A drilling-rate formula for rollercone bits is derived from rock cratering mechanisms. This formula holds for "perfect cleaning", which is defined as the condition where all of the rock debris is removed between tooth impacts. Under these conditions, the drilling rate is directly proportional to the rotary speed and to the bit weight squared, and inversely proportional to the bit diameter squared and to the rock strength squared. Under imperfect cleaning conditions, such as those usually present in field drilling, regrinding of the cuttings occurs under the bit, and the drilling rates fall below those for perfect cleaning.

Craters were produced by firing spherical steel projectiles of loand I.-in. diam into sandstone and granite at velocities ranging from 300 to 6000 ft/sec. Impact angles of 30, 60, and 90 deg were used for the sandstone and 90 deg for the granite. The craters are formed by two mechanisms: (a) crushing of material in front of the projectile and (b) fracturing which takes place as fractures are initiated by a constant impulse in steplike fashion in front of the projectile and propagated along logarithmic spirals of maximum shear to the free surface of the rock. The volume of the material removed by crushing varies as the first power of the impact velocity and the volume removed by fracturing, as the second power of the impact velocity. Penetration varies linearly with the impact velocity and is inversely proportional to the specific acoustic resistance of the target material, the proportionality constant being dependent upon the shape of the projectile.

A study of bit-tooth penetration, or crater formation, under simulated borehole conditions has been made. Pressure conditions existing when drilling with air, water and mud have been simulated for depths of 0 to 20,000 ft. These crater tests showed that a threshold bit-tooth force must be exceeded before a crater is formed. This threshold force increased with both tooth dullness and differential pressure between the borehole and formation fluids. At low differential pressures, the craters formed in a brittle manner and the cuttings were easily removed. At high differential pressures, the cuttings were firmly held in the craters and the craters were formed by a pseudoplastic mechanism. With constant force of 6,500 lb applied to the bit teeth, an increase in differential pressure (simulated mud drilling) from 0 to 5,000 psi reduced the crater volumes by 90 per cent. A comparable increase in hydrostatic fluid pressure (simulated water drilling) produced only a 50 per cent decrease in volume while changes in overburden pressure (simulated air drilling) had no detectable effect on crater volume. Crater tests in unconsolidated. sand subjected to differential pressure showed that high friction was present in the sand at high pressures. Similar friction between the cuttings in craters produces the transition from brittle to pseudoplastic craters. INTRODUCTION The number of wells drilled below 15,000 ft increased from 5 in 1950 to 308 in 1964.1 Associated with these deep wells are low drilling rates and high costs. High bottom-hole pressures produce low drilling rates by increasing rock strength and by creating bottom-hole cleaning problems. This paper describes an experimental investigation of crater formation under bottom-hole conditions simulating air, water and mud drilling. Although numerous investigators have studied bit-tooth penetration (cratering) at atmospheric pressure conditions, only limited work has been done on cratering in rocks subjected to pressures existing in oil wells. Payne and Chippendale2 have studied cratering in rocks subjected to hydrostatic pressure using spherical penetrators. Garner et al.3 conducted crater tests in dry limestone by varying overburden pressure and borehole fluid pressure independently and using atmospheric formation-fluid pressure. Gnirk and Cheathem4,5 have studied crater formation in several dry rocks subjected to equal overburden and borehole pressure and atmospheric formation pressure. Podio and Gray6 studied the effect of pore fluid viscosity on crater formation using atmospheric borehole and formation-fluid pressure and varying overburden pressure. Although these studies have provided useful information On crater formation under pressure, they were limited in that the three bottom-hole pressures could not be varied independently and, therefore, that many drilling conditions could not be simulated. The pressure chamber used in this study allowed visual observation of the cratering mechanism and independent control of the borehole, formation and confining pressures. By using different fluids in the chamber, pressure conditions existing in air, water and mud drilling to depths of 20,000 ft were simulated. The mechanisms involved in cratering at these different pressure conditions were studied for teeth of varying dullness and at different loading rates. High-speed movies (8,000 frames/sec) and closed-circuit television were used to visually study the crater mechanism under pressure. EXPERIMENTAL PROCEDURE PRESSURE CHAMBER The pressure chamber in Fig. 1 was used to simulate bottom-hole pressure conditions. This chamber has been pressure-tested to 22,500 psi and is normally operated at pressures up to 15,000 psi. The chamber contains four lucite windows7 used for illuminating and observing the crater mechanism under pressure. A closed-circuit television and a Fastax camera (8,000 frames/sec) have been used in these studies. Cylindrical rock specimens (8-in. diameter×6-in. long) were subjected to three independently controlled pressures simulating overburden, borehole fluid and formation-fluid pressures. Overburden pressure, which corresponds to the stress induced by the overlying earth mass, was applied by exerting fluid pressure against a rubber sleeve surrounding the rock. Borehole pressure, which is the pressure exerted by the column of mud in the wellbore, was simulated by applying pressure to the fluid overlying the rock in the chamber. Formation pressure was simulated by applying pressure to the water saturating the rock. The borehole and formation pressures were equal except when mud was used in the chamber, in which case the differential pressure between these fluids acted across the mud filter cake. PRESSURE CHAMBER The pressure chamber in Fig. 1 was used to simulate bottom-hole pressure conditions. This chamber has been pressure-tested to 22,500 psi and is normally operated at pressures up to 15,000 psi. The chamber contains four lucite windows7 used for illuminating and observing the crater mechanism under pressure. A closed-circuit television and a Fastax camera (8,000 frames/sec) have been used in these studies. Cylindrical rock specimens (8-in. diameter×6-in. long) were subjected to three independently controlled pressures simulating overburden, borehole fluid and formation-fluid pressures. Overburden pressure, which corresponds to the stress induced by the overlying earth mass, was applied by exerting fluid pressure against a rubber sleeve surrounding the rock. Borehole pressure, which is the pressure exerted by the column of mud in the wellbore, was simulated by applying pressure to the fluid overlying the rock in the chamber. Formation pressure was simulated by applying pressure to the water saturating the rock. The borehole and formation pressures were equal except when mud was used in the chamber, in which case the differential pressure between these fluids acted across the mud filter cake.

