B.V.Drill bit, system, and method for drilling a borehole in an earth formationWO2004104362A12003-05-262004-12-02Shell Internationale Research Maatschappij B.V.Percussive drill bit, drilling system comprising such a drill bit and method of drilling a bore holeWO2004111381A12003-06-122004-12-23Shell Internationale Research Maatschappij B.V.Percussive drill bitUS6918455B21997-06-302005-07-19Smith InternationalDrill bit with large insertsUS20050269139A12004-04-302005-12-08Smith International, Inc.Shaped cutter surfaceUS7104344B22001-09-202006-09-12Shell Oil CompanyPercussion drilling headFigure 2.8. The impact of force on bit displacement under different rock conditions.
From Liu, H.Y., Kou, S.Q., Lindqvist, P.A., January 2008. Numerical studies on bit-rock fragmentation mechanisms. International Journal of Geomechanics 8 (1).