This paper describes recent advances in horizontal drilling technology including multibrach wells, geosteering, advanced drilling bits and motors; new short-radius tools, and small diameter drilling systems. Multibranch horizontal wells can reduce horizontal drilling costs by 20 to 30 % and the size and number of offshore platform by up to 50%. Geosteering systems utilize logging-while-drilling tools to accurately guide horizontal wells to avoide obstacles and gas water contacts and to significantly increase horizontal well productivity. Improved PDC and TSP bits and new high-power motors have potential wells. New short-radius motor systems 12 to 30 m (40 to 100 ft) radii allow 305 to 457 m (1000 to 1,500 ft) horizontal wells to be drilled more rapidly. Advantages of short-radius wells includesidetracking wells below gas caps or troublesome shales,intersecting fractures close to vertical wellboressignificantly reducing the length of the build sectionreducing drilling time and cost andallowing pumps to be placed lower in the wells New small diameter systems allow 92 mm (3 5/8 in) single and multibranch horizontal wells to be sidetracked from 114 mm (4 ?) casing Implementation of these new technologies should significantly increase the use of horizontal wellsworld wide. Multibranch Wells One of the most important developments with horizontal drilling is the increased use of multibranch wells. Figure 1 shows a Soviet (1) multibranch well with ten lateral. This well was drilled in the 1950's with nurbodrills. The laterals produced a 17-fold increase in oil production in this well. Figure 2 shows a Soviet dual branch well drilled in the Macova field in 1968. This well cast 1.23 times more than a conventional well and produced 10.5 times more useful length in the pay zone. By 1975, the Soviets had drilled over 30 multibranch and horizontal wells(2). They typically drilled vertical well just above the pay zone and then used turbodrills or electrodrills to drill five or six stand or horizontal branches 61 to 305 m (200 to 1,000 ft) into the pay zone. These multibranch wells cost 30 to 80% more han vertical wells, but produced up to 17 times more oil or gas. From 1953 to 1980, the Soviets had drilled over 30 multibranch and horizontal well2. Including:57 development36 exploratory8 relief wells10 injection wells A total 329 slant and horizontal branches were drilled in these wells with an accumulated pay-zone penetration of 175,260 m (575,000 ft.). This included 2120 sharply curved brances sidetracked from open holes (i. e. no cement plugs) with a total extension of 21,336 m (70,000 ft). Build rates up to 10 °/30 m (10 °/100 ft). Build rates up to horizontal branches were drilled with a maximum length of 640 m (2,100 ft) and a total length o 4,724 m (15,500) ft). Petro-Huun 3 used a dual branch well to drain an entire Austin Chalk lease as shown in Figures 3 and 4.

Interest in underbalanced drilling is growing worldwide at a rate not seen for anew drilling technology since the introduction of horizontal drilling. Compressible, multi-phase fluids, oftenpresent in well bores, make underbslsnced drilling dfllcult. This is a result of the intentional introduction of gas into the fluid either at the surface or through parasite or concentric strings or because of fluid influxes into the wellbore fknn the formation. Many underbalsnced drilling problems would be ehinated by the auccedd implementation of the incompressible, lightweight drilling fluid described in this paper